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High Performance CMOS Light Detector
with Dark Current Suppression in
Variable-Temperature Systems
Wen-Sheng Lin, Guo-Ming Sung * and Jyun-Long Lin
Department of Electrical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan;
[email protected] (W.-S.L.); [email protected] (J.-L.L.)
* Correspondence: [email protected]; Tel.: +886-2-27-712-171 (ext. 2121); Fax: +886-2-27-317-187
Academic Editor: Vittorio M. N. Passaro
Received: 26 September 2016; Accepted: 9 December 2016; Published: 23 December 2016
Abstract: This paper presents a dark current suppression technique for a light detector in a
variable-temperature system. The light detector architecture comprises a photodiode for sensing the
ambient light, a dark current diode for conducting dark current suppression, and a current subtractor
that is embedded in the current amplifier with enhanced dark current cancellation. The measured
dark current of the proposed light detector is lower than that of the epichlorohydrin photoresistor or
cadmium sulphide photoresistor. This is advantageous in variable-temperature systems, especially
for those with many infrared light-emitting diodes. Experimental results indicate that the maximum
dark current of the proposed current amplifier is approximately 135 nA at 125 ◦ C, a near zero dark
current is achieved at temperatures lower than 50 ◦ C, and dark current and temperature exhibit
an exponential relation at temperatures higher than 50 ◦ C. The dark current of the proposed light
detector is lower than 9.23 nA and the linearity is approximately 1.15 µA/lux at an external resistance
RSS = 10 kΩ and environmental temperatures from 25 ◦ C to 85 ◦ C.
Keywords: light detector; dark diode (DD); photodiode (PD); current amplifier (CA); dark current
cancellation; variable-temperature system
1. Introduction
With the ever-increasing demand for eco-design, environmental legislation for electronics is
focused on two major requirements. One is the internationalization of the restriction of hazardous
substances and the waste of electrical and electronic equipment; the other is a new directive
especially for energy using products and the registration, evaluation, authorization, and restriction of
chemicals [1]. Lifetime extension is an important eco-design strategy for mitigating the environmental
burden of developing new devices. Organic semiconductors increase the lifetime of large-area,
low-cost image sensors by several thousand hours, and they monolithically integrate with photonic
microsystems [2]. Miniaturized power supply units tend to have a smaller environmental impact at
the production stage than traditional electronics do [1]; integrated circuitry can miniaturize power
supplies. Colace reported on the first silicon-integrated, 2-D, light-sensitive array fabricated with
CMOS technology and readout electronics. The proposed chip includes a light-sensitive array,
analog-to-digital converters, dark current cancellation circuitry, and facilities for testing and calibration.
It operates as a near-infrared camera [3].
Dark current cancellation is another important eco-design element for photosensors. In reference [4],
a hybrid CMOS microfluidic microsystem was proposed for electrochemiluminescence-based biochemical
sensing. In the CMOS imager, a two-transistor reset path technique is employed to attenuate the
subthreshold leakage current and to reduce the dark current. The imager achieves a low photodiode
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(PD) dark current of 3.6 nA/cm2 , but the reset voltage is as high as 2.3 V. However, sub-dark current
measurement
by subtracting the dark signal frame from the captured frame,
Sensors 2017, is
17, completed
15
2 ofand
12 the
dark signal frame is stored off chip. Furthermore, a prototype integrated phototransistor-based CMOS
current
measurement
completed by
subtracting
dark signal
frame imaging.
from the captured
activedark
pixel
sensor
circuit withisscintillating
material
was the
presented
for X-ray
Cancellation
and
the dark
framephototransistor
is stored off chip.
Furthermore,
a prototype
integrated
of theframe,
leakage
current
usingsignal
a dummy
technique
was tested
and proved
efficient [5].
phototransistor-based CMOS active pixel sensor circuit with scintillating material was presented for
In reference [6], an ultralow dark signal was presented for an embedded active-pixel CMOS image
X-ray imaging. Cancellation of the leakage current using a dummy phototransistor technique was
sensor. To achieve in-pixel dark current cancellation, a combined photogate and PD photon-sensing
tested and proved efficient [5]. In reference [6], an ultralow dark signal was presented for an
device
was developed [6]. Specifically, the dark current was cancelled using a sensing device that was
embedded active-pixel CMOS image sensor. To achieve in-pixel dark current cancellation, a
fabricated
withphotogate
a large area.
Inphoton-sensing
the current study,
dark
cancellation
wasthe
performed
using a
combined
and PD
device
was current
developed
[6]. Specifically,
dark current
current
amplifier
(CA).
A
CA
is
a
good
photo-detection
circuit,
which
is
easily
fabricated
with
CMOS
was cancelled using a sensing device that was fabricated with a large area. In the current study, dark
technology.
cancelling
the
dark current,
can not
only enhance
of the CA but also
current By
cancellation
was
performed
usingwe
a current
amplifier
(CA). Athe
CAsensitivity
is a good photo-detection
circuit,
which is
easily fabricated
with
CMOS technology.
By cancelling
darkperformance
current, we can
notsmall
reduce
the power
consumption.
The
proposed
light detector
provides the
a high
and
only enhance the sensitivity of the CA but also reduce the power consumption. The proposed light
chip area.
detector
provides arange
high performance
and smallconverter
chip area. (LFC) chip is proposed with dark current
A
high dynamic
light-to-frequency
A high dynamic
range light-to-frequency converter (LFC) chip is proposed with dark current
◦
suppression up to 125 C. By regulating the cathode voltage of the PD and using a replica amplifier,
suppression up to 125 °C. By regulating the cathode voltage of the PD and using a replica amplifier,
the dark current is reduced. Measurements show that the output frequency is more insensitive
the dark current is reduced. Measurements show that the output frequency is more insensitive to the
to theambient
ambient
temperature and process variation. Supply voltage variation is also minimized by
temperature and process variation. Supply voltage variation is also minimized by
implementing
a
delay
Figure11shows
showsa ademonstration
demonstration
board
design
which
is
implementingconstant
a constant
delaymodule
module [7].
[7]. Figure
board
design
which
is
implemented
withwith
the light-dependent
resistor
(LDR),
thethe
light-emitting
diode
(LED),
and
ananLED
implemented
the light-dependent
resistor
(LDR),
light-emitting
diode
(LED),
and
LEDdriver
printed
circuit
board
(PCB).
The(PCB).
typicalThe
LED
driver
with
the LDR
operates
with a large
current.
driver
printed
circuit
board
typical
LED
driver
with the
LDR operates
with dark
a large
dark
current.
In
this study, a CA with enhanced dark current cancellation was fabricated using 0.18 µm 1P2M
In this study,
a CA
with enhanced
dark
current
was fabricated
using 0.18and
µ m 1P2M
CMOS technology
with
a chip
area of 762
µm
× 452cancellation
µm, including
pads. Simulated
measured
CMOS
technology
with
a
chip
area
of
762
μm
×
452
μm,
including
pads.
