Avalanche PhotoDiode
Photoconductive Detector
The concentration of electrons and holes in a
semiconductor is changed if light is absorbed
by the semiconductor. The light must have
photon energy large enough to cause
excitation, either by raising electrons across
the forbidden band gap or by activating
impurities present within the band gap. The
increased number of charge carriers leads to
an increase in the electrical conductivity of the
semiconductor. The change in electrical
conductivity leads to an increase in the current
flowing in the circuit..
Photovoltaic Detector
Introduction
When a photon strikes a semiconductor, it can
promote an electron from the valence band
(filled orbitals) to the conduction band (unfilled
orbitals) creating an electron(-) - hole(+) pair.
The concentration of these electron-hole pairs
is dependent on the amount of light striking the
semiconductor, making the semiconductor
suitable as an optical detector
Schematic of
semiconductor detector
Photoemissive Detector
Photomultiplier tubes containing a
photocathode which emits electrons
when illuminated, the electrons are then
amplified by a chain of dynodes.
Photoconductive—light increases the flow of electrons and reduces the resistance
Photovoltaic—light makes electrons move between layers, producing a voltage and a
current in an external circuit.
.
Photoemissive—light knocks electrons from a cathode to an anode,
making a current flow through an external circuit
Avalanche Photo Diode
Operation of APD
When light enters a photodiode,electron –hole
pairs are generated if the light energy is higher
than the band gap energy.
By applying a high reverse bias voltage
(typically 100-200 V in silicon), APDs show an
internal current gain effect (around 100) due to
impact ionization (avalanche effect).
At a critical value of field (10^5V/cm),these
electrons will collide with lattice.
It will result in the generation of electron –hole
pair.
These electron-hole pair will create additional
pairs in a process just like a chain reaction.
This is known as avalanche multiplication of
photcurrent.
Thus there is an avalanche of electrons and
holes moving through the detector. These
current pulses are then detected in an external
circuit.
some silicon APDs employ alternative doping
and beveling techniques compared to traditional
APDs that allow greater voltage to be applied (>
1500 V) before breakdown is reached and
hence a greater operating gain (> 1000)
Avalanche photodiodes are preferred
over many other candidate
photodetectors, including PIN diodes,
particularly due to their high internal gain
characteristics and improved signal-to-
noise ratio.
. The electrical signal output from the
APD is coupled to an amplifier for
amplification.
The basic parameters characterizing the operation of
APDs are:
- quantum efficiency QE;
- avalanche gain M;
- excess noise factor F;
- dark current Id = Is + M·Ib, where Is and Ib are the
surface leakage current and the
bulk generation current respectively;
- capacitance Cd and series resistance Rs , which
determine the response function of the APD;
- operating voltage U.
APD noise is given by the formula:
In^2=2q(Il+Idg)BM^2F+2qIdsB
q:electron charge
In:Photocurrent at M=1
Idg:Dark current component to be multiplied
Ids: Idg:Dark current component to be
multiplied
B:bandwidth
M:multiplication RatioF:excess noise factor
Comparison with
photomultipliers
Advantages compared to photomultipliers:
Excellent linearity of output current as a function of incident
light
Spectral response from 190 nm to 1100 nm (silicon), longer
wavelengths with other semiconductor materials
Low noise
Ruggedized to mechanical stress
Low cost
Compact and light weight
Long lifetime
High quantum efficiency, typically 80%
No high voltage required
Disadvantages compared to
photomultipliers:
Small area
No internal gain (except avalanche
photodiodes, but their gain is typically 10²–10³
compared to up to 108 for the photomultiplier)
Much lower overall sensitivity
Photon counting only possible with specially
designed, usually cooled photodiodes, with
special electronic circuits
Response time for many designs is slower
Applications
A typical application for APDs is laser
rangefinders and long range fiber optic
telecommunication. New applications
include positron emission tomography
and particle physics. APD arrays are
becoming commercially available.