 is many times larger than primary photocurrent. The APD designs are more 53.3.2 The characteristics of optical detectors complex than those for p-i-n detectors but essentially a p-i-n detec- tor can function as an APD with the i-layer both absorbing the The device parameters of a p-i-n detector relevant to the design of photons and accelerating the photoelectrons and holes. Of course an optical receiver are the quantum efficiency nq, or responsivity R, such a device is not as good as could be made and special designs the capacitance Cd, the series resistance Rs, the dark current Id and have evolved for APDs. The current in the output of an APD is given the device bandwidth. The nq or R depend on the material and the by Equation 53.45 where M is the current gain of the device. device design. For well designed devices nq of the order of 80% to 95% is possible. Since p-i-n detectors operate reversed biased the (53.45) device capacitance can be approximated to that of a plane capacitor, given by Equation 53.46 where E0 is the free space permittivity, er the relative permittivity, Ad the device junction area and W the width Clearly for M=1 the output current equals the photocurrent as in of the i-layer. a p-i-n detector. The maximum usable value of M depends on the material, the device structure and the quality of the semiconductor (53.46) processes used to produce the device. Typical Si and InGaAsP designs as shown in Figure 53.20. The Si device, known as reach through APD, offers good quantum effi- The value of C d depends on the material ( e ), the device area (Ag) and the width of the i-layer width. For fast devices with 50um Contact AR coating diameter the capacitance can be as low as 0.5pF. The series resist- ance is the resistance of the material and of the contact bonding. In well designed detectors the series resistance is of the order of 5 to 20 ohms. The dark current of the device is the current flowing in the circuit with bias applied but without light. This current is small because the device is reversed