An avalanche photodiode has been operated as a photon-counting detector with 2 to 3 times the sensitivity of currently-available photomultiplier tubes. APD (avalanche photodiodes) detection probabilities that exceed 27% and approach 50% have been measured at an optimum operating temperature which minimizes noise. The sources of noise and their dependence on operating temperature and bias voltage are discussed

Metscher, B. D.

Robinson, D. L.

English

NASA Technical Reports Server

TDA Progress Report 42-87 
N 8 I - 15 3 3 4 f-7 
July-September 1986 
A Cooled Avalanche Photodiode With High Photon 
Detect ion Pro babi I ity 
D. L. Robinson and B. D. Metscher 
Communications Systems Research Section 
A n  avalanche photodiode has been operated as a photon-counting detector with 2 to  
3 times the sensitivity of currently-available photomultiplier tubes. APD detection proba- 
bilities that exceed 27% and approach 50% have been measured at an optimum operating 
temperature which minimizes noise. The sources of nois’e and their dependence on operat- 
ing temperature and bias voltage are discussed. 
1. Introduction 
The use of optical wavelengths for communication over 
planetary distances requires detectors sensitive enough to  
detect light pulses containing only a few photons. Presently 
the only detectors with high enough gain and low enough 
noise to be useful for detecting such faint signals are photo- 
multiplier tubes (PMTs); however, their low quantum effi- 
ciency, the fragility of vacuum tube packages, and the high 
anode voltages required are important disadvantages for a 
space-based optical receiver. As a possible alternative. ava- 
lanche photodiodes (APDs) have been tested in a single photon 
detection mode. When cooled to reduce the number of ther- 
mal carriers, an APD can be reverse-biased beyond its break- 
down voltage to give i t  a very high ( l o 7  to los)  internal gain. 
APDs have a higher quantum efficiency and the reliability of 
solid state devices; thus they may be a useful alternative to 
PMTs. 
The theory of operation, statistical models of signal detec- 
tion and noise generation, and quenching systems have been 
investigated by others (Refs. 1-6). The present work centers 
on understanding the operational characteristics of APDs in ‘i 
photon counting mode. and in particular on measuring the 
probability of photon detection as a function of temperature 
and bias voltage. 
The theory of operation of APDs, the sources of noise. 
and the problem of quenching have been discussed in previous 
work (Ref. 7). The experimental procedures used to examine 
the characteristics of APDs and to measure detection proba- 
bilities are described here. Experimental results are then pre- 
sented and discussed. 
II. Experimental Setup and Procedure 
A schematic of the experimental setup is shown in Fig. 1 .  
The APD is cooled in an MMR Technologies K7701 System I 
microminiature refrigerator, in which the diode is mounted on 
a cold finger inside a small evacuated chamber and connected 
to the quenching circuit by manganin wire leads. The light 
source is a Mitsubishi ML-3101 laser diode emitting at 822 nm 
at 20 mA operating current. The laser may be operated as 
41 
https://ntrs.nasa.gov/search.jsp?R=19870005901 2020-03-20T12:24:48+00:00Z
either a pulsed or a continuous source. A calibrated beam- 
splitter directs a known fraction of the laser light to a United 
Detector Technology S550 optical power meter to  monitor 
the light level incident on the detector being tested. The 
light intensity is attenuated to a few photons per laser pulse 
by neutral density filters calibrated at 822 nm, then a second 
beamsplitter divides the beam between the APD and PMT. 
Two different PMTs were used. an RCA model C31034 and 
an RCA C31034A-02, both with GaAs photocathodes. These 
were cooled to  240 K in a TE refrigerator housing and were 
operated at 1600 V. All optical components were contained 
in a light-tight enclosure. Data collection and reduction were 
accomplished using a pair of HP 5370A time interval counters, 
a Tektronix 7834 storage oscilloscope, and an IBM PC AT 
(Fig. 2). 
The APDs tested were RCA type C30902S IR-sensitive 
silicon diodes. This model is similar to  the one tested in pre- 
