TABLE OF CONTENTS An optoelectronic device is characterized by its I V characteristics, spectral response, quantum efficiency, speed of response, and power consumption. In Chapter 3, we considered the first two features. There are two important aspects to the speed of response of a diode. There is an intrinsic response speed associated with optical absorption or emission. There is also an extrinsic response that characterizes the electrical coupling of the diode to an external circuit such as a driver or amplifier. In this chapter, we will consider the response time of diode-based devices like photodiodes and light-emitting diodes (LEDs). The intrinsic response time is related to the minority carrier mobility in the case of a photodiode, and the minority carrier recombination time in the case of an LED, whereas the extrinsic response time is related to the device capacitance. The case of the photodiode is a bit more complicated than that of the LED because the carriers are initially distributed throughout the diode by the absorption of photons. In the LED, the photons are all generated in a narrow region at the p-n junction. If we can understand the photodiode response, then the LED behavior follows as a special case.
In almost every case, the response time of a photodiode or an LED will be determined by the product of its capacitance and series resistance. Photodiodes are operated in reverse bias, and the diode capacitance in reverse bias is much less than the diode capacitance in forward bias. Therefore, photodiodes tend to...
