TABLE OF CONTENTS There are a number of solid-state devices that can generate an electric signal when they are illuminated. We can divide all these devices into two categories. In one category are the devices that convert 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 converted to heat, and the rise in temperature is converted by the thermocouples 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 current, regardless of their energy above the threshold value. Photodiodes fall into this second category. Photons can be absorbed in a photodiode if their energy exceeds the band gap energy of the photodiode 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 nearly 100% efficient.
Photodiode detectors were developed along with the transistor. Silicon 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...
