Appendix
Operation of the C30902S and C30921S
in the Geiger Mode
Passive-Quenching Circuit
The simplest, and in many cases a perfectly adequate method
of quenching a breakdown pulse, is through the use of a
current limiting load resistor. An example of such a "passive"
quenching circuit is shown in Figure 9. The load-line of the
circuit is shown in Figure 10. To be in the conducting state at
Introduction
When biased above the breakdown voltage, an avalanche
photodiode will normally conduct a large current. However,
if the current is such that the current is limited to less than
a particular value (about 50 µA for these diodes), the
current is unstable and can switch off by itself. The
explanation of this behavior is that the number of carriers in
the avalanche region at any one time is small and
fluctuating wildly. If the number happens to fluctuate to
zero, the current must stop. It subsequently remains off
until the avalanche pulse is retriggered by a bulk- or photo-
generated carrier.
V
two conditions must be met:
BR
1. The avalanche must have been triggered by either a
photoelectron or a bulk-generated electron entering the
avalanche region of the diode. (Note: holes are inefficient at
starting avalanches in silicon.) The probability of an avalanche
being initiated is discussed above.
2. To continue to be in the conducting state a sufficiently large
current, called the latching current I
, must be passing
LATCH
through the device so that there is always an electron or hole in
The C30902S and C30921S are selected to have small
bulk-generated dark-current. This makes them suitable for
the avalanche region. Typically in the C30902S and C30921S,
I
I
=50 µA. For currents (V -V )/R , much greater than
BR
LATCH
B
L
low-noise operation below V
or of photon-counting
BR
in the Geiger mode. In this so-called Geiger
, the diode remains conducting. If the current (V -
LATCH
R
above V
BR
V
)/R , is much less than I
BR
, the diode switches almost
LATCH
L
mode, a single photoelectron (or thermally-generated
immediately to the non-conducting state. If (V -V )/R , is
BR
B
L
electron) may trigger an avalanche pulse which discharges
approximately equal to I
, then the diode will switch at an
LATCH
the photodiode from its reverse voltage V to a voltage
R
arbitrary time from the conducting to the non-conducting state
depending on when the number of electrons and holes in the
avalanche region statistically fluctuates to zero.
slightly below V . The probability of this avalanche
BR
occurring is shown in Figure 8 as the "Photoelectron
Detection Probability" and as can be seen, it increases with
reverse voltage V . For a given value of V -V , the
When R is large, the photodiode is normally nonconducting,
L
R
R BR
Photoelectron Detection Probability is independent of
temperature. To determine the Photon Detection
and the operating point is at V - I R in the non-conducting
R
ds L
state. Following an avalanche breakdown, the device recharges
to the voltage V - I R with the time constant CR where C
Probability, it is necessary to multiply the Photoelectron
Detection Probability by the Quantum Efficiency, which is
shown in Figure 2, the Quantum Efficiency also is relatively
independent of temperature, except near the 100 nm cutoff.
R
ds L
L
is the total device capacitance including stray capacitance.
Using C = 1.6 pF and R = 200.2 KΩ a recharge time constant
L
of 0.32 microseconds is calculated, in reasonable agreement
with observation as shown in Figure 9. As is also evident from
The C30902S and C30921S can be used in the Geiger
mode using either "passive" or "active" pulse quenching
circuits. The advantages and disadvantages of each are
discussed below.
Figure 9, the rise-time is fast, 5 to 50 ns, decreases as V -
R
V
increases, and is very dependent on the capacitances of
BR
the load resistors, leads, etc. The jitter at the half-voltage point
is typically the same order of magnitude as the rise-time. For
timing purposes where it is important to have minimum jitter,
the lowest possible threshold of the rising pulse should be
used.
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
