Signal and noise response of high speed germanium avalanche photodiodes

Germanium avalanche photodiodes, providing gain at microwave frequencies, have been fabricated and tested. The diodes employ a guard ring structure to achieve a uniform, microplasma-free, multiplying region with an active diameter of 40 microns. Low-frequency chopped light current gains of greater than 200, and small-signal 6 GHz current gains of greater than 10 have been obtained at room temperature for a carrier wavelength of 1.15 microns. In the normal operating range, the signal output power is found to vary as the square of the multiplication, and the noise is found to vary as the cube of the multiplication. This limits the maximum useful multiplication of the diode to that level which gives a diode noise equal to the receiver noise. A small-signal equivalent circuit with lumped elements corresponding to the physical processes occurring within the diode, is introduced to describe the small signal behavior. The model is valid over the entire multiplication range, up to frequencies of about 10 GHz.