Voltage controlled minority carrier collection—the exponentially retrograded photodiode

Abstract

The large retarding field in an exponentially retrograded photodiode is shown to significantly reduce the base-generated photocurrent at low reverse-bias voltages. Increasing the reverse voltage on the diode reduces the length over which this retarding field is effective, thereby increasing the Photo-Transmission Coefficient of the diode. From avalanche breakdown considerations, the largest ratio of change in photocurrent caused by this effect is shown to be only a function of the resistivity and bulk-lifetime in the base of the diode. This voltage-controlled collection effect is observable only in diodes with high background resistivity (i.e. ϱ silicon > 25 Ω cm.). It is theoretically possible to obtain voltage-controlled collection ratios of better than 50:1 in, typically, a 10 V junction swing. Experimental results compare well with theoretical models.