Simulated
and
measured
results were obtained with white LED light and characterized using a lux meter at a supply voltage
results were obtained with white LED light and characterized using a lux meter at a supply voltage
VCC of 3.3 V, an external resistance RSS of 10 kΩ, and an environmental temperature varying from
VCC of 3.3 V, an external resistance RSS of 10 kΩ, and an environmental temperature varying from
−40 ◦ C to 125 ◦ C. The rest of this paper is organized as follows. Section 2 elucidates the proposed
−40 °C to 125 °C. The rest of this paper is organized as follows. Section 2 elucidates the proposed
circuit
topology
of the
light
detector.
thesimulated
simulated
and
measured
results,
and in
circuit
topology
of the
light
detector.Section
Section 33 presents
presents the
and
measured
results,
and in
Section
4, conclusions
areare
drawn.
Section
4, conclusions
drawn.
Figure 1. Demonstration board design implemented with the LDR, LED, and LED driver PCB.
Figure 1. Demonstration board design implemented with the LDR, LED, and LED driver PCB.
2. Proposed Circuit Topology of the Light Detector
2. Proposed Circuit Topology of the Light Detector
In this section, we propose a CA with dark current cancellation that is composed of a dark diode
In
thisand
section,
we propose
CADD
with
darkthe
current
that
is composed
of a the
dark
(DD)
a photodiode
(PD).aThe
senses
dark cancellation
current in dark
environment,
while
PDdiode
(DD)senses
and athe
photodiode
The
DD senses
the Figure
dark current
inthe
dark
environment,
while
the PD
photodiode(PD).
current
in electric
lighting.
2 presents
schematic
cross-section
of an
senses
the photodiode
current
in electric
lighting.
Figure
2 presents
schematic
n+/p-substrate
PD and
its equivalent
circuit.
The PD
is built
with a p-nthe
junction,
whichcross-section
is sensitive toof an
the environmental
Figure
3 shows
the simulated
PD adark
current aswhich
a function
of
n+/p-substrate
PD andtemperature.
its equivalent
circuit.
The PD
is built with
p-n junction,
is sensitive
environmental
temperature
(°C)
and
bias
voltage
(V)
in
dark
conditions.
According
to
the
simulated
to the environmental temperature. Figure 3 shows the simulated PD dark current as a function of
results, the PD
dark current
(nA)
is highly
sensitive(V)
to the
environmental
temperature,
but
environmental
temperature
(◦ C)
and
bias voltage
in dark
conditions.
According
toinsensitive
the simulated
to the bias voltage.
results, the PD dark current (nA) is highly sensitive to the environmental temperature, but insensitive
to the bias voltage.
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17, 17,
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2017,
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3 of
12 12
Anode
Anode
Cathode
Cathode
N-well
N-well
P-substrate
P-substrate
(a)
(a)
Anode
Anode
PD
PD
Cathode
Cathode
Anode
Anode
IIpp
IId
d
Cathode
Cathode
(b)
(b)
Figure 2. n+/p-substrate PD: (a) cross-section; and (b) the equivalent circuit.
Figure
PD:(a)
(a)cross-section;
cross-section;and
and(b)
(b)the
theequivalent
equivalentcircuit.
circuit.
Figure 2.
2. n+/p-substrate
n+/p-substrate PD:
Figure 3. Simulated PD dark currents versus the environmental temperature (°C) and bias voltage (V)
in dark conditions.
Figure
Figure 3.
3. Simulated
Simulated PD
PD dark
dark currents
currents versus
versus the
the environmental
environmental temperature
temperature (°C)
(◦ C) and
and bias
bias voltage
voltage (V)
(V)
in
conditions.
in dark
dark
conditions.
Furthermore, the sensing area of the PD is another important factor. The simulated dark current
is approximately proportional to the area ratio, which is defined as the ratio of the PD area to 300 μm
Furthermore,
the sensing
area of
PD
another
important factor.
The
dark
Furthermore,
sensing
of the
thedark
PD is
iscurrent
another
factor.
The simulated
simulatedtemperature
dark current
current
× 300
μm. Figurethe
4 shows
thearea
simulated
as important
a function of
the environmental
is
approximately
proportional
to
the
area
ratio,
which
is
defined
as
the
ratio
of
the
PD
area
to 300
is approximately
proportional
to
the
area
ratio,
which
is
defined
as
the
ratio
of
the
PD
areaμm
to
(°C) in dark conditions. The smaller the area ratio is, the smaller the dark current is. All simulated
×300
300µm
μm.
Figure
4
shows
the
simulated
dark
current
as
a
function
of
the
environmental
temperature
300 µm.
Figure
4 shows
simulated
dark current
as model,
a function
of is
theprovided
environmental
dark ×
current
levels
are settled
to the
0.3 pA
due to imperfect
diode
which
from
(°C)
in dark conditions.
The
smaller the
area
ratio is,
the
smaller
thethe
darkcurrent
current
is.dark
All simulated
temperature
(◦ C) in
dark
conditions.
The
smaller
area
ratio
is,
the
current
foundry. Figure
5 shows
the CA of the
PD,
which the
operates
without
darksmaller
cancellation.
The is.
dark
current oflevels
areRPD
settled
to
pAvoltage
due
imperfect
diode
which
is
provided
from
the current
PD
is equal
tosettled
the
VPDtoinimperfect
the
PD model,
divided
by its current
. The
Allresistance
simulated
dark
levels
are0.3
to 0.3todrop
pA
due
diode model,
which
isILight
provided
foundry.
FigureFigure
shows
theisCA
theofPD,
operates
without
dark dark
current
The
light
current
of5the
PD
ILight
amplified
with
aPD,
current
mirror,
whose
magnification
iscancellation.
approximately
from
foundry.
5 shows
theofCA
thewhich
which
operates
without
current
cancellation.
resistance
of
the
PD
R
PD
is
equal
to
the
voltage
drop
V
PD
in
the
PD
divided
by
its
current
I
Light
.
The
to (W2/W
× (WPD
4/WR
3) × (W
5) with
length (L)
for V
allPD
MOSFETs.
Thus,
the output
Theequal
resistance
of1)the
equal
to equal
the voltage
drop
in the PD
divided
by itscurrent
current
PD is6/W
light
current
of
the
PD
I
Light
is
amplified
with
a
current
mirror,
whose
magnification
is
approximately
IO.can
belight
expressed
with
A:
ILight
The
current
of the
thetotal
PD current
ILight is gain
amplified
with a current mirror, whose magnification is
equal
to
(W
2/W1) × (W4/W3) × (W6/W5) with equal length (L) for all MOSFETs. Thus, the output current
approximately equal to (W 2 /W 1 ) × (WW
)
×
(W
/W
length (L) for all MOSFETs. Thus,
4 /W
3
6

W L  5 )WwithL equal
2 L2 
 A: 4 4  6 6  I Light
Ithe
O can be expressed with the totalI current
gain
O 
output current IO can be expressed
with
the
total
current
gain
A:
W1 L1  W3 L3  W5 L5 
(1)

W
L2  W4 L4  W6 L6 
2 I

A
Light
I O(
W2 /L2 ) (W4 /L4 ) (W6 /L6 )I Light
W1 L1 × W3 L3  ×W5 L5  × ILight
IO =
(1)
W1 /L
/L3 ) (the
W5 /L
(
) (W3includes
5)
1current,
where ILight, also called the photodiode
illumination
current ILux and the dark(1)
 A  I Light
current IDark, which flow from=VA
DD ×
toIthe
ground in a dark environment. The photocurrent ILight and
Light
darkILight
current
are amplified
A times.current,
If the CAincludes
operatesthe
in aillumination
variable-temperature
where
, alsoIDark
called
the photodiode
current Isystem
Lux andwithout
the dark
dark
current
cancellation,
the
dark
current
causes
malfunction.
