Transit-Time Considerations in p—i—n Diodes

Linear equations for the carrier concentrations and photocurrent density in the i region of a p—i—n photodiode have been obtained. The frequency response of this photodiode has been calculated taking account of the difference in mobility of the carriers, and of the nonuniform generation of carriers in the i region. For high frequencies, the frequency response of the diode is largely dependent on the faster moving of the two carriers. For a Si p—i—n photodiode using ruby laser light, a transit-time limitation of 5 Gc/sec has been previously estimated by the authors. On the basis of the above analysis, the transit time limitation is estimated to be 15 Gc/sec. In both cases these estimates refer to the frequency of the half-power point.