High Speed Photodetection in Germainum and Silicon Cartridge-Type Point-Contact Photodiodes

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Abstract
The purpose of this paper is to discuss the capabilities of a new high-speed encapsulated point-contact photodiode as a detector of microwave-modulated light. The laser-forming of germanium and silicon diodes and its influence on the diode characteristics is discussed. Measurements are presented on the static voltage-current characteristics for various light levels and on the small-signal microwave impedance characteristics of these diodes. These data show that the point-contact photodiode can have an effective device quantum efficiency greater than unity and an equivalent microwave impedance typically of several hundred to several thousand ohms. Data on the sensitivity of the diode as a direct demodulator of light which is intensity-modulated at 3 Gc/sec are also presented. It is shown that the silicon diode, operated at avalanche breakdown, is more sensitive than the germanium diode, and that both the germanium and silicon point-contact diodes are at least as sensitive as a good germanium surface-junction PIN photodiode. Finally, experimental results relating to the heterodyne mixing characteristics of the diodes are given when the diode is illuminated with microwave-modulated light and biased with a microwave local oscillator signal. Under these conditions it is found that the point-contact photodiode has greater sensitivity than that obtained previously using a simple bulk photoconductor.

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