C30902E, C30902S, C30921E, C30921S
Active-Quenching Circuit
Dark Current
Until the C30902S or C30921S is recharged, the probability
of detecting another incoming photoelectron is relatively
low. To avoid an excessive dead-time when operating at a
Both the C30902S and C30921S have been selected to have a
low dark-count rate. Cooling to -25°C can reduce this by a factor
of 50, since the dependence of dark-count rate on temperature
is exponential.
large voltage above V , an "actively quenched" circuit can
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be used. The circuit temporarily drops the bias voltage for a
fraction of a microsecond following the detection of an
avalanche discharge. This delay time allows all electrons
and holes to be collected, including most of those
temporarily "trapped" at various impurity sites in the silicon.
When the higher voltage is reapplied, there are not
electrons in the depletion region to trigger another
avalanche or latch the diode. Recharging can now be very
rapid through a small load resistor. Alternatively, the bias
voltage can be maintained but the load resistor is replaced
by a transistor which is kept off for a short time after an
avalanche, and then turned on for a period sufficient to
recharge the photodiode.
The dark-count increases with voltage following the same curve
as the Photoelectron Detection Probability until a voltage where
after-pulsing is responsible for a feedback mechanism which
dramatically increases the dark-count rate. This maximum
voltage is circuit dependent, and is not warranted other than the
values listed on page 3. In most cases, with a delay time of 300
ns, the diode can be used effectively at V up to V
+ 25V.
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R
The C30902S and C30921S should not be forward biased or,
when unbiased, exposed to strong illumination. These
conditions result in a greatly enhanced dark-count which
requires up to 24 hours to return to its nominal value.
After-Pulsing
An after-pulse is an avalanche breakdown pulse which follows
a photon-generated pulse and is induced by it. An after-pulse is
8
usually caused by one of the approximately 10 carriers which
pass through the diode because of the first avalanche. This
electron or hole is captured and trapped at some impurity site in
the silicon, as previously described. When this charge-carrier is
liberated, usually in less than 100 ns but sometimes several
milliseconds later, it may start another avalanche. The
probability of an after-pulse occurring more than one
microsecond later is typically less than 2% at 2 volts above
V
, using the circuit shown in Figure 9. After-pulsing
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increases with bias voltage. If it is necessary to reduce after-
pulses, it is recommended that one keep V -V low, use an
R
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actively-quenched circuit with a long delay-lime (See Figure
12), or a passively-quenched circuit with a long R C constant.
L
Stray capacitances must also be minimized. Electronic gating of
the signal can be performed in certain situations. Should after-
pulses be a serious complication in a particular application,
operation below V
considered.
with a good amplifier might be
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For more information e-mail us at opto@perkinelmer.com or visit our web site at www.perkinelmer.com/opto
PerkinElmer Optoelectronics
22001 Dumberry Road,
Vaudreuil, Québec
Canada J7V 8P7
All values are nominal; specifications subject to change without notice.
Phone: (450) 424-3300
Fax: (450) 424-3411
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