Page 67 - Photonics Essentials an introduction with experiments

Source: Photonics Essentials

Chapter
4

Electrical Response
Time of Diodes

4.1 Introduction

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 absorp-
tion or emission. There is also an extrinsic response that character-
izes 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 recombi-
nation 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 pho-
tons. 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 resist-
ance. Photodiodes are operated in reverse bias, and the diode capaci-
tance in reverse bias is much less than the diode capacitance in for-
ward bias. Therefore, photodiodes tend to operate much faster than
LEDs of the same size.

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