Avalanche photodiode A User Guide
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Avalanche photodiode detectors (APD)
have and will continue to be used in
many diverse applications such as laser
range finders, data communications or
photon correlation studies. This paper
discusses APD structures, critical
performance parameter and excess noise
For low-light detection in the 200- to
1150-nm range, the designer has three
basic detector choices - the silicon PIN
detector, the silicon avalanche
photodiode (APD) and the photomultiplier
APDs are widely used in instrumentation
and aerospace applications, offering a
combination of high speed and high
sensitivity unmatched by PIN detectors,
and quantum efficiencies at >400 nm
unmatched by PMTs.
What is an Avalanche Photodiode?
In order to understand why more than one APD structure exists, it is important to appreciate
the design trade-offs that must be accommodated by the APD designer. The ideal APD would
have zero dark noise, no excess noise, broad spectral and frequency response, a gain range
from 1 to 106 or more, and low cost. More simply, an ideal APD would be a good PIN
photodiode with gain! In reality however, this is difficult to achieve because of the need to
trade-off conflicting design requirements. What some of these trade-offs are, and how they are
optimized in commercially available APDs, are listed below.
Consider the schematic cross-section for a typical APD structure shown in Figure 1. The basic
structural elements provided by the APD designer include an absorption region "A", and a
multiplication region "M". Present across region "A" is an electric field "E" that serves to
separate the photo-generated holes and electrons, and sweeps one carrier towards the
multiplication region. The multiplication region "M" is designed to exhibit a high electric field so
as to provide internal photo-current gain by impact ionization.
This gain region must be broad enough to provide a useful gain, M, of at least 100 for silicon
APDs, or 10-40 for germanium or InGaAs APDs. In addition, the multiplying electric field profile
must enable effective gain to be achieved at field strength below the breakdown field of the
Figure 1 shows the "reach-through" structure used by PerkinElmer which offers the best
available combination of high speed, low noise and capacitance, and extended IR response.
Critical Performance Parameters
An APD differs from a PIN photodiode by providing internal photo-electronic signal gain.
Therefore, output signal current, Is, from an APD equals Is = M×Ro (l)×Ps, where Ro(l) is the
intrinsic responsivity of the APD at a gain M=1 and wavelength l, M is the gain of the APD,
and Ps is the incident optical power. The gain is a function of the APDs reverse voltage, VR,
and will vary with applied bias. A typical gain-voltage curve for a silicon APD manufactured by
PerkinElmer is shown in Figure 2.
Types of APDs
Avalanche photodiodes are commercially available that span the wavelength range from
300nm to 1700nm. Silicon APDs can be used between 300nm to 1100nm, germanium
between 800nm and 1600nm, and InGaAs from 900nm to 1700nm.
Although significantly more expensive than germanium APDs, InGaAs APDs are typically
available with much lower noise currents, exhibit extended spectral response to 1700nm, and
provide higher frequency bandwidth for a given active area. A germanium APD is
recommended for environments applications in high electro-magnetic interference (EMI),
where amplifier noise is significantly higher than the noise from an InGaAs APD, or for
applications where cost is of primordial consideration.
Understanding the Specifications
Responsivity and Gain
APD gain will vary as a function of applied reverse voltage, as shown in Figure 2. In addition,
for many APDs, it is not possible, or practical, to make an accurate measurement of the
intrinsic responsivity, Ro(l), at a gain M=1. It is therefore inappropriate to state typical gain and
diode sensitivity at M=1 as a method for specifying diode responsivity at a given operating
voltage. In order to characterize APD response, one must specify APD responsivity (in
Amps/Watt) at a given operating voltage. However, because of diode to diode variations in the
exact gain voltage curve of each APD, the specific operating voltage for a given responsivity
will vary from one APD to another. Manufacturers should therefore specify a voltage range
within which a specific responsivity will be achieved.
Excess Noise Factor
All avalanche photodiodes generate excess noise due to the statistical nature of the avalanche
process. The 'Excess Noise Factor’ is generally denoted as 'F'. As shown in the noise
equation (Equation 2), ÖF is the factor by which the statistical noise on the APD current (equal
to the sum of the multiplied photocurrent plus the multiplied APD bulk dark current) exceeds
that which would be expected from a noiseless multiplier on the basis of Poissonian statistics
(shot noise) alone.
The excess noise factor is a function of the carrier ionization ratio, k, where k is usually defined
as the ratio of hole to electron ionization probabilities (k £ 1).
In both modes of APD operation, i.e. Linear and Geiger, APDs have and will continue to be
used in many diverse applications. In the linear mode operation, the APD is well suited for
applications which require high sensitivity and fast response times. For example, laser range
finders which incorporate APD detectors result in more sensitive instruments than ones which
use conventional PIN detectors. In addition, APDs used in this application can operate with
lower light levels and shorter laser pulses, thus making the range finder more 'eye safe'.
Other applications for linear mode APDs include fast receiver modules for data
communications, high speed laser scanner (2D bar code reader), speed gun, ceilometers
(cloud height measurement), OTDR (Optical Time Domain Reflectometry), PET Scanner,
confocal microscopy and particle detection.
Silicon APDs operated in the Geiger mode are used to detect single photons for photon
correlation studies and are capable of achieving very short resolving times. Operated in this
mode, PerkinElmer SLiKTM detector provides gains of up to 108 and quantum efficiencies of -
70% at 633nm and 50% at 830nm. Other applications in which APDs operated in this mode
are used include: Lidar, Astronomical observations, Optical range finding, Optical fiber test and
fault location, ultra sensitive fluorescence, etc