Download Photo Detector Principle of Operation

Photo Detector
Ip
Photodiode
Electron
Conduction
band
Incident light
Features
2.
n
RL
A photodiode is a semiconductor device which
converts the light to electric energy when exposed to
light. It comes in a package for use as an electronic
component.
1.
VR
p
ｈν
Excellent linearity of the output current with
respect to incident light
Eg(eV)
ｈν
Depletion layer
P-layer
Quick response
I-layer
h: Planck’s constant
Eg: Energy gap
Hole
Band gap
Valence band
N+-layer
v: Frequency of light
3.
Wide spectral response
4.
Long life
Fig. 1: Action of a PIN photodiode
5.
Small output change with respect to temperature
changes
6.
Small output current
up to the conduction band, thus leaving holes in the
valance band and generating carriers. The carriers
thus generated in the depletion layer separate due to
electrical field conductivity, so that the electrons move
to the N region, and the holes move to the P region. At
this time, the number of carriers generated is basically
in proportion to the luminance of the light, and a
photocurrent flows through the photodiode. The size of
the photocurrent is roughly proportional to the intensity
of the light.
Phototransistor
A phototransistor is designed to amplify the output
from a photodiode using a transistor. Compared to the
photodiode, it provides higher output signal levels.
Characteristics of a phototransistor
1.
Large photocurrents
2.
Slow response
3.
Long life
4.
Poor output current linearity with respect to
incident light
A PIN photodiode is a device that has a significantly
thick I-layer between the PN junction. This widens the
depletion layer when a reverse bias is applied, and
allows the diode to be used at a high reverse bias
voltage. The high electrical field conductivity in the
wide depletion layer prompts the carriers to move, and
has the effect of increasing response.
5.
Large output changes
temperature changes
Phototransistor
with
respect
to
The phototransistor amplifies a photocurrent using the
amplification function of a transistor, to compensate for
the low sensitivity of photodiodes and PIN
photodiodes. It has a structure that basically combines
a photodiode and a transistor (see Fig. 2). When light
strikes the depletion layer between the base and
collector and its surrounding area, the photocurrent
that is generated becomes the base current for the
transistor and is amplified by the transistor’s
amplification factor ß. However, the photocurrent
between the base and collector flows slowly, because
it flows with the diffusion of the carriers. It also has a
slower response than a photodiode or PIN photodiode,
because the large diffusion of the carriers causes the
Principle of Operation
Photodiode
The photodiode has a PN junction consisting of a
P-type semiconductor region on the light receiving
side and an N-type semiconductor region on the
substrate side (see Fig. 1). Applying a reverse bias to
the PN junction creates a depletion layer between the
P-layer and N-layer, so-called because it has no
mobile carriers. When light that has greater energy
than the semiconductor band gap energy strikes the
region near the junction of the photodiode (the
depletion layer and the area surrounding it), the
electrons in the valance band absorb the light and rise
2008 年 10 月、Oct 2008
スタンレー電気株式会社 Stanley Electric Co., Ltd
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high frequency component inside the photocurrent to
short-circuit.
Reverse voltage [VR]
The allowable amount of reverse voltage that can be
applied from the cathode to the anode side of a
photodiode. [V]
Incident light
Base
hν
Emitter
Collector-emitter voltage [VCEO]
ｎ
The allowable amount of voltage that can be applied
from the collector side to the emitter side of a
phototransistor. [V]
p
ｎ
Emitter-collector voltage [VECO]
The allowable amount of voltage that can be applied
from the emitter side to the collector side of a
phototransistor. [V]
Collector
Fig. 2: Structure of a phototransistor
Collector current [IC]
Glossary of Terms
The allowable amount of current that can flow from the
collector side to the emitter side of a phototransistor.
[mA]
Vertical PIN photodiode
Vertical phototransistor
Collector loss [PC]
A type of photodiode and phototransistor that is
mounted on a printed substrate by inserting the leads
through holes on the substrate. The photo detector is
mounted on the lead frame and sealed with resin.
The allowable amount of power that can be consumed
by the photocurrent and collector voltage in a
phototransistor. [mW]
Dark current [ID]
The current that flows through a photodiode when a
reverse voltage is applied with light blocked out. [nA]
Dark current [IC]
The current that flows through a phototransistor when
a positive voltage is applied with light blocked out. [µA]
Vertical lamp types
Chip phototransistor
Photocurrent [IP]
A type of phototransistor that is mounted on the
surface of a printed substrate, or substrate-type
phototransistors.
The current that flows from the cathode to the anode
side of a photodiode when incident light is applied
under prescribed conditions. [µA]
Photocurrent [IC]
Substrate types
The collector current that flows through a
phototransistor when incident light is applied under
prescribed conditions. [mA]
Power dissipation [Pd]
Radiation luminance [Ee]
The radiant flux that is applied per unit area. [W/cm2]
The allowable amount of power that can be consumed
by the photocurrent and reverse voltage in a
photodiode. [mW]
2008 年 10 月、Oct 2008
スタンレー電気株式会社 Stanley Electric Co., Ltd
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Peak sensitivity wavelength [λP]
The wavelength at which photosensitivity is greatest.
[nm]
Inter-terminal capacity [CT]
The capacitance between the cathode and anode
terminals of a photodiode. [pF]
Collector-emitter saturated voltage
[VCE(sat)]
The voltage between the collector and emitter of a
phototransistor under prescribed saturation conditions.
[V]
Response speed [tr⋅tf]
The amount of time it takes for a photocurrent to rise
from 10% to 90% or fall from 90% to 10% of its
maximum value with respect to a pulsed light input. [ns
or µs]
Half intensity angle [Δθ]
The distribution characteristic of photosensitivity in the
direction of the optical axis of a lens. It is expressed as
the range of angle through which the response is 50%
of its peak value. [deg]
Sensitivity [S]
The amount of current that flows per energy of
radiation from a single wavelength of light. [A/W]
2008 年 10 月、Oct 2008
スタンレー電気株式会社 Stanley Electric Co., Ltd
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