nductor that has heavily doped in-type or p-type material. This type of photodetector is commonly used for infrared detection. During operation, a photoconductor's electrical conductivity increases linearly with temperature. Thus, the photoconductor provides an increase in current when the detector is exposed to radiation. In contrast, the absence of radiation produces a current referred to as the "dark current." PHOTODIODES A diode represents a region of a single-crystal semi- conductor material that has a p-n junction, resulting in electrons in the conduction band on the n side of the junction drifting across to combine with the holes on the p side of the junction. After a period of time the n- type material near the junction is depleted of conduction electrons while the p-type material near the junction is depleted of holes. This results in the generation of a potential difference across the p-n junction. Chapter Four When a photodiode receives optical radiation with a wavelength short enough to bridge the bandgap at the p-n junction, additional electrons from the n region flow across the p-n junction and combine with holes in the p region. This results in a widening of the depletion region and causes the potential across the p-n junction to widen until electrostatic forces again act to stop the flow of electrons. When the gap widens, cur- rent ceases to flow through the diodes, resulting in the generation of a voltage that is proportional to the intensity of the received optical radia- tion. This action results in an open circuit; thus, the voltage across the p- n junction is called the open-circuit voltage Voc. oc A photodiode can also operate in a short-circuit condition by short- circuiting both sides of the detector to prevent voltage from occurring across the diode. This action results in a current flowing through the diode, referred to as the short-circuit current Isc Here Isc is proportional to the incident intensity and provides a second mechanism for converting an optical signal into an electrical si