Page 43 - Photonics Essentials an introduction with experiments

Source: Photonics Essentials

Chapter
3

Photodiodes

3.1 Introduction

There are a number of solid-state devices that can generate an elec-
tric signal when they are illuminated. We can divide all these de-
vices into two categories. In one category are the devices that con-
vert the energy in a beam of light into an electric signal. An example
of this is the bolometer. This is really a collection of thermocouples
inside an efficient photon absorber. The energy of the photons is con-
verted to heat, and the rise in temperature is converted by the ther-
mocouples into an electric signal. These devices are energy detectors.
The electrical current is proportional to the energy in the optical
beam. In the second group are quantum threshold detectors. Photons
can be absorbed in these devices if the energy of a photon exceeds a
certain threshold value. All absorbed photons generate the same cur-
rent, regardless of their energy above the threshold value. Photodi-
odes fall into this second category. Photons can be absorbed in a
photodiode if their energy exceeds the band gap energy of the photo-
diode material. In principle, each photon absorbed contributes one
electron to the current. This is a direct exchange of quanta—one
electron for one photon. In most photodiodes, this exchange is near-
ly 100% efficient.
Photodiode detectors were developed along with the transistor. Sili-
con is the most common photodiode material for two reasons. Silicon
photodiodes are sensitive to a range of light wavelengths that include
the region of visible light. Silicon photodiode manufacture benefits
from the same advanced processing technology used to make silicon
integrated circuits.

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