This
is
the
reverse
of
the
characteristic
where
I
,
also
called
the
photodiode
current,
includes
the
illumination
current
I
and
Lux
Light
current IDark, which flow from VDD to the ground in a dark environment. The photocurrent the
ILightdark
and
of
p-i-n
PDs.
For
a
reverse
biased
diode,
the
reverse
bias
current
I
R can beThe
defined
by
the
following
current
I
,
which
flow
from
V
to
the
ground
in
a
dark
environment.
photocurrent
I
and
Dark IDark are amplified ADD
dark current
times. If the CA operates in a variable-temperature system Light
without
equation
with
reverse
bias
voltage
V
[8]:
dark current
current cancellation,
IDark are amplified
A times.
If causes
the CAmalfunction.
operates in aThis
variable-temperature
system
without
dark
the dark
current
is the reverse of the
characteristic
dark
current
cancellation,
the
dark
current
causes
malfunction.
This
is
the
reverse
of
the
characteristic
of p-i-n PDs. For a reverse biased diode,
IR can be defined by the following
I R the
 I Sreverse
 exp qVbias
nkTcurrent
 1
(2)
of p-i-n PDs.
a reverse
biased diode,
equation
withFor
reverse
bias voltage
V [8]: the reverse bias current IR can be defined by the following
equation with reverse bias voltage V [8]:
I R  I S  exp qV nkT  1
(2)
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IR = IS × exp(qV/nkT − 1)
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where
IS Iis
the saturation current, q is the electronic charge, kT is the Boltzman energy, and n is a
where
S is the saturation current, q is the electronic charge, kT is the Boltzman energy, and n is a
where
I
S is the saturation current, q is the electronic charge, kT is the Boltzman energy, and n is a
parameter
ofof
which
fromboth
bothdiffusion
diffusionand
andsurface-generation
surface-generation
parameter
whichthe
thevalue
valuedepends
dependson
on contributions
contributions from
parameter
ofa which
the
value depends
on contributions
from
both
diffusion
and to
surface-generation
currents.
For
4H-SiC
avalanche
photodiode
(APD),
the
dark
current
is
close
the
measurement
currents. For a 4H-SiC avalanche photodiode (APD), the dark current is close to the
measurement
currents. For a 4H-SiC avalanche photodiode (APD), the dark current is close to the measurement
limit
at atlow
At high
high bias,
bias,the
thedark
darkcurrent
current
relatively
insensitive
to temperature,
limit
lowbias
biaslevels.
levels. At
is is
relatively
insensitive
to temperature,
a
limit at low bias levels. At high bias, the dark current is relatively insensitive to temperature, a
a characteristic
that signifies
signifies tunneling.
tunneling.IfIfwe
weassume
assumethat
thatthe
thetunneling
tunnelingdark
darkcurrent
current
undergoes
characteristic that
undergoes
characteristic that signifies tunneling. If we assume that the tunneling dark current undergoes
avalanche
photocurrentatathigh
highbias,
bias,the
thetotal
totaldark
dark
current
avalanchemultiplication
multiplicationsimilar
similar to
to that
that of the photocurrent
current
is is
avalanche multiplication similar to that of the photocurrent at high bias, the total dark current is
expressed
byby
reference
expressed
reference[9]:
[9]:
expressed by reference [9]:
 Gain(V,
V , ) ×Itunneling
IdarkIIdark
= Gain
 Gain V ,T
T  I Itunneling
dark


(3) (3)
(3)
tunneling
where
Gain(V,
an additional
additionalcorrection
correction
factor
obtained
through
photocurrent
measurement.
where
Gain(V,T)
T) is
is an
factor
obtained
through
photocurrent
measurement.
The
where Gain(V, T) is an additional correction factor obtained
through photocurrent measurement. The
3/2 with the device area A and the effective bias
The
tunneling
currentItunneling
Itunneling
is proportional
to ×A V×3/2
V
tunneling
current
is proportional
to A
with
the
device
area
A
and
the
effective
bias
tunneling current Itunneling is proportional to A × V3/2 with the device area A and the effective bias
voltage
[9].The
The objectiveof
of thisstudy
study
to obtain
order
toto
achieve
higher
voltage
VV
[9].
obtainaaazero
zerodark
darkcurrent
currentin
order
achieve
higher
voltage
V
[9]. Theobjective
objective ofthis
this study was
was to obtain
zero
dark
current
ininorder
to achieve
higher
photo
resistance
sensitivity.
photo
resistance
sensitivity.
photo resistance sensitivity.
Figure
SimulatedPD
PD (dark) current
current versus standardized
dark
Figure
4.4.
standardizedarea
arearatio
ratioin
darkconditions.
conditions.
Figure
4.Simulated
Simulated PD(dark)
(dark) current versus
versus standardized
area
ratio
inindark
conditions.
V
VDD
DD
M
M11
M
M22
M
M55
M
M66
IILight
Light
IIO
O
M
M33
M
M44
V
VSS
SS
Figure 5. CA without dark current cancellation.
Figure5.5.CA
CA without
without dark
dark current
Figure
currentcancellation.
cancellation.
Figure 6 shows a CA with dark current cancellation that is composed of a dark diode (DD) and
Figure
6shows
showsaa CA
CA with dark
thatthat
is composed
of a dark
(DD) and
Figure
darkcurrent
currentcancellation
is composed
of a diode
dark diode
(DD)
a PD
[10].6The
DD senseswith
the dark
current
Icancellation
Dark in dark environments, while the PD senses the
a PD [10]. The DD senses the dark current IDark in dark environments, while the PD senses the
and
a
PD
[10].
The
DD
senses
the
dark
current
I
in
dark
environments,
while
the
PD
senses
Dark
photodiode current ILight in electric lighting. The photodiode
size of PD (APD) is two times of that of
photodiode current ILight in electric lighting. The photodiode size of PD (APD) is two times of that of
theDD
photodiode
current
ILightsize
in of
electric
lighting.
The
PD (W/L)
(APD )M2is= two
times
(ADD); and the
MOSFET
M2 is two
times that
of photodiode
M1, e.g., APD = size
2 × Aof
DD and
2 × (W/L)
M1. of
DD (ADD); and the MOSFET size of M2 is two times that of M1, e.g., APD = 2 × ADD and (W/L)M2 = 2 × (W/L)M1.
that
of DD
);
and
the
MOSFET
size
of
M
is
two
times
that
of
M
,
e.g.,
A
=
2
×
A
and
Thus,
the (A
drain
current
I
D3
of
M
3
is
equal
to
the
illumination
current
I
Lux
and
the
output
current
I
O
is
DD
1
PD
Thus, the drain
current ID3 of M3 is equal to the2illumination current ILux and
the output
currentDD
IO is
(W/L)
=
2
×
(W/L)
.
