Download 12-MOSFET1

Budapest University of Technology and Economics
Department of Electron Devices
Microelectronics, BSc course
Field effect transistors 2:
The MOSFETs
http://www.eet.bme.hu/~poppe/miel/en/12-MOSFET1.ppt
http://www.eet.bme.hu
Budapest University of Technology and Economics
Department of Electron Devices
The abstraction level of our study:
SYSTEM
MODULE
+
GATE
CIRCUIT
Vin
Vout
DEVICE
G
S
n+
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
D
n+
2
Budapest University of Technology and Economics
Department of Electron Devices
Field effect transistors 1
►
FET = Field Effect Transistor – the flow of charge carriers is
influenced by electric field
Flow
Channel
depletion layer
JUNCTION FET: depletion layers of pnjunctions close the channel
Most important parameter:
U0 pinch-off voltage
►
►
transversal
field is used to
control
Unipolar device: current is conducted by majority carriers
Power needed for controlling the device  0
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
3
Budapest University of Technology and Economics
Department of Electron Devices
Field effect transistors 2
► MOSFET:
oxide
Metal-Oxide-Semiconductor FET
-
depletion layer
oxide
+
Bulk
First type: depletion mode device
Most important parameter:
U0 pinch off voltage
inversion layer
Bulk
Second type: enhancement mode
device
Most important parameter:
VT threshold voltage
Most frequently used today
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
4
Budapest University of Technology and Economics
Department of Electron Devices
Field effect transistors 3
► Symbols:
n channel
n channel
enhancement
mode
p channel
p channel
enhancement
mode
depletion mode
p channel
13-11-2008
n channel
depletion mode
p channel
depletion mode
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
n channel
enhancement
mode
5
Budapest University of Technology and Economics
Department of Electron Devices
MOSFETs
► More
realistic cross-sectional view of enhnacement
mode MOSFETs:
Gate oxide
Polysilicon
Gate
Source
n+
Drain
n+
p substrate
Field-Oxide
(SiO2)
p+ stopper
Bulk contact
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
6
Budapest University of Technology and Economics
Department of Electron Devices
The most modern MOSFETs:
► 2007/2008,
13-11-2008
Intel:
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
7
Budapest University of Technology and Economics
Department of Electron Devices
How is it manufactured?
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
8
Budapest University of Technology and Economics
Department of Electron Devices
Poli-Si gate self-aligned device
PSG
Structure:
Source/drain doping
thin oxide
poli-Si gate
Layout:
metallization,
contact window
W
L
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
9
Budapest University of Technology and Economics
Department of Electron Devices
Steps of the self-aligned poli-Si
gate process
1) Open window for the active region
M
 photolitography, field oxide etching
2) Growth of thin oxide
3) Window for hidden contacts
M
 Contacts the poli-Si gate (yet to be deposited) with the
active region (after doping).
3) Deposit poli-Si
4) Patterning of poli-Si
M
5) Open window through the thin oxide (etching
only)
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
10
Budapest University of Technology and Economics
Department of Electron Devices
Steps of the self-aligned poli-Si
gate process
6) n+ doping:
Form source and drain regions as well as wiring by diffusion
lines. Through the hidden contact poli-Si gate will also be
connected to diffused lines.
7) Deposit phosphor-silica glass (PSG) as insulator
8) Open contact windows through PSG-n
M
9) Metallization
10) Patterning metallization layer
M
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
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Budapest University of Technology and Economics
Department of Electron Devices
Further topics:
►
►
►
►
13-11-2008
Overview of operation of MOS transistors
Characteristics
Secondary effects
Models
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
12
Budapest University of Technology and Economics
Department of Electron Devices
Operation of MOSFETs
► The
simplest (logic) model:
 open (off) / short (on)
| VGS |
Source
(of carriers)
Open (off) (Gate = ‘0’)
Gate
Drain
(of carriers)
enhancement mode
device
Closed (on) (Gate = ‘1’)
Ron
13-11-2008
| VGS | < | VT |
| VGS | > | VT |
open
short
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
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Budapest University of Technology and Economics
Department of Electron Devices
Operation of MOSFETs
► n-channel
device:
 electrons are flowing
► p-channel
device:
 holes are flowing
 same operation, change of the signs
► Normally
OFF device: at 0 gate (control) voltage
the are "open" (enhancement mode device)
► Normally ON device: at 0 gate (control) voltage the
are "short" (depletion mode device)
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
14
Budapest University of Technology and Economics
Department of Electron Devices
Overview of MOSFET types
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
15
Budapest University of Technology and Economics
Department of Electron Devices
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
16
Budapest University of Technology and Economics
Department of Electron Devices
Overview of the operation
►
The operation is based on the so called
MOS capacitance:
 As a result of electrical field perpendicular to
the gate surface
• positive charges accumulate at the metal (gate)
• in the p-type semiconductor
– first the positive charges are "swept" out and a
depletion layer is formed
– further increasing the electric field, negative
carriers are collected from the bulk under the
metal
– if the voltage at the surface exceeds a threshold
value, the type of the semiconducter gets
"inverted": an inversion layer is formed
 VT threshold voltage – the minimal voltage
needed to form the inversion layer; depends on:
• the energy levels of the semiconductor material
• the thickness and the dielectric constant of the
oxide (SiO2)
• the doping level and dielectric constant of the
semiconductor (Si)
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
17
Budapest University of Technology and Economics
Department of Electron Devices
Overview of the operation
►
Surface phenomena in case of the MOS capacitance
Accumulation
Depletion
Inversion
Strong inversion:
UF = 2  F
 Wi  WF 
p  ni  exp 

