TECHNICAL NOTE: V805
Separate absorption, charge, and multiplication (SACM)
Avalanche Photodiode Design
Introduction
This article reviews basic avalanche photodiode (APD) design theory.
APDs are notable among
photodetectors in that they provide internal gain. Avalanche multiplication can amplify a small initial
photocurrent by a factor of over 200. Thus, APDs are the most sensitive of the common solid state
photodetectors available. To date, silicon APDs that are responsive between 190 - 1100 nm have been
widely commercialized, and products based upon InGaAsP on InP that operate between 1000 - 1650 nm
are becoming more popular. APDs have also been fabricated in a wide variety of other material
systems, AlGaInAs on InP among them.
Figure 1 schematically illustrates the avalanche photodiode. In this device, incoming photons produce
electron-hole pairs in the absorption region, as with any other photodiode. However, the APD is
operated with a large reverse bias. The high internal field accelerates the photon-generated electrons.
Figure 1: Schematic cross section (not to scale) of a typical APD structure
I.7:schematic
Layer schematic
and band
diagram
showinghow
how the
the SACM
Figure Figure
2: Layer
and band
edgeedge
diagram
showing
SACM design
design
(right)
reduces
tunneling
leakage
in
long-wavelength
APDs.
(right) reduces tunneling leakage in long-wavelength APDs.
These collide with the atomic lattice releasing additional electrons via secondary ionization. These
secondary electrons are also accelerated, resulting in an avalanche of carriers, hence the name.
Si APDs for short-wavelength applications are commonly manufactured by dopant diffusion, but
the junctions in long-wavelength InP-based APDs are often grown epitaxially. Epitaxial growth allows an
extra degree of control in device structure, which is fortunate, for long-wavelength APDs suffer from
problems related to control of internal field strength. Avalanche multiplication relies upon extremely
high internal electric fields on the order of 105 V cm-1 to drive ionizing collisions of carriers with the
lattice.i The smaller bandgap of materials sensitive to long-wavelength light means that such high fields
can cause tunneling between bands: an unwanted source of dark current. The SACM design mitigates
this problem by putting the low-bandgap material necessary to absorb long-wavelength light in a single
dedicated absorption layer. Doping is used in an adjacent charge layer to keep the potential across the
absorption material low, so that only the multiplication layer experiences extreme fields (Figure 2).
APD Layer Design
Design of an SACM APD consists of selecting doping levels throughout the structure as well as
appropriate thicknesses for the three functional layers (absorption, charge, and multiplication).
Voxtel Inc.
TECHNICAL NOTE: V805 - Page 2
Parameters to be adjusted by design include efficiency, speed, noise performance, and maximum gain;
constraining material parameters are absorption coefficient, saturation velocity, ionization coefficient,
and breakdown field strength.
Absorption Layer
Absorption layer thickness is dictated by a tradeoff between efficiency and bandwidth. Once incident
light reaches an absorption layer characterized by the absorption coefficient α, the efficiency of its
absorption during one pass through a thickness L is given by:
 abs  1  e  L
(1)
Thus, for a given absorption coefficient – 1.84 × 104 cm-1 at 1.3 μm, but only 5.5 × 103 cm-1 at 1.6 μm for
InGaAs lattice-matched to InP – it is evident that thicker absorbers imply greater absorption efficiency.
Designing a detector for high speed operation places a countervailing requirement upon
absorption length. A p-i-n detector’s intrinsic frequency response to a small-signal modulation at
angular frequency ω depends upon carrier saturation velocity via the transit times τe and τh. In the limit
where both recombination and diffusion can be neglected, the photocurrent resulting from such a signal
can be written as [1]:
I ω 
1
α L  i ω τe
 1  e i ω τ e 1  e α L 
1

 

