Avalanche photodiode quenching circuit wtih resetting means havi

Details Avalanche photodiode quenching circuit wtih resetting means havi Avalanche photodiode quenching circuit wtih resetting means havi

1992-02-20

1993-03-16

Nelms, David C.

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

307311, H01J 4014

Patent

active

051947277

DESCRIPTION:

BRIEF SUMMARY
This invention relates to an avalanche photodiode quenching circuit for use in photon counting measurements.
Photon counting measurements were originally, and are to some extent presently, carried out using photomultiplier tubes for photon detection. A typical photomultiplier is however relatively fragile, bulky and expensive. The search for a more convenient alternative has led to the use of photodiodes operated in the so-called avalanche Geiger mode. This mode entails reverse-biasing the photodiode with a bias voltage typically a few volts greater than the photodiode breakdown voltage V.sub.BR. V.sub.BR is the voltage at which a single photon absorption produces complete electrical breakdown of the photodiode active region by cascaded collision ionisation. It is analogous to the ionisation processes occurring in the gas phase in a Geiger-Muller tube.
Avalanche photodiodes are comparatively inexpensive and rugged, and exhibit high quantum efficiencies. They are not however without disadvantages. In particular, for the purposes of achieving high quantum efficiencies, it is necessary to operate at reverse voltages at least bordering on that capable of producing a self-sustaining avalanche in the photodiode. If the photodiode avalanche current reaches a value referred to as I.sub.latch, typically 50 microamps, the avalanche is self-sustaining in the absence of further photons. This may produce catastrophic failure. The photodiode is substantially insensitive to photons while in the avalanche condition. Furthermore, it experiences temperature stress which, after the avalanche is terminated by removing the bias voltage, manifests itself as an increased dark current in subsequent operation. This reduces measurement accuracy and sensitivity, since dark current counts must be subtracted from the total count in a measurement, and both are subject to Poissonian statistics. Furthermore, a sustained current through the photodiode in excess of I.sub.latch tends to fill normally empty defect sites or traps in the photodiode semiconductor material. These traps have long lifetimes compared to the minimum time between counts or dead-time of the photodiode. Trapped charge carriers are therefore released considerably later than, but are correlated with, a photon absorption responsible for the avalanche creating them. The release produces so-called after pulses which are detected by the measuring circuitry monitoring the photodiode. This is a serious problem in the field of photon correlation spectroscopy in particular, since it means that the detection system introduces a degree of correlation between detected pulses which is absent in the original light beam. The measured autocorrelation function will therefore exhibit spurious features which affect or even invalidate the measurement results.
To circumvent these difficulties, the approach in the art has been to provide means for quenching an avalanche as soon as possible after initiation and detection. One particularly simple approach is referred to as passive quenching. It involves arranging the photodiode in series with a comparatively large series resistor, e.g. 220 Kohm, and applying the bias voltage across the series arrangement. Prior to photon absorption, i.e. when the photodiode is quiescent, the bias appears across the substantially non-conducting photodiode. After absorption, the resistor limits the maximum current taken by the photodiode to a value below I.sub.latch when the falling voltage across the photodiode becomes equal to V.sub.BR. The avalanche is therefore automatically terminated. This arrangement is adequate for comparatively low photodetection rates up to 250 KHz and light intensity fluctuation frequencies up to the same value. However, its disadvantage is that the photodiode is comparatively slow to recover from a detection event. The photodiode must recharge its capacitance through the large resistance before it returns to the quiescent or photosensitive state and this leads to a dead-time in the order of 1 .mu.sec. Furthermore, during recharge, th

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Applied Optics, vol. 26, No. 12, Jun. 15, 1987, New York, U.S., R G. W. Brown et al, "Characterization of Silicon Avalanche Photodiodes for Photon (con't) Correlation Measurements. 2: Active Quenching", pp. 2383-2389.

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