 kT 
 WF  Wv 
p ~ exp  

kT 

13-11-2008
F 
Wi  WF kT p
N

ln  U T ln a
q
q
ni
ni
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
18
Budapest University of Technology and Economics
Department of Electron Devices
The MOS transistor
► MOS
capacitance completed by two electrodes at its
two sides:
► n-channel
► p-channel
13-11-2008
device: current conducted by electrons
device: current conducted by holes
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
19
Budapest University of Technology and Economics
Department of Electron Devices
Qualitative operation of the MOSFET
►
If VGS > VT, inversion layer is
formed
 the n+ region at the source can
inject electrons into the
inversion channel
 the positive potential at the
drain induces flow of electrons
in the channel,
 the positive potential of the
drain reverse biases the pn
junction formed there
 the electrons drifted there are
all sank in the n+ region and the
circuit is closed
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
20
Budapest University of Technology and Economics
Department of Electron Devices
Qualitative operation of the MOSFET
 the charge density in channel
depends on the VGS voltage
 there is a voltage drop in the
channel, thus, the thickness of
the inversion layer will deminish
along the channel
 at a given VDSsat saturation
voltage the thickness will reach
0, this is the so called pinch-off
VDSsat = VGS - VT
After this voltage is reached, the MOSFET operates in saturation mode,
the drain voltage does not influence the drain current any longer.
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
21
Budapest University of Technology and Economics
Department of Electron Devices
Qualitative operation of the MOSFET
In the pinch-off region the charge transort takes place by means of
diffusion current.
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
22
Budapest University of Technology and Economics
Department of Electron Devices
I-V charactersitics
• output charactersitic: ID=f(UDS), parameter: UGS
• input characteristc: ID=f(UGS)
Output characteristic:
In saturation:
W  n  ox
ID 
VGS  VT
L 2 tox