Thus,
the
drain
current
I
of
M
is
equal
to
the
illumination
current
ILux
expressed
by:
3
M2
M1
D3
expressed
by:
and the output current IO is expressed by:
II D3  II Light  II Dark  II Dark  II Lux  II Dark  II Lux
(4)
(4)
D3
Light
Dark
Dark
Lux
Dark
Lux
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ID3 = ILight − IDark = IDark + ILux − IDark = ILux
(4)
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5 of 12
(W/L)6
× ILux = B × ILux
(5)
(W/L
)L5
W
W L
6



6  I Lux  B  I Lux
(5)
IIOO 
 IBLuxisthe
B current
I Lux gain. That is, (W/L) = (W/L)
(5)
WMnL
L,and
where (W/L)n is the metric ratio of transistor
M3
M4
5
W
5
and (W/L)M6 = B × (W/L)M5 . The output current IO operates without the dark current IDark . That is,
where (W/L)
(W/L)nn is
is the
the metric
metric ratio
ratio of
of transistor
transistor M
Mnn,, and
and B
B is
is the
the current
current gain.
gain. That is,
is, (W/L)
(W/L)M3
M3 = (W/L)M4
= (W/L)
M4
thewhere
dark current
is cancelled
in Figure
6. However,
it is difficult
to achieve That
a zero illumination
current
and
(W/L)
M6 = B × (W/L)M5. The output current IO operates without the dark current IDark. That is, the
M6 = B × (W/L)M5. The output current IO operates without the dark current IDark. That is, the
in and
light(W/L)
application
because of the MOSFET mismatch in the CA. When both the DD and PD operate
dark current
current is
is cancelled
cancelled in
in Figure 6.
6. However,
However, it
it is
is difficult
difficult to
to achieve
achieve aa zero
zero illumination
illumination current
current in
in
dark
only
with
dark current,
theofFigure
two
equivalent
resistors
ofthe
theCA.
DDWhen
and PD
are
expressed
as operate
RDD and
light
application
because
the
MOSFET
mismatch
in
both
the
DD
and
PD
light
application and
because
of the MOSFET
in the
CA. ,When
both, the
DD and
PD operate
RPD
, respectively,
thethe
current
symbols,mismatch
IDark1 , IDark2
, IDD
and
denote
the
currents
Dark3and
only
with dark
dark current,
current,
two equivalent
equivalent
resistors
of the
the
PDIDark4
are expressed
expressed
as R
Rdark
DD and RPD,
only
with
the two
resistors
of
DD and
PD
are
as
DD and RPD,
of M
and the
M4 ,current
respectively,
in dark
conditions. As shown in Figure 7, the output current I
1 , M2 , M3 , and
respectively,
symbols,
Dark1, IDark2, IDark3, and IDark4, denote the dark currents of M1, M2, O
respectively,
and the
current symbols,
IIDark1
, IDark2, IDark3, and IDark4, denote the dark currents of M1, M2,
is achieved:
M3, and M4, respectively, in dark conditions. As shown in Figure 7, the output current IO is achieved:
IO =
M3, and M4, respectively, in dark conditions. As shown in Figure 7, the output current IO is achieved:
IOII O=
BB
B×
(6) (6)
Dark 4
 IIDark4
(6)
O
Dark 4
where
B is
the
current
gain
and is approximately
approximately10,000
10,000times.
times.The
The
output
where
B is
is the
the
current
gaindisplayed
displayedin
inEquation
Equation (6)
(6) and
output
where
B
current
gain
displayed
in
Equation
(6)
is approximately 10,000
times. The
output
current
IOIIincludes
the
Thiscircuit
circuitoperates
operateswith
witha large
a large
dark
current.
current
O includes
theamplified
amplifieddark
darkcurrent
current of M44.. This
dark
current.
current
O includes the amplified dark current of M4. This circuit operates with a large dark current.
VDD
V
DD
M2
M
2
M1
M
1
0.5·IIDark
0.5·
Dark
PD
PD
DD
DD
(Photo diode)
(Photo diode)
(Dark diode)
(Dark diode)
M4 ILux
M3 M
4 ILux
IILux M
3
II Dark Lux
Dark
=B IILux
IIOO=B
Lux
+IDark
Lux+I
IILux
Dark
M5
M
5
M6
M
6
VSS
V
SS
Figure 6. CA with
with dark current
cancellation.
Figure
currentcancellation.
cancellation.
Figure6.6. CA
CA with dark current
VDD
V
DD
M2 IIDark2
M4
M3 M
M
M
2 Dark2
4
3
Dark3
Dark4
IIDark3
IIDark4
=BIDark4
IIOO=BI
Dark4
M1
M
1
Dark1
IIDark1
RDD
R
DD
RPD
R
PD
+IDark3
Dark2+I
IIDark2
Dark3 M5
M5
M6
M
6
VSS
V
SS
Figure 7. All dark currents of MOSFETs in Figure 6 in dark conditions.
Figure7.7.All
Alldark
darkcurrents
currents of
of MOSFETs
MOSFETs in
Figure
in Figure
Figure66in
indark
darkconditions.
conditions.
The dark
dark current
current in
in Figure
Figure 77 must
must be
be reduced.
reduced. Figure
Figure 88 displays
displays aa CA
CA with
with enhanced
enhanced dark
dark
The
The dark
current The
in Figure
7 must
be
reduced. Figure 8 displays a CA with enhanced dark
current
cancellation.
MOSFET
size
of
M
3 is two times of that of M4, e.g., (W/L)M3 = 2 × (W/L)M4. As
current cancellation. The MOSFET size of M3 is two times of that of M4, e.g., (W/L)M3 = 2 × (W/L)M4. As
current
cancellation.
MOSFET
size
M3 is two times
of that
of M4 , e.g.,
(W/L)
=Dark
2 ,×which
(W/L)isM4 .
M3
shown
in Figure
Figure 8,
8,The
current
steering
is of
implemented
to draw
draw
different
current,
Light
shown
in
current
steering
is
implemented
to
aa different
current,
IILight
−− IIDark
, which is
Asequal
shown
in
Figure
8, current
steering
is implemented
toIOdraw
a different
current,
ILight
−total
IDark ,
to
the
illumination
current
I
Lux. The
output
current
can
then
be
expressed
with
the
equal to the illumination current ILux. The output current IO can then be expressed with the total
which
is
equal
to
the
illumination
current
I
.
The
output
current
I
can
then
be
expressed
with
the
current gain
gain A
AEE,, if
if (W/L)
(W/L)M1
M1 = (W/L)M2, (W/L)M5
(W/L)M6
M6, and (W/L)M8 = O
AEE ×× (W/L)
(W/L)M7
M7. The output current
Lux
current
= (W/L)M2, (W/L)M5
== (W/L)
, and (W/L)M8 = A
. The output current
total
current
AE , if without
(W/L)M1
= dark
(W/L)
in
Figuregain
operates
without
the
dark
current.
M2 , (W/L)M5 = (W/L)M6 , and (W/L)M8 = AE × (W/L)M7 .
IIOO in
Figure
88 operates
the
current.
The output current IO in Figure 8 operates
without
the dark current.
W L
I O  W L 88  I Light  I Dark   AE  I Lux
(7)
I O  W L   I Light  I Dark  AE  I Lux
(7)
(W/LW
)8 L 77
IO =
× ILight − IDark = A E × ILux
(7)
W/L)7 only with
When both
both the
the DD
DD and
and PD
PD (operate
operate
dark currents,
currents, the
the two
two equivalent
equivalent resistors
resistors of
of the
the
When
only with dark
DD and
and PD
PD are
are expressed
expressed as
as R
RDD
DD and RPD, and the current symbols, IDark1, IDark2, and IDark3, denote the
DD
and RPD, and the current symbols, IDark1, IDark2, and IDark3, denote the
dark
currents
of
M
1, M2, and M3, respectively. As shown in Figure 9, the output current IO is obtained.
dark currents of M1, M2, and M3, respectively. As shown in Figure 9, the output current IO is obtained.
