Noise in phototransistors

The investigation reported here presents both analytical and experimental results on the noise performance of phototransistors. The noise performance of phototransistors depends on fluctuations traceable to two main sources: 1) the random fluctuations in the rate of photon arrival; and 2) mechanisms inherent in the device such as fluctuations in the generation of free carriers, diffusion and recombination fluctuations, and 1/f mechanisms, to cite a few. On the basis of corpuscular arguments, it is shown that the important fluctuations may be represented by simple, partially correlated shot noise current generators in parallel with the device junctions for a wide range of frequencies. The hybrid-pi transistor model and these noise generators have been used to derive an expression for the noise current to the short-circuited output of a phototransistor. Attention has been primarily focused on the low-frequency expression since correction terms due to high-frequency effects are negligible. The output noise spectra of commercially available silicon phototransistors and silicon units fabricated at the Microelectronics Laboratory of the University of Florida have been measured. The measurements are in good agreement with the model. The current gains and beta cutoff frequencies of these devices have been computed by interpreting the measured spectra with the aid of the analytical results. Independent ac and dc measurements of these parameters are in satisfactory agreement with values determined from noise measurements. It is thereby ascertained that noise measurements on phototransistors can be used to characterize them in terms of current gain and beta cutoff frequency.