K
 n ox
tox

2
current
constant
The circuit designer can
change the geometry only:
the W width and the L length
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
23
Budapest University of Technology and Economics
Department of Electron Devices
Example
Calculate the saturation current of a MOSFET for UGS=5V if
K
 n ox
tox
 110A / V 2
VT =1V, and the geometry
a) W= 5μm, L=0.4μm ,
b) W= 0.8μm, L=5μm !
a)
ID 
W K
5 110 6
(U GS  VT ) 2 
10 (5  1) 2  1110 3 A  11mA
L 2
0.4 2
b)
ID 
W K
0.8 110 6
(U GS  VT ) 2 
10 (5  1) 2  14110 6 A  141A
L 2
5 2
By changing the W/L ratio the drain current can be changed by
orders of magnitude
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
24
Budapest University of Technology and Economics
Department of Electron Devices
I-V charactersitics
6
X 10-4
VDSsat = VGS - VT
5
Voltage
controlled
current source
voltage controlled
resistor
4
VGS = 2.5V
VGS = 2.0V
3
lineáris
szaturáció
2
VGS = 1.5V
1
VGS = 1.0V
0
cut-off
0
0.5
1
1.5
2
2.5
VDS (V)
nMOS tranzisztor, 0.25um, Ld = 10um, W/L = 1.5, VDD = 2.5V, VT = 0.4V
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
25
Budapest University of Technology and Economics
Department of Electron Devices
Overview of the
physics:
►
Charges and potentials at the surface
► The threshold voltage
► The charateristics
► Secondary effects
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
26
Budapest University of Technology and Economics
Department of Electron Devices
Potentials of the MOS
structure
U GB  U ox  U F   MS
QG  QSC  QSS  Qi
C0 
 ox
d ox
QG  C0U ox
QSC  qN a S
oxide
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
semiconductor
27
Budapest University of Technology and Economics
Department of Electron Devices
Potentials of the MOS
structure
U GB  U ox  U F   MS
Qi  QG  QSC  QSS 
QG  QSC  QSS  Qi
 C0U ox  2 s qN a U F  QSS
QG  C0U ox
Qi  C0 (U GB  U F   MS ) 
QSC  qN a S
 2 s qN a U F  QSS
QSC  qN a
13-11-2008
2 s
U F  2 s qN a U F
qN a
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
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Budapest University of Technology and Economics
Department of Electron Devices
The threshold voltage of the MOSFET
Inversion
U F  2 F
13-11-2008
U F  2 F  U SB
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
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Budapest University of Technology and Economics
Department of Electron Devices
The threshold voltage of the MOSFET
Qi  C0 (U GB  2 F  U SB   MS )  2 s qN a 2 F  U SB  QSS
VT  UGS Q 0
i
VT  2 F   MS
13-11-2008
Qi  C0 U GS  VT 
2 s qN a
QSS


C0
C0
2 F  U SB
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
30
Budapest University of Technology and Economics
Department of Electron Devices
The threshold voltage of the MOSFET
VT  2 F   MS
2 s qN a
QSS


C0
C0
2 F  U SB
Flat-band potential:
 FB   MS
QSS

C0
Bulk constant:
P
2 s qN a
C0
VT 2F FB  P 2 F  USB
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
31
Budapest University of Technology and Economics
Department of Electron Devices
The charactersitics of an enhnacement
mode MOSFET
inversion layer
Later we shall calculate these!
triode
region
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
saturation
33
Budapest University of Technology and Economics
Department of Electron Devices
Derivation of the charactersitic
inversion layer
U(0) = UGS , U(L) = UGD
Qi(U) = Qi[U(x)]
I D  Qi W v
dU
v   E   
dx
dU
I D  Qi (U )W
dx
13-11-2008
L
L
dU
 I D dx  W  Qi dx dx
0
0
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
34
Budapest University of Technology and Economics
Department of Electron Devices
Derivation of the charactersitic
L
L
dU
 I D dx  W  Qi dx dx
0
0
I D L  W
inversion layer
U GD
 Qi (U ) dU
U GS
Qi  C0 U ( x)  VT 
W
W  C0
2 U GS
U  VT 
I D     C0 U  VT  dU 
U GD
L U
L 2
U GD

W  C0
2
2
U GS  VT   U GD  VT 
ID 
L 2
GS
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008

35
Budapest University of Technology and Economics
Department of Electron Devices
Derivation of the charactersitic

W  C0
U GS  VT 2  U GD  VT 2
ID 
L 2

inversion layer
W  C0
F (U GS )  F (U GD )
ID 
L 2
(U  VT ) 2
F (U )  
0

ha U  VT
ha U  VT
For all regions of operation!
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
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Budapest University of Technology and Economics
Department of Electron Devices
The saturation region
inversion layer
W  C0
F (U GS )  F (U GD )
ID 
L 2
(U  VT ) 2
F (U )  
0

ha U  VT
ha U  VT
Saturation: UGD < VT
For all regions of operation!
W  C0
2
U GS  VT 
ID 
L 2
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
37
Budapest University of Technology and Economics
Department of Electron Devices
Overview of all types of MOSFETs
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
38
Budapest University of Technology and Economics
Department of Electron Devices
Depletion mode MOSFET
Like an enhance mode MOSFET with a
negative threshold voltage
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
39
Budapest University of Technology and Economics
Department of Electron Devices
Capacitances of the MOSFET
inversion layer
Bulk
QG  fG (U GS ,U GD ,U GB )
Qi  fi (U GS ,U GD ,U GB )
S/D – B capacitance: reverse
biased PN junction
13-11-2008
C gs
QG