Sensors 2017, 17, 15
6 of 12
When both the DD and PD operate only with dark currents, the two equivalent resistors of the DD
and PD are expressed as RDD and RPD , and the current symbols, IDark1 , IDark2 , and IDark3 , denote the
Sensors
2017,
66 of
dark
currents
Sensors
2017, 17,
17,of15
15M1 , M2 , and M3 , respectively. As shown in Figure 9, the output current IO is obtained.
of 12
12
IO =
× (IDark2
IIDark3)=AA
EA
E I× Io f f
II OA
I 2−
O  AE
E  I Dark
Dark2  I Dark
Dark33   AE
E  I off
off
(8)
(8)
(8)
where
A isE is
the current
gain, as
in Equation
(7). The
The outputcurrent
current I can
be reduced
to approximately
where
whereEA
AE is the
the current
current gain,
gain, as
as in
in Equation
Equation (7).
(7). The output
output current IIOOOcan
can be
be reduced
reduced to
to approximately
approximately
zero
if
the
dark
current
of
M
is
approximately
equal
to
that
of
M
(I
I
). The proposed
≈
zero
if
the
dark
current
of
22 is approximately equal to that of M3 (IDark2
≈
I
Dark3). The
proposed
circuit
3
Dark2
Dark3
zero if the dark current of M2 is approximately equal to that of M3 (IDark2 ≈ IDark3). The proposed
circuit
circuit
can
also
suppress
the
dark
current
induced
by
the
environmental
temperature,
as
described
can
also
suppress
the
dark
current
induced
by
the
environmental
temperature,
as
described
can also suppress the dark current induced by the environmental temperature, as described in
in in
Equation
(3).
Equation
(3).
Equation (3).
V
VDD
DD
IILight
Light
M
M11
M
M22
M
M33
IIDark
Dark
IILight
Light
IIDark
Dark
PD
PD
M
M55
M
M66
IIOO=A
=AEEI
ILux
Lux
M
M44
IILight
-IDark=I
=ILux
Light -IDark
Lux
DD
DD
0.5·
IIDark
0.5·
Dark
M
M77
M
M88
V
VSS
SS
Figure
CA
with
enhanced
Figure
darkcurrent
currentcancellation.
cancellation.
Figure8.8.
8.CA
CA with
with enhanced
enhanced dark
dark
current
cancellation.
V
VDD
DD
IIDark1
Dark1
||||
IIDark2
Dark2
M
M11
M
M
Dark2
M22 IIDark2
M33
IIDark3
Dark3
IIDark3
Dark3
R
RPD
PD
M
M55
M
M66
M
IIDark3
M44 0.5·
0.5·
Dark3
IIOO=A
=AEEI
Ioff
off
IIDark2
-IDark3=I
=Ioff
Dark2-IDark3
off
R
RDD
DD
M
M77
M
M88
V
VSS
SS
Figure
9. Dark
currents
all
MOSFETs
in
the
CA with
enhanced dark
current
cancellation.
Figure
Darkcurrents
currentsofof
ofall
allMOSFETs
MOSFETs in
in the
the proposed
proposed
current
cancellation.
Figure
9. 9.
Dark
proposed CA
CAwith
withenhanced
enhanceddark
dark
current
cancellation.
3. Simulated
Simulated and
and Measured
Measured Results
Results
3. 3.
Simulated
and
Measured
Results
Figure
Figure 10
10 presents
presents the
the simulated
simulated PD
PD dark
dark current
current versus
versus temperature
temperature for
for CAs.
CAs. Three
Three types
types of
of
Figure
10 presents
the the
simulated
PD
dark
current
versus
temperature
for CAs.
Three
types
of
CA
were
compared.
First,
proposed
CA
with
enhanced
dark
current
cancellation
is
indicated
by
CA were compared. First, the proposed CA with enhanced dark current cancellation is indicated by
CAFigure
were 10a.
compared.
First,
the
proposed
CA
with
enhanced
dark
current
cancellation
is
indicated
by
Figure 10a. Next,
Next, aa CA
CA without
without dark
dark current
current cancellation
cancellation is
is denoted
denoted by
by Figure
Figure 10b.
10b. Finally,
Finally, aa CA
CA with
with
Figure
10a.
Next,
a CA without
dark current
cancellation
is denoted
by Figure results,
10b. Finally,
a CA with
dark
current
cancellation
is
denoted
by
Figure
10c.
According
to
the
simulated
the
proposed
dark current cancellation is denoted by Figure 10c. According to the simulated results, the proposed
dark
current
cancellation
is
denoted
by Figureoperates
10c. According
to
the simulated
results,
the
proposed
CA
with
CA with
with enhanced
enhanced dark
dark current
current cancellation
cancellation operates
with the
the lowest
lowest dark
dark current.
current. This
This means
means that
that
CAthe
with
enhanced
dark
current
cancellation
operates
with
the
lowest
dark
current.
This
means
that
the dark
dark current
current can
can be
be cancelled
cancelled perfectly
perfectly with
with the
the proposed
proposed CA.
CA. The
The CA
CA without
without dark
dark current
current
thecancellation,
dark current
can is
cancelled
perfectly
the with
proposed
CA.
The
CA without
dark
which
by
10b,
operates
aa larger
dark
current.
Note
the
dark
cancellation,
which
isbedenoted
denoted
by Figure
Figure
10b,with
operates
with
larger
dark
current.
Note that
that
the current
dark
cancellation,
which
is
denoted
by
Figure
10b,
operates
with
a
larger
dark
current.
Note
that
current
level
of
curve
Figure
10c
is
higher
than
that
of
Figure
10b
below
approximately
50
°C,
because
current level of curve Figure 10c is higher than that of Figure 10b below approximately 50 °C, becausethe
the
magnification
of
curve
of
curve
Figure
dark
level of
10c is
higherthan
thanthat
that
Figure
10b10b.
below approximately 50 ◦ C,
thecurrent
magnification
ofcurve
curve Figure
Figure 10c
10c
isishigher
higher
than
that
ofof
curve
Figure
10b.
11
total
IIDD
of the
proposed
as
function
of
becauseFigure
the magnification
of curve Figure
10c is higher
than
that of CA
curve
10b.
Figure
11 shows
shows the
the simulated
simulated
total current
current
DD of the
proposed
CA
as aaFigure
function
of illumination
illumination
at
resistances,
namely
R
=1
SS = of
10 kΩ,
and
= 25
shown
in
Figure
Figureexternal
11 shows
the simulated
total
as aAs
function
at three
three
external
resistances,
namely
RSS
SS current
=1 kΩ,
kΩ, R
RIDD
SS = 10the
kΩ,proposed
and R
RSS
SS = CA
25 kΩ.
kΩ.
As
shown of
in illumination
Figure 11,
11,
a
lower
R
SS
expands
the
illumination
detection
range.
