Fundamental investigation of vacuum PD tube [photodiode]

As a fundamental study of photodiodes (PDs) for electron bombardment, two types of PDs have been experimentally investigated to be applied in electron tubes. A PD bombarded from the front surface (FB-PD), where a pn junction of planar structure existed, was evaluated to measure the fast response characteristics such as a 2.1 ns rise time. However, more than an order of magnitude increase of dark current was also confirmed after a long term stability test of 1000 hours. On the other hand, a PD bombarded by electrons from the rear surface (RB-PD) showed no increase of dark current after the stability test and a fast rise time of 2.7 ns. However, it was clarified that the rise time of RB-PD depended on the applied voltage to the PD, and an applied voltage of 200 V was necessary to achieve such a fast response. Since it was a much higher voltage than expected, some modifications may be necessary to achieve a fast response with a lower applied voltage, considering the yield of the PDs. Comparisons of the two types of PDs for some other characteristics are discussed. Preliminary test results of an electron bombardment APD are also discussed