利用无源抑制技术，研究了盖革模式下雪崩光电二极管(APD)的电流-电压特性。发现光电流和暗电流的一个显著区别是暗电流不反映贯穿特性，这是光生载流子和热载流子有不同统计分布的实验证据，也说明在盖革模式下，暗计数增加比光子探测效率增加更快的原因是由于载流子收集效率不同引起的。根据其贯穿特性适当选择盖革模式下APD的反偏压可提高单光子探测器的信噪比。

A passive quenching circuit is used to study the punch-through characteristics of avalanche photodiodes under the Geiger mode. It is found that the great difference between the photocurrent-voltage curve and the dark current-voltage curve is that the photocurrent-voltage curve indicates clearly the punch-through voltage while the dark current-voltage curve is insensitive to the punch through. The experiments demonstrate different distributions of the carriers. It is due to the different collection efficiency that the dark carriers counts increase much faster than the photo carriers counts. A proper selection of the bias can increase the SNR of the single photon detector.