Germanium microwave photodiodes exhibiting micro-plasma-free carrier multiplication

Germanium, microplasma-free, avalanche photodiodes have been fabricated using guard ring mesa structures. The diode fabrication procedure used a 5 micron Sb diffusion in the guard ring and a 0.3 micron As diffusion in the 0.002" diameter avalanche region. Uniform breakdown voltages of 16 to 40 volts were obtained by varying the bulk resistivity. Room temperature leakage current below breakdown was typically one tenth of a microampere. Room temperature dc multiplication factors of more than 100 were measured at a wavelength of 1.15 microns. At 3 Gc a power gain greater than 20db was obtained. Microwave signal power was measured as a function of multiplication and can be described in terms of a small signal equivalent circuit. Signal-to-noise ratios and maximum achievable power gains were measured as a function of frequency. Thus, for the first time, enough gain has been achieved to overcome receiver noise at room temperature in a microwave near-infrared detector at moderate light levels.