Two
higher
resistances,
R
SS
=
10
kΩ
and
R
SS
at athree
external
resistances,
namely RSSdetection
= 1 kΩ, range.
RSS = 10
kΩ,higher
and Rresistances,
Figure
lower
RSS expands
the illumination
Two
SS =shown
10 kΩ in
and
RSS == 11,
SS = 25 kΩ.RAs
25
kΩ,
sharply
force
the
proposed
CA
into
saturation
at
280
lux
and
140
lux,
respectively.
Thus,
25 kΩ,Rsharply
force the proposed
CA into
saturation
at 280
luxhigher
and 140
lux, respectively.
for
a lower
illumination
detection
range.
Two
resistances,
RSS = Thus,
10 kΩfor
and
SS expands
wide
application,
selecting
a
low
R
SS is beneficial. Figure 12 shows the setup of the proposed CA that
selecting
a low
RSS is beneficial.
Figure 12 shows
of the
CA that
RSSwide
= 25application,
kΩ, sharply
force the
proposed
CA into saturation
at 280 the
lux setup
and 140
lux,proposed
respectively.
Thus,
used
for
aa current
meter
voltage
(V),
and
the
design
under
was
used
for measurement,
measurement,
with
current
meter (A),
(A),Figure
voltage12meter
meter
(V),
and
the of
design
under test
testCA
forwas
wide
application,
selectingwith
a low
RSS is beneficial.
shows
the
setup
the proposed
(DUT)
chip.
The
external
resistance
R
SS was used to measure the total current IDD, which was varied
(DUT)
chip.
The
external
resistance
R
SS
was
used
to
measure
the
total
current
I
DD
,
which
was
varied
that was used for measurement, with a current meter (A), voltage meter (V), and the design under test
by
illumination
(lux)
LED
by applying
applying
illumination
(lux) with
withRwhite
white
LED light.
light.
(DUT)
chip. The
external resistance
SS was used to measure the total current IDD , which was varied
by applying illumination (lux) with white LED light.
Sensors 2017, 17, 15
7 of 12
Sensors 2017, 17, 15
Sensors 2017, 17, 15
7 of 12
7 of 12
Sensors 2017, 17, 15
7 of 12
Figure
10.
Simulated PDdark
dark currents versus
versus temperatures for
various
CAs.
(a)(a)Proposed
CA with
Figure
10.10.
Simulated
forvarious
variousCAs.
CAs.(a)
Proposed
with
Figure
SimulatedPD
PD darkcurrents
currents versus temperatures
temperatures for
Proposed
CACA
with
enhanced
dark
current
cancellation;
(b)
the
CA
without
dark
current
cancellation;
and
(c)
the
CACA
with
enhanced
dark
current
cancellation;
(b)
the
CA
without
dark
current
cancellation;
and
(c)
the
with
enhanced dark current cancellation; (b) the CA without dark current cancellation; and (c) the CA with
Figure
10. Simulated
PD dark currents versus temperatures for various CAs. (a) Proposed CA with
dark
current
cancellation.
dark
current
cancellation.
dark current cancellation.
enhanced dark current cancellation; (b) the CA without dark current cancellation; and (c) the CA with
dark current cancellation.
Figure 11. Simulated total currents versus illumination (lux) for the proposed CMOS light detector.
Figure
11.Simulated
Simulatedtotal
totalcurrents
currents versus illumination
(lux)
for
the
proposed
CMOS
light detector.
Figure
11.
(lux)for
for
the
proposed
CMOS
detector.
Figure
11. Simulated
total currentsversus
versus illumination
illumination (lux)
the
proposed
CMOS
lightlight
detector.
VCC
VVCC
CC
A
A
A
VDD
DD
VVDD
VDD
V
VDD
DD
DUT
DUT
DUT
V
SS
VVSS
SS
V
V
V
V
SS
VVSS
SS
RSS
SS
RRSS
IDD
DD
IIDD
GND
GND
GND
Figure
CAfor
formeasurement.
measurement.
Figure12.
12.Setup
Setupof
of the
the proposed
proposed CA
Figure
proposed
CAfor
formeasurement.
measurement.
Figure12.
12. Setup
Setup of the proposed
CA
Tables
1 and
2 2summarize
measureddark
darkcurrent
currentIDark
IDarkand
and
total
current
Tables
1 and
summarizethe
thesimulated
simulated and measured
total
current
IDDIDD
of of
Tables
1 and
andmeasured
measureddark
darkcurrent
current
IDark
current
Tables
1 and2 2summarize
summarizethe
the simulated
simulated and
IDark
andand
totaltotal
current
IDD ofIDD
proposed
CAwith
withenhanced
enhanceddark
darkcurrent
current cancellation
cancellation at
× 300
μm
and
a a
thethe
proposed
CA
atthe
thePD
PDarea
areaofof300
300μm
μm
× 300
μm
and
of the
theproposed
proposedCA
CA
with
enhanced
dark
current
cancellation
the
PDofarea
of 300
µmμm
× and
300 aµm
with
enhanced
dark
current
cancellation
at theatPD
area
300 μm
× 300
Sensors 2017, 17, 15
8 of 12
17, 15
of 12
andSensors
a PD 2017,
to DD
area ratio of 2 times. Table 1 indicates that the measured dark currents are8smaller
than those of the simulations. This is because the PD model was always adopted in the simulations,
PD to DD area ratio of 2 times. Table 1 indicates that the measured dark currents are smaller than
but is not proven in the silicon process. However, the measured total currents are larger than those
those of the simulations. This is because the PD model was always adopted in the simulations, but is
of the simulations. The total light current is approximately linear as a function of illumination in the
not proven in the silicon process. However, the measured total currents are larger than those of the
simulation and the measurements in Table 2.
simulations. The total light current is approximately linear as a function of illumination in the
simulation
and the and
measurements
in Table
2. versus temperature for the proposed CA with enhanced
Table 1. Simulated
measured dark
currents
dark current cancellation.
Table 1. Simulated and measured dark currents versus temperature for the proposed CA with
enhanced dark current cancellation.◦
Temperatures ( C)
Simulations
Measurements
25
13.6 nA
1.24 nA
Temperatures
(°C)
Simulations
Measurements
85
21.6
nA
9.23
nAnA
25
13.6
nA
1.24
105
77.0 nA
36.4 nA
85
21.6 nA
9.23 nA
105
77.0 nA
36.4 nA
Table 2. Simulated and measured total currents versus illumination (lux) of the proposed CA with
enhanced
dark
currentand
cancellation.
Table 2.
Simulated
measured total currents versus illumination (lux) of the proposed CA with
enhanced dark current cancellation.