α L  α L  i ω τh
 i ω τe


 e  α L e i ω τ h  1 e α L  1 



i ω τh
α L 

(2)
Of course, this is not the final frequency response of the complete device – the RC time constant of the
equivalent circuit, the transport of electrons from the absorption layer to the multiplication layer, and
the time it takes for both primary and secondary carriers to clear the multiplication layer all result in
additional terms. However, the qualitative effect of absorption layer thickness on bandwidth can easily
be seen in figure 1, where normalized plots of the hole component of (Eq.2) are presented for varying
values of L.
Voxtel Inc.
TECHNICAL NOTE: V805 - Page 3
Figure 4: Schematic diagram of the photocurrent I-V characteristic of a properly designed
SACM APD.Figure
The brief
following intrinsic
punch-through
marks
the unity
gain for
photocurrent,
IV.1: plateau
Plot of normalized
frequency
response
amplitude
Figure 3 Plot of normalized
intrinsic
frequencydepletion
responseof
amplitude
for varying
absorption layer
and
corresponds
to
complete
the
absorption
layer.
varying absorption layer thicknesses (L). Frequency is in units such that
thicknesses (L). Frequency is in units such that ω τh = 1 when L = 1.
ω τh = 1 when L = 1.
In principle, (Eq.1) and (Eq.2) allow optimization of absorption layer design provided that the saturation
e
h
e
velocities v sat
and v sat
are well known. Transistor researchers have measured v sat
to be 2.6 × 107 cm s-1
h
[3], and v sat
is estimated to be around 6 × 106 cm s-1 in InGaAs.ii These numbers suggest an intrinsic 3
dB bandwidth on the order of 50 GHz for a 1 μm absorber that is operated as a p-i-n detector. However,
without a good handle on the other relevant parameters (RC parasitics, electron injection rate, and
multiplication layer transit time), it is difficult to perform a quantitative optimization based upon these
calculations.
In practice, it is found that APDs with 1 μm absorption layers can be operated above 10 GHz;
such devices have a calculated single-pass absorption efficiency around 84% at 1.31 μm and 45% at 1.55
μm. In applications where both higher bandwidths and better absorption efficiencies are desired,
thinner absorption layers can be used so long as coupling between incident light and the absorber can
be enhanced. One solution is to place the absorber at the standing wave peak inside a short optical
cavity.iii Another possibility is to propagate the incident light in the plane of the wafer rather than at
normal incidence, so that the path length can be made quite long; this is akin to the placement strategy
of an active region inside an IPL.iv
Voxtel Inc.
TECHNICAL NOTE: V805 - Page 4
Charge Layer
The role of the charge layer is to control electric field strength inside the APD. A first requirement is to
keep field strength in the InGaAs absorber well below the breakdown field of about 250 kV cm -1 even
when the APD is biased near the limit of its operation (breakdown in the multiplication layer). The
higher the total charge (implying either higher dopant concentration or a thicker charge layer), the
lower the field in the absorber. Of course, it is possible to err on the other side. A second requirement
is that “punch-through” – the bias at which the depletion region reaches the absorption layer – not
happen too late. As reverse bias across an APD is increased, the depletion region extends through the
absorption layer, sweeping up more of the photocarriers generated there and steadily increasing the
photocurrent. If punch-through occurs comfortably before the onset of avalanche multiplication (which
hopefully is confined to the multiplication layer), the photocurrent I-V characteristic of the device has a
plateau that identifies unity multiplication gain (Figure 4). In a device with late punch-through, the unity
gain point is difficult to discern because avalanche multiplication starts before the collection of
generated photocarriers has reached a maximum. Another quantitative consideration is that the field
inside the absorber be sufficiently high for photocarriers to reach their saturation velocity; in InGaAs,
this is approximately 50 kV cm-1. Thus, one wants a charge layer that prevents breakdown or tunneling
in the absorption layer without concealing the onset of avalanche multiplication; at the same time, it
must maintain a field in the absorption layer that is strong enough to facilitate rapid photocarrier
collection.
Voxtel Inc.
TECHNICAL NOTE: V805 - Page 5
For very high speed applications it is best to use a thin charge layer with higher doping so as to
minimize carrier transit time. Also, as higher doping levels (in the 1018 cm-3 range) are easier to
reproduce by MBE than lower levels (1017 cm-3 range), and since thickness can in general be more
accurately controlled than dopant concentration, it makes sense to use a thin charge layer.
Computational solutions to the Poisson equation are helpful to evaluate internal electric field strengths
inside SACM APDs with different charge layer designs (Figure 5).
