The PHOTOCONDUCTIVE or reversed biased p-n device is designed to detect high speed light pulses or the high frequency modulation of a continuous light beam. The reverse voltage increases the junction field strength to accelerate electron/hole transit times and reduces the junction capacity, thereby minimizing capacitive loading effects on the frequency response. PC photodiodes operate over a frequency range from DC to 100 MHz with rise times in the 3 to 12 nanosecond range. The noise current generated by the PC photodiode is a combination of shot noise, excess noise, and, in the case of a guard ring device, Johnson noise. Shot noise is produced by the reverse bias current and exhibits a l/f excess noise characteristic below 1 kHz. The Johnson noise is generated by the channel resistance between the active and guard ring diodes.
The PHOTOVOLTAIC or zero bias detector is designed for ultra low noise, low frequency, instrument applications. The PV frequency response, shunt resistance and junction capacity are active area dependent. The equivalent noise current generated by the device at zero voltage is virtually a flat Johnson noise spectrum from DC to the cutoff frequency.
The design decision to use a PV or a PC photodiode is predicated primarily on the frequency response requirements of the given application. Below 100 kHz, the PV photodiode provides better signal-to-noise performance than that obtained from an equivalent active area PC photodiode; below 1 kHz, the PV silicon photodiode is far superior in signal-to-noise performance.