vious work (Ref. 7 ;  RCA C30817), but the C30902 diodes are 
selected by the manufacturer for performance as photon 
counters and are sold as “optimized” for operation in a 
photon counting mode. 
The APDs were tested using a passive quenching circuit 
shown schematically in Fig. 3. The recovery time of the detec- 
tor using this circuit, i.e., the time between a detection event 
and restoration of the bias voltage across the diode to its 
original value, was approximately 6 P S .  
An active quenching circuit devised by Cova, et al., (Ref. 5) 
was built and tested, but effective implementation of their 
discrete-component, hgh-frequency feedback circuit proved 
impractical. However, since active quenching can reduce the 
detector’s dead time to as little as 10 ns, further consideration 
of active quenching circuits is planned. 
The frequency of dark noise counts was measured for each 
diode as a function of temperature and reverse bias voltage, 
and dark noise was measured for the PMT. These data were 
collected by simply counting noise events with a time interval 
counter and reading and storing the count data with an IBM PC. 
To measure the APD detection probabilities, the light 
source was pulsed at 15 kHz with pulses of about Sons  
duration. The voltage pulse from the laser power source was 
used to  gate a logic circuit (Fig. 4) which transmits either the 
APD output counts occurring during a laser pulse (“true” 
counts), those occurring outside a laser pulse (“false” counts), 
or all output counts. 
The PMT measured the beam intensity after attenuation. 
From this measurement, using the quantum efficiency of the 
PMT and the measured efficiency of the beamsplitter and 
other optics, the light intensity incident on the APD was 
calculated. Detection probabilities were calculated from this 
incident intensity and the measured number of APD detec- 
tion events, 
111. Results and Discussion 
A. Dark Noise 
Figure 5 summarizes the data for dark count frequencies 
for one RCA APD. Measurements were made at temperatures 
between 77 K and room temperature. As can be seen in these 
plots, the dark noise from the APDs increased more slowly 
with increasing bias voltage at intermediate temperatures 
than it did at higher and lower temperatures. The optimum 
operating temperature for these devices appears to  be about 
200 K.  At this temperature, the dark noise counts were less 
than 100 per second for a bias voltage of more than 1OV 
above the breakdown voltage. Operating the detector at the 
highest possible bias voltage is desirable in order to obtain 
the greatest sensitivity. 
The statistical frequency distribution of the lengths of the 
time intervals between noise counts was compared with a 
Poisson distribution using the chi-squared goodness-of-fit test. 
It was found that the dark noise counts were well-approxi- 
mated by a Poisson process at very low (near 80 K) and high 
(near 300 K) temperatures. At intermediate temperatures 
(between 120 K and 200 K) the chi-squared test failed t o  
show a good fit of the dark noise distribution to a Poisson 
distribution. 
We interpret these results as being dependent on the dead 
time of the passive quenching circuit and may differ with the 
use of an active quenching circuit where dead times are approxi- 
mately 10-20 ns. At high temperatures 0 200 K) the thermal 
carriers effectively dominate (due to  the circuit dead time) 
while at lower temperatures the dominant noise is due to  
trapped carriers or secondary avalanches. Trapping of carriers 
occurs during an avalanche, presumably at imperfection sites 
in the semiconductor crystal lattice. Carriers trapped after a 
detection or noise event can remain in the lattice and dis- 
lodge, initiating a secondary avalanche, after a time ranging 
from nanoseconds to  tens of minutes. Both the thermal gen- 
eration of noise carriers and the release of trapped carriers are 
expected to be Poisson processes. 
At high temperatures (i.e., near room temperature), since 
electrons are more mobile in the crystal lattice, trapped car- 
riers are dislodged more quickly and most initiate secondary 
avalanches within the 6 ps dead time of the APD quenching 
circuit. Thus only a few avalanches due to  trapped carriers are 
