 biased but it depends on temperature, device structure and material quality. In general devices for long wave- Guard lengths (because of the smaller band gap) have larger dark currents Depletion than those for short wavelength devices. For example Si p-i-n have layer much lower I that both Ge and InGaAsP, pAs rather that nAs. The device operates reversed biased and consequently the device resist- ance, that is the resistance of the junction, is large and is usually ignored in designs. The equivalent small signal circuit for normally biased photodiodes is shown in Figure 53.21. The bandwidth of the p-i-n detector is dictated by the transit time of the carrier across the i-layer and the circuit time constant estab- Contact lished by Co and any other external capacitance and the total resistive load. The bandwidth due to the transit time effect is given by Equation 53.47 where W the depletion layer of the device, usually equal to the width of the i-layer, and the carrier velocity corresponding to the InP (P ) reverse bias. Usually this is the carrier saturation velocity. InP (n) (53.47) InGaAs (n) InP (h) The bandwidth imposed by the circuit time constant is given by Equation 53.48 where C, the total capacitance. Multiplication absorption A/CR - 2r RC (53.48) layer layer C consists of the diode capacitance, the parasitic circuit capacit- Figure 53.20 Some typical APD device structures: ance and any other capacitance external to the detector. Therefore (a) silicon (b) InGaAsP for high bandwidth all these capacitances must be as small as permitted by the technology used. Usually inequality 53.49 applies. Optical detectors 53/17 Figure 53.21 Equivalent circuit for p-i-n and APD detectors: (a) p-i-n; (b) APD However for very high bandwidth, that is in excess of 3GHz to The bandwidth of an APD depends on the circuit time constant but 5GHz, the transit time starts limiting the device bandwidth and as a also on the gain-bandwidth product of the device, as given by result the width of the i-layer is reduced with a subseque