Narrow Base Germanium Photodiodes

The operation of germanium photodiodes at room temperature both as reverse-biased and photovoltaic detectors is analyzed. This analysis takes into account generation of hole-electron pairs in the base as an exponentially decreasing function of distance from the surface. General expressions are derived for the steady-state and the time-varying detector signal components. The intrinsic frequency behavior (that associated with carrier diffusion from the point of generation to the p-n junction) is the same for reverse-biased and photovoltaic operation. The frequency-cutoff behavior is compared with that of a homogeneous base transistor and, for the case of small loss of photogenerated carriers by surface recombination, it is shown that the cutoff frequencies are essentially the same for identical base width. Photodiodes may be useful at modulating frequencies well above this cutoff frequency if the radiation is penetrating since for this type of radiation the intrinsic frequency response does not decrease rapidly above cutoff. The equivalent circuits for both reverse-biased and photovoltaic operation are obtained as is the noise-equivalent circuit for reverse-biased operation. It is shown that at frequencies where 1/f noise may be neglected, for most small-signal applications reverse-biased operation is greatly superior to photovoltaic operation. With a reduction in base width the intrinsic frequency cutoff will be increased, the bulk and surface recombination loss of photogenerated carriers decreased, and the diffusion capacitance associated with the p-n junction in photovoltaic operation reduced. Thus, improvements in both reverse-biased and photovoltaic operation are realized.