absorbe
r
multiplication
Figure IV.3: Calculated band edge diagrams for a SACM APD with various
Figure 5: Calculated
band
for a toSACM
doping
levelsedge
in the diagrams
charge layer, biased
achieveAPD
a fieldwith
of 400 various
kV cm-1 in doping levels in the
17-1 -3
charge layer, biased
to achieve
a field
ofdevice,
400 NkV
the multiplication
layer. For this
the multiplication
layer.
For this
10 cmin allows
too strong of a
A = cm
-3
absorber,
andstrong
NA = 2 × of
1017acm
results
in a late
punch-through
device, NA = 1017field
cmin-3the
allows
too
field
in the
absorber,
and NA = 2 × 1017 cm-3
and slow carrier collection.
results
in a late punch-through and slow carrier collection.
Voxtel Inc.
TECHNICAL NOTE: V805 - Page 6
Multiplication Layer
The multiplication layer is designed to provide the desired gain while suppressing noise. Detailed Monte
Carlo simulations of avalanche multiplication have been performed which confirm that SACM APDs with
thin multiplication layers have reduced multiplication noise as a result of the so-called “dead-space”
effect.v Ionization events tend to be localized inside thin multiplication regions because following each
ionizing collision, carriers must pick up energy across a certain distance – the dead-space – before they
are capable of causing subsequent ionizations. The resulting correlation between ionization events is
the source of the noise reduction observed for such APDs. Dead-space-multiplication theory (DSMT) is a
convenient mathematical treatment of these effects which allows one to analyze APD noise data with a
minimum of computational effort, and good fits with experimental data have been obtained.vi
Although DSMT and its variants provide an adequate model of multiplication noise in an APD, its
specific application to device optimization has been limited. The reason for this is that although DSMT
does a good job of predicting the multiplication noise associated with a given APD design, its main
conclusion is that noise suppression gets better as the ratio of dead space to multiplication layer
thickness increases – in other words, thinner is better. The thinner one makes a multiplication layer, the
lower the gain it can provide for a given field strength. Beyond a certain point, higher field strengths
lead to run-away dark current as the result of tunneling or other leakage paths. Although this critical
field strength is slightly higher for thinner multiplication layers (Figure 6), shrinking the multiplication
Figure IV.4: Breakdown voltage and critical field versus thickness as
Figure 6: Breakdown voltage and critical field versus thickness as
measured for InAlAs multiplication layers. The data was extracted by
measured for InAlAs multiplication layers.
Xiaoguang Zheng from device measurements originally published in [8].
Voxtel Inc.
TECHNICAL NOTE: V805 - Page 7
layer beyond a certain point greatly curtails the gain at which a SACM APD can operate. Thus, unless
one has a specific need for a certain excess noise factor, multiplication layers are designed to be as thin
as possible while delivering the desired level of gain. Experience has shown that InAlAs multiplication
layers on the order of 200 nm are a good compromise, affording multiplication gains above 100 with less
multiplication noise than the best commercial long-wavelength APDs; devices with multiplication layers
as short as 150 nm are reported in this thesis.
References
i
X. G. Zheng, J. S. Hsu, J. B. Hurst, X. Li, S. Wang, A. L. Holmes, Jr., J. C. Campbell, A. S. Huntington, and L. A.
Coldren, “A 12 × 12 In0.53Ga0.47As – In0.52Al0.48As avalanche photodiode array,” IEEE Journal of Quantum
Electronics, vol. 38, no. 11, pp. 1536 – 1540, 2002.
ii J. C. Campbell, W. S. Holden, G. J. Qua, and A. G. Dentai, “Frequency response of InP/InGaAsP/InGaAs avalanche
photodiodes with separate absorption ‘grading’ and multiplication regions,” IEEE Journal of Quantum Electronics,
vol. QE-21, no. 11, pp. 1743 – 1746, 1985.
iii C. Lenox, H. Nie, P. Yuan, G. Kinsey, A. L. Holmes, Jr., B. G. Streetman, and J. C. Campbell, “Resonant-cavity
InGaAs-InAlAs avalanche photodiodes with gain-bandwidth product of 290 GHz,” IEEE Photonics Technology
Letters, vol. 11, no. 9, pp. 1162 – 1164, 1999
iv C. Cohen-Jonathan, L. Giraudet, A. Bonzo, and J. P. Praseuth, “Waveguide AlInAs/GaAlInAs avalanche photodiode
with a gain-bandwidth product over 160 GHz,” Electronic Letters, vol. 33, no. 17, pp. 1492 – 1493, 1997
v F. Ma, S. Wang, X. Li, K. A. Anselm, X. G. Zheng, A. L. Holmes, Jr., and J. C. Campbell, “Monte Carlo simulation of
low-noise avalanche photodiodes with heterojunctions,” Journal of Applied Physics, vol. 92,no. 8, pp. 4791 - 4795,
2002.
vi M. A. Saleh, M. M. Hayat, P. P. Sotirelis, A. L. Holmes, Jr., J. C. Campbell, B. E. A. Saleh, and M. C. Teich, “Impactionization and noise characteristics of thin III-V avalanche photodiodes,” IEEE Transactions on Electron Devices, vol.
48, no. 12, pp. 2722 – 2731, 2001
Voxtel Inc.
TECHNICAL NOTE: V805 - Page 8