Excess noise, gain, and dark current in Ge avalanche photodiodes

Measurements have been carried out on a p+ n type Ge avalanche photodiode (typical guard ring structure with channel stoppers) to obtain the excess noise factor, F; multiplication, M; and the dark current, Id over a wide range of temperatures (143–323 K). The multiplication initially increases with the illumination wavelength and peaks in the 1550 nm region because of more holes being injected into the avalanche region as the absorption depth becomes much longer than the width of the p+ region. The excess noise factor, F, at À = 1550 nm was typically slightly less than multiplication, M, and showed very little dependence on temperature. The effective ratio of electron to hole ionization coefficients was approximately 0.7–0.8, the dark current activation energy close to room temperature was about Eg, indicating that the dark current is controlled by the diffusion of minority carriers. Below ~ 232 K, dark current activation energy is about Eg/2, indicative of carrier generation in the depletion region. M varied approximately as T−n, with n depending strongly on the reverse bias. In the low-frequency range below ~ 200 Hz, the noise current followed a 1/f behavior.