Illuminations (lux)
Illuminations
(lux)
10
100
10
200
100
200
Simulations
Simulations
8.78 µA
99.3
8.78 µA
μA
194.0
µA
99.3 μA
194.0 μA
Measurements
Measurements
18.2 µA
145.0
18.2µA
μA
236.0
145.0µA
μA
236.0 μA
Figure 13 shows a 65,536-point FFT simulation with a noise level of 80 dB and an external
13 shows a 65,536-point FFT simulation with a noise level of 80 dB and an external resistor
resistor RFigure
SS of 1 kΩ for the proposed CMOS light detector. This figure presents a simulated frequency
R
SS of 1 kΩ for the proposed CMOS light detector. This figure presents a simulated frequency
response of 10 Monte Carlo samples with random noises at node VSS , which is shown in Figure 12.
of 10current
Monte Carlo
samples
noises atofnode
VSSlux
, which
is shown in Figure
12. Ifsine
If a response
photodiode
signal,
whichwith
is a random
superposition
a 250
(peak-to-peak)
noiseless
a photodiode current signal, which is a superposition of a 250 lux (peak-to-peak) noiseless sine wave
wave with a frequency of 210 Hz and a DC 500 lux, is fed to the proposed CMOS light detector, a plot
with a frequency of 210 Hz and a DC 500 lux, is fed to the proposed CMOS light detector, a plot of
of the frequency response of the simulated 10 Monte Carlo samples with random noises is made in
the frequency response of the simulated 10 Monte Carlo samples with random noises is made in
Figure 14 at node VSS for the proposed light detector. The current signal with frequency of 210 Hz
Figure 14 at node VSS for the proposed light detector. The current signal with frequency of 210 Hz
performs
without
any any
random
noises.
TableTable
3 shows
the simulated
total harmonic
distortions
(THDs),
performs
without
random
noises.
3 shows
the simulated
total harmonic
distortions
signal-to-noise
ratios
(SNRs),
and
signal-to-noise+distortion
ratios
(SNDRs)
of
the
10
Monte
Carlo
(THDs), signal-to-noise ratios (SNRs), and signal-to-noise+distortion ratios (SNDRs) of the 10 Monte
samples
with
random
noises
at
node
V
of
Figure
14
for
the
proposed
light
detector.
As
shown
SS VSS of Figure 14 for the proposed light detector. As shown in
Carlo samples with random noises at node
Table
those
simulated
results
of THD,
SNR,
and
consistent.The
The
Monte-Carlo
in 3,
Table
3, those
simulated
results
of THD,
SNR,
andSNDR
SNDRare
are roughly
roughly consistent.
Monte-Carlo
approach
cancan
capture
very
nonlinear
approach
capture
very
nonlinearnoise
noisebehaviors.
behaviors.
Figure
Frequency
response
simulated10
10Monte
Monte Carlo
Carlo samples
samples with
atat
node
VSSV
Figure
13. 13.
Frequency
response
ofof
simulated
withrandom
randomnoises
noises
node
SS
of
Figure
12
for
proposed
light
detector,
with
R
SS = 1 kΩ.
of Figure 12 for proposed light detector, with RSS = 1 kΩ.
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9 of 12
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Figure
Frequency
response of
simulated 10Monte
Monte Carlosamples
sampleswith
withrandom
random noises
noises and
and aa 210
210 Hz
Figure
14. 14.
Frequency
response
Figure
14.
Frequency
response of
of simulated
simulated10
10 MonteCarlo
Carlo samples
with random
noises and
a 210
Hz
sine
wave
at
node
V
SS of Figure 12 for the proposed light detector, with RSS = 1 kΩ.
sinesine
wave
at node
VSSVof
Figure 12 for the proposed light detector, with RSSR=SS 1= kΩ.
Hz
wave
at node
SS of Figure 12 for the proposed light detector, with
1 kΩ.
Table
3. Simulated
THDs,
SNRs,and
andSNDRs
SNDRs of
of the
the 10
Monte
with
random
noises
at at
Table
3. Simulated
Simulated
THDs,
SNRs,
10 Monte
MonteCarlo
Carlosamples
samples
with
random
noises
Table
3.
THDs,
SNRs,
and SNDRs
of the
10
Carlo
samples
with
random
noises
at
node VSS of Figure 14 for the proposed light detector.
node VVSSSSofofFigure
Figure14
14for
forthe
theproposed
proposedlight
lightdetector.
detector.
node
Samples
1
2
3
4
5
6
7
8
9
10
Samples
2−39.82
33
44
55
66
77
8
10
Samples
1
2
99 −40.7510
THD (dB) 1−39.84
−39.84
−40.41
−39.85
−39.75
−40.27
−40.52
−40.06
THD
(dB)
−39.84
−39.82
−39.84
−40.41
−39.85
−39.75
−40.27
−40.52
−40.06
−40.75
SNR(dB)
52.60
51.16
50.99
50.15
51.86
52.41
50.39
50.05
51.43
52.64
THD (dB)
−39.84 −39.82 −39.84 −40.41 −39.85 −39.75 −40.27 −40.52 −40.06 −40.75
SNDR(dB)52.60
39.61 51.16
39.51 50.99
39.52
39.97
39.58
39.52
SNR(dB)
50.15
51.86
52.41
50.39
50.05
51.43 40.48
52.64
SNR(dB)
52.60
51.16
50.99
50.15
51.86
52.41 39.87
50.39 40.07
50.05 39.76
51.43
52.64
SNDR(dB) 39.61
39.61 39.51
39.51 39.52
39.52
39.97
39.58
39.52
39.87
40.07
39.76
40.48
SNDR(dB)
39.97
39.58
39.52
39.87
40.07
39.76
40.48
Figure 15 shows the measured dark current versus temperature for the proposed CA with
enhanced
dark current
cancellation. dark
The maximum versus
dark current is approximately
135 nA at 125
°C.
Figure
Figure 15
15 shows
shows the
the measured
measured dark current
current versus temperature
temperature for
for the
the proposed
proposed CA
CA with
with
This
measurement
proves
that
the
proposed
CA
performs
with
low
dark
current
as
a
function
of
the
◦ C.
enhanced
dark
current
cancellation.
The
maximum
dark
current
is
approximately
135
nA
at
125
enhanced dark current cancellation. The maximum dark current is approximately 135 nA at 125°C.
environmental
temperature.
According
to the
measurements
inlow
Figure
15,current
the device
performs
with
This
measurement
proves
that
the
proposed
CA
performs
with
dark
as
a
function
of
the
This measurement proves that the proposed CA performs with low dark current as a function of the
zero dark current at temperatures lower than 50 °C and exhibits an exponential relation at temperatures
environmental
temperature.
According
to
the measurements
measurements in
in Figure
Figure 15,
15, the
the device
device performs
performs with
environmental
temperature.
According
to the
with
higher than 50
°C. Notice that
two measured
dark currents are consistent
in Figure
15, although chip
◦
zero
dark
current
at
temperatures
lower
than
50
°C
and
exhibits
an
exponential
relation
at
temperatures
zero#1dark
temperatures
lower maximum
than 50 Cdark
and currents
exhibits an
exponential
relation at temperatures
and current
chip #2 at
perform
with different
at 125
°C.
◦ C.Notice
higher
higherthan
than50
50°C.
Noticethat
thattwo
twomeasured
measureddark
darkcurrents
currentsare
areconsistent
consistentininFigure
Figure15,
15,although
although chip
chip
◦
#1
and
chip
#2
perform
with
different
maximum
dark
currents
at
125
°C.
#1 and chip #2 perform with different maximum dark currents at 125 C.
Figure 15. Measured dark currents versus temperatures for the proposed CAs with enhanced dark
current cancellation.
Figure 15.
15. Measured
Measured dark
dark currents
currents versus
versus temperatures
temperaturesfor
for the
the proposed
proposed CAs
CAs with
with enhanced
enhanced dark
dark
Figure
current cancellation.
cancellation.
current
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Table 4 summarizes the measured dark currents versus temperature of the proposed CAs, Chap
#1 and
Chip4 #2.