detected. At low temperatures (80 K to  120 K), trapped car- 
42 
riers remain trapped longer, and noise events due to them 
dominate those due to thermal carriers. whose numbers are 
greatly reduced at low temperatures. 
At intermediate temperatures, detected events include a 
significant proportion of both the Poisson-distributed primary 
(thermal) events and the conditionally Poisson-distributed 
secondary (trapping) events. Thus the noise at intermediate 
temperatures does not fit a simple Poisson distribution. 
B. Detection Probabilities 
The probability of single photon detection was taken as 
the ratio of the number of photons detected per second by 
the APD to the number of photons per second incident on 
its active region. The intensity of incident light was deter- 
mined from two independent measurements, one by the UDT 
photodiode power meter, the other by the PMT. From the 
intensity of the beam at each of these two points. using the 
measured transmission and reflection efficiencies of the two 
beamsplitters and the measured transmittance of the filters 
and other optics, the number of photons falling on the APD 
was calculated. Incident light intensities on the APD calculated 
from the photodiode data agreed within approximately 30% 
(or about 1/3 photon since we are detecting single photons) 
of that calculated from the PMT output data. As a further 
verification of the incident light intensity, a second PMT 
was used. The two PMT count rates were within approxi- 
mately 25% agreement for similar incident light intensities 
of a few photons. This discrepancy reflects the uncertainties 
in the manufacturer’s specifications for the quantum effi- 
ciencies of the PMTs. Despite this, we are reasonably confident 
that the light levels incident on the detectors are equal to or 
less than one photon per pulse for the experiment described 
here. 
The output count frequency measurements of the “true,” 
“false” and dark APD counts and the PMT output counts are 
summarized graphically in Fig. 6 for one RCA APD. Results 
have been scaled to represent a constant light intensity to  both 
the PMT and APD. (Experimentally, approximately 1.7 times 
more light was incident on the PMT than on the APD.) 
As can be seen in Fig. 6, dark counts remain less than 100 
counts per second even with an over-bias of 10 volts (as 
described earlier). When light pulses are incident on the APD, 
more trapped carriers are generated. These carriers detrap 
causing a secondary avalanche after the primary avalanche has 
occurred. These secondary avalanches or false pulses increase 
the effective dark count rate. The rate of occurrence of these 
false counts is dependent on the over-bias of the APD. As the 
over-bias is increased, the false count rate also increases. It 
is expected that the false count rate is also a function of the 
light intensity incident on the APD. This will be further 
investigated at a later time. As can be seen in Fig. 6, the 
APD is still quite effective in spite of the generation of sec- 
ondary avalanches since false count rates remain below 100 
counts per second for an over-bias of about 1.5 volts on the 
APD. Even at this low over-bias, the APD true detection 
count rate exceeds the PMT count rate as shown in Fig. 6. As 
expected, the true count detection rate increases with over- 
bias voltage. 
From this true count detection rate. a single-photon detec- 
tion probability was calculated (as described above) for each 
voltage tested. Results are summarized in Fig. 7 ,  and the 
appropriate false count levels are indicated. The uncertainty 
in this curve is approximately 30%, due mainly to uncertain- 
ties in the sensitivities of the PMTs. For a false count rate or 
effective dark count rate of 100 counts per second and an 
over-bias of about 1.5 volts, single-photon detection proba- 
bilities of approximately 27% were achieved. This is over twice 
that achievable with the C31034 PMT (quantum efficiency 
-12% at 822 nm, cooled to 240 K). As can be seen from 
Fig. 7 ,  single-photon detection probability approaches 50% 
for a 10 volt over-bias; however, the effective dark count rate 
nearly equals the signal count rate. 
IV. Conclusion 
Commercial avalanche photodiodes have been operated as 
single photon detectors by optimizing their operating tem- 
