Anomalous noise behavior in wide-bandwidth photodiodes in heterodyne and background-limited operation

When the frequency response of a semiconductor photodiode is limited by carrier diffusion to the junction, anomalous noise behavior can result. Although the signal current decreases, the noise current remains constant since the junction shot noise is only a function of the dc current flow. Under these conditions, there is a marked decrease in high-frequency sensitivity and the noise spectrum is not a true measure of the frequency response.