Table 5 summarizes
thedark
measured
output
with respect
to the illumination
Table
summarizes
the measured
currents
versuscurrents
temperature
of the proposed
CAs, Chap of
#1
the
proposed
CAs.
According
to
the
measured
output
currents
in
Table
5,
the
proposed
CA
performs
and Chip #2. Table 5 summarizes the measured output currents with respect to the illumination
of the
with
goodCAs.
linearity
and wide
illumination
proposed
According
to the
measured range.
output currents in Table 5, the proposed CA performs with
good linearity and wide illumination range.
Table 4. Measured dark currents versus temperature of the proposed CAs, Chip #1 and Chip #2.
Table 4. Measured
dark currents
Temperatures
(°C)versus temperature
Chip #1of the proposed CAs,
ChipChip
#2 #1 and Chip #2.
25
1.39 nA
Temperatures (◦ C)
Chip #1
85
9.7 nA
25
1.39 nA
105
35 nA
1.24 nA
9.23 nA
1.24 nA36.4 nA
Chip #2
85
9.7 nA
9.23 nA
105
35 nA
36.4 nA
Table 5. Measured output currents with respect to illumination with RSS of 1 kΩ for the proposed
CAs.
Table 5. Measured output currents with respect to illumination with RSS of 1 kΩ for the proposed CAs.
Illumination (lux)
Chip #1
Illumination
(lux)
10
17.2Chip
μA #1
100
142 17.2
μA µA
10
100
200
237 142
μA µA
200
237 µA
300
426 μA
300
426 µA
600
776 776
μA µA
600
750
943 943
μA µA
750
900
900
975 975
μA µA
Chip #2
Chip #218.2 μA
18.2 µA145 μA
145 µA 236 μA
236 µA
430 μA
430 µA
776 µA 776 μA
947 µA 947 μA
980 µA 980 μA
Figure
Figure 16
16shows
showsthe
thetotal
totalcurrent
currentas
asaafunction
functionof
ofillumination
illumination(lux)
(lux)for
forthe
theproposed
proposedCMOS
CMOSlight
light
detector
with
dark
current
suppression
at
room
temperature.
The
larger
R
SS is, the lower the total
detector with dark current suppression at room temperature. The larger RSS is, the lower the total
current.
in Figure
Figure16,
16,two
twocurves
curves
with
RSS
= 10
and
SS = 25 kΩ indicate saturation at
current. As
As shown
shown in
with
RSS
= 10
kΩkΩ
and
RSSR=
25 kΩ indicate saturation at low
low
illumination,
but
the
curve
with
R
SS = 1 kΩ does not display saturation until 1000 lux. The
illumination, but the curve with RSS = 1 kΩ does not display saturation until 1000 lux. The measured
measured
totalI currents
IDD match those in Figure 11. Figure 17 shows the microphotograph of the
total currents
DD match those in Figure 11. Figure 17 shows the microphotograph of the proposed
proposed
CMOS
light
detector
darksuppression
current suppression
in a variable-temperature
Note
CMOS light detector with darkwith
current
in a variable-temperature
system. system.
Note that
the
that
the
PD
area
A
PD is twice as large as the DD area ADD. This arrangement not only lowers the dark
PD area APD is twice as large as the DD area ADD . This arrangement not only lowers the dark current
current
by contracting
DD but also enhances the luminous efficiency by enlarging APD.
by contracting
A butAalso
enhances the luminous efficiency by enlarging A .
DD
PD
Figure16.
16.Measured
Measuredtotal
totalcurrents
currentsversus
versusillumination
illumination(lux)
(lux)for
forthe
theproposed
proposedCMOS
CMOSlight
lightdetector.
detector.
Figure
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Sensors 2017, 17, 15
11 of 12
Figure
Figure 17.
17. Microphotograph
Microphotograph of
of the
the proposed
proposed CMOS
CMOS light
light detector.
detector.
4. Conclusions
4.
Conclusions
This study
study presents
presentsaacurrent
currentsteering
steeringmethod
method
drawing
current
difference
lowering
This
forfor
drawing
thethe
current
difference
andand
lowering
the
the dark
current
is applied
tolight
the light
detector
in a variable-temperature
system.
The maximum
dark
current
thatthat
is applied
to the
detector
in a variable-temperature
system.
The maximum
dark
dark current
the proposed
CA is approximately
nA◦ C;
at and
125 that
°C; the
andlight
that detector
the lightproposed
detector
current
of the of
proposed
CA is approximately
135 nA 135
at 125
proposed
in
this
study
performs
with
a
low
dark
current
as
a
function
of
the
environmental
in this study performs with a low dark current as a function of the environmental temperature.
temperature. Furthermore,
the device
a zeroatdark
current atlower
temperatures
lower
than 50 °C
Furthermore,
the device obtains
a zero obtains
dark current
temperatures
than 50 ◦ C
and exhibits
an
◦
and
exhibits
an
exponential
relation
at
temperatures
higher
than
50
°C.
In
addition,
the
larger
R
SS
is,
exponential relation at temperatures higher than 50 C. In addition, the larger RSS is, the lower the
the
lower
the
total
current
is.
Two
curves
with
R
SS
=
10
kΩ
and
R
SS
=
25
kΩ
indicate
saturation
at
low
total current is. Two curves with RSS = 10 kΩ and RSS = 25 kΩ indicate saturation at low illumination,
illumination,
with
RSSnot
= 1display
kΩ does
not display
saturation
until external
700 lux. resistance
Low external
but
the curve but
withthe
RSScurve
= 1 kΩ
does
saturation
until
700 lux. Low
Rss
resistance
Rss
results
in
a
wide
detected
range
of
illumination
(lux).
The
PD
area
A
PD
is
twice
as large
results in a wide detected range of illumination (lux). The PD area APD is twice as large as the DD
area
asDD
the
DD area
ADDnot
. This
design
not
only
lowers
theby
dark
current by
contracting
ADD but also
enhances
A
. This
design
only
lowers
the
dark
current
contracting
ADD
but also enhances
the luminous
the
luminous
efficiency
by
enlarging
A
PD
.
The
chip
area
of
the
light
detector
is
762
μm
×
452pads.
μm,
efficiency by enlarging APD . The chip area of the light detector is 762 µm × 452 µm, including
including
pads.
According
to
the
measured
results,
the
proposed
CA
successfully
eliminates
dark
According to the measured results, the proposed CA successfully eliminates dark current from 50 ◦ C
current
from 50 °C to 125 °C.
to
125 ◦ C.
Acknowledgments: The authors would like to thank
thank James
James Zhang
Zhang and
and Wayne
Wayne Wei for the project sponsorship,
sponsorship,
Ann Lain for the layout, and Bryan Chan for providing
providing silicon
silicon test
test support.
support.
Author
W.-S.L. and
Author Contributions:
Contributions: This
This study
study is
is completed
completed with
with three
three authors.
authors. W.-S.L.
and J.-L.L.
J.-L.L. conceived
conceived and
and designed
designed
the experiments; W.-S.L. performed the experiments; G.-M.S. analyzed the data and wrote the paper.
the experiments; W.-S.L. performed the experiments; G.-M.S. analyzed the data and wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
Conflicts of Interest: The authors declare no conflict of interest.
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article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).