peratures and bias voltages. When operated under these con- 
ditions, the sensitivity of an APD can exceed that of a photo- 
multiplier tube by more than a factor of two with less than 
100 noise counts per second. At this sensitivity, the APD 
probability of detection was approximately 27 f 8% at an 
over-bias of 1.5 volts. Thus APDs represent an attractive 
alternative to PMTs for applications in deep-space optical 
communications, as well as in other applications requiring 
sensitive, reliable, small or lightweight detectors, such as 
astronomy, remote sensing and fiberoptic communications. 
43 
References 
1. McIntyre, R. J., The Distribution of Gains in Uniformly Multiplying Avalanche Photo- 
diodes: Theory, IEEE Trans. Electron. Dev., ED-19, pp. 703-713, 1972. 
2. McIntyre, R. J.,  On the Avalanche Initiation Probability of Avalanche Diodes above 
the Breakdown Voltage, IEEE Trans. Electron. Dev., ED-20, No. 7 ,  pp. 637-641, 
July 1973. 
3. Estrom, P. A., Triggered-Avalanche Detection of Optical Photons, J. Appl. Phys. 5 2 ,  
pp. 69744979,1981.  
Avalanche Diodes, IEEE Trans. Electron. Dev., ED-19, pp. 1056-1060, 1972. 
4. Oldham, W. G., Samuelson, R. R., and Antognetti, P., Triggering Phenomena in 
5. Cova, S., Longoni, A., and Ripamonti, G., Active-quenching and Gating Circuits for 
Single-photon Avalanche Diodes (SPADs), IEEE Trans. Nucl. Sci., NS-29, pp. 599- 
601,1982. 
6. Tan, H. H. Avalanche Photodiode Statistics in Triggered Avalanche Detection Mode, 
TDA Progress Report 43-79, Jet Propulsion Laboratory, Pasadena, Calif., pp. 69-80, 
July-September 1984. 
7. Robinson, D. L., and Hays, D. A., Photon Detection with Cooled Avalanche Photo- 
diodes: Theory and Preliminary Experimental Results, TDA Progress Report 42-81, 
Jet Propulsion Laboratory, Pasadena, Calif., pp. 9-16, January-March 1985. 
OPAQUE 
PARTITION 
POWER METER 
PHOTODIODE 
NEUTRAL 
FILTERS 
PMT IN 
REFRIGERATOR 
HOUSING 
MICROSCOPE CUBE 
OBJECTIVE BEAMSPLITTER 
COOLER 
PASSIVE 
QUENCHING 
CIRCUIT 
LASER 
DIODE o---o 
LOGIC - CIRCUIT 
Fig. 1. Arrangement of optical components inside dark enclosure 
- ~ STORAGE 
OSCl LLOSCOPE 
UDT POWER 
METER 
DARK 1 
PHOTODIODE I 
I I 
I 1 I I , I ,  1 
LASER PULSE I PMT P R E r P  I 
GENERATOR 
I LASER APD 
I i d i A  
c c 
TIME INTERVAL 
COUNTER PC AT I 
Fig. 2. Schematic representation of APD detection probability data acquisition and reduction 
apparatus. The amplifier for the APD output signal is necessary only for input to the TTL logic 
circuit. In general, further amplification of an APD signal pulse is not needed. 
45 
+VB 
APD L 
OUTPUT 0 I 
0.22 pF 
100 kn 
- n ss 
U I-OOUTPUT 
10 pF I 
ApDT 
Fig. 3. Passivequenching circuit. Deadtime 
of the detector using this circuit is about 6 ps. 
0 
z 
8 
W 
In c
z 
3 
r 
8 
W 
5!? 
0 z 
Y 
LT 
d 
OUTPUT 0
PULSE 
Fig. 4. Logic circuit for determining coincidence of APD output pulses with 
laser input pulses 
APD REVERSE BIAS VOLTAGE, V 
Fig. 5. Measured frequency of dark noise counts as a function of APD bias voltage and temperature (horizontal axis is discontinuous) 
46 
l o 5 i  LASER RATE 
-4 
L 
PMT COUNTS 
Id - 
V Ill 
2 -  
l- z 
3 
0 
V 
102 - 
PMT DARK COUNTS 
101 - 
APPROXIMATE APD 
BREAKDOWN VOLTAGE I 
1001 I I I 1 I I I 1 
175 177 179 181 183 185 187 189 191 
APD BIAS VOLTAGE 
0.7 
0.6 
r- 
1 - 
m 
0 
a L
z 
0 0.4 
l- 
V u
2 0.5 
t, 
0 0.2 
z 
0 
I- 
O 
I 
w 1
W 
4 0.2 
v, 0.1 
c 
I I I I I I I 
*FALSE COUNTS 
PER SECOND 
/ / @  
0. .” 
_--- ---- 
31034PMT QUANTUM EFFICIENCY -12% 
APPROXIMATE APD /- BREAK DOWN VOLTAGE 
V I  I I I I I I 
~ ~~ 
75 177 179 181 183 185 187 189 191 
APD BIAS VOLTAGE 
Fig. 7. APD single-photon detection probability as a function of 
bias voltage at 200 K. Effective dark (“false”) count rates are 
indicated. Uncertainty (dashed curves) in this curve is estimated to 
be 30%. 
Fig. 6. APD output count frequencies as functions of bias voltage 
at 200 K for an RCA 30902s APD. Laser pulse rate was 15 kHz. The 
PMT (RCA 31034, quantum efficiency -lWo) count rate has been 
scaled to the same incident light intensity as the APD. APD break- 
down voltage was approximately 175.5 V at 200 K. 
47 


A cooled avalanche photodiode with high photon detection probability

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