U GS
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
40
Budapest University of Technology and Economics
Department of Electron Devices
The gate capacitance:
Polysilicon gate
Source
Drain
xd
n+
xd
Ld
W
n+
Gate-bulk
overlap
Top view
Gate oxide
tox
n+
L
n+
Cross section
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
41
Budapest University of Technology and Economics
Department of Electron Devices
Secondary effects
► Channel
length reduction
► Narrow channel operation
► Temperature dependence
► Subthreshold current
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
42
Budapest University of Technology and Economics
Department of Electron Devices
Dependence of threshold voltage on
geometry
Short channel: VT
decreases
13-11-2008
Narrow channel:
VT increases
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
43
Budapest University of Technology and Economics
Department of Electron Devices
Velocity saturation
► Influences
10
the operation of short channel devices
5
Velocity saturation the speed of
carriers (due to the collisions)
becomes constant
0
c=1.5
0
3
(V/m)
In a L = 0.25m channel device a few Volts of D-S voltage may already
result in velocity saturation.
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
44
Budapest University of Technology and Economics
Department of Electron Devices
Velocity saturation
► In
short channel device velocity saturation takes
place sooner (at lower voltage)
10
0
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
45
Budapest University of Technology and Economics
Department of Electron Devices
Short channel charactersitics
2.5
X 10-4
Early velocity saturation
VGS = 2.5V
2
VGS = 2.0V
1.5
Linear
1
Saturation
VGS = 1.5V
VGS = 1.0V
0.5
0
0
0.5
1
1.5
2
2.5
VDS (V)
nMOS transistor, 0.25um, Ld = 10um, W/L = 1.5, VDD = 2.5V, VT = 0.4V
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
46
Budapest University of Technology and Economics
Department of Electron Devices
Temperature dependence
W  C0
U GS  VT 2
ID 
L 2
1 d
 0,003...  0.006 / o C
 dT
VT
 1,5...  4 mV / o C
T
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
47
Budapest University of Technology and Economics
Department of Electron Devices
Subthreshold current
►
Assuming a given VT is rough model; in reality the current
vanishes exponentially with the gate voltage:
10-2
linear region
quadratic region
subthreshold,
exponential
region
VT
10-12
0
13-11-2008
ID ~ IS e (qVGS/nkT) where n  1
VGS (V)
0.5
1
1.5
2
2.5
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
48
Budapest University of Technology and Economics
Department of Electron Devices
Subthreshold current
►
Continuous transition between the ON and OFF states
 Subthreshold is undesired: strong deviation from the switch model
I D ~ I 0e
qVGS
nkT
CD
, n  1
Cox
► I 0, n –
► Slope
empirical parameters, n is typically 1.5
factor: S = n (kT/q) ln (10)
(tipically: 60 ..100 mV/decade) – the smaller the better,
depends on n értékétől függ.
Can be reduced by SOI:
Si
e.g. SIMOX process
13-11-2008
SiO2
Si substrate
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
49
Budapest University of Technology and Economics
Department of Electron Devices
Subthreshold ID(VGS) charactersitic
VDS : 0 .. 0.5V
I D  I 0e
13-11-2008
qVGS
nkT
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
qV
 DS

1  e kT






50
Budapest University of Technology and Economics
Department of Electron Devices
Subthreshold ID(VDS) charactersitic
VGS : 0 .. 0.3V
I D  I 0e
13-11-2008
qVGS
nkT
qV
 DS

1  e kT


Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008

1    VDS 


51
Budapest University of Technology and Economics
Department of Electron Devices
MOS transistor models
►
►
Neded for circuit simulators (SPICE, TRANZ-TRAN, ELDO,
SABER, stb)
Different levels of complexity:
 level0, 1, 2, ...n,
 EKV,
 BSIM3, BSIM4
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
52
Budapest University of Technology and Economics
Department of Electron Devices
Examples for MOSFETs
inversion layer
Photograph by
optical microscope
SGD
SGD
Micro-photograph by SEM
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
53
Budapest University of Technology and Economics
Department of Electron Devices
Some more complex MOS circuits
n- & p-channel
devices :
CMOS circuit, see
later
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
54
Budapest University of Technology and Economics
Department of Electron Devices
Some more complex MOS circuits
Designed by CAD tools
13-11-2008
Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET 2008
55