Light Detectors

Photoresistors

Low cost, two lead photoresistors using a cadmium sulfide (CdS) element show a dramatic decrease in resistance when illuminated and are commonly used as a light sensing element for security lights, etc. so that they will turn on at night. In the dark, the resistance is very high, in the 1 megohm range. When illuminated, the resistance may drop to a few hundred ohms. They are typically more sensitive to green light, and they have a very high sensitivity compared to other types of sensors. They cannot be practically used for high speed switching or modulation for communications purposes because there is a "memory effect", requiring on the order of a second for the resistance to rise to its dark resistance.

Solar Cells

Silicon photovoltaic cells are typically thought of as voltage supplies, but they are also useful as sensitive detectors of light and near infrared. Solar cells are silicon wafers which are doped to produce a p-n junction. Commonly used "cells" are produced as wafers of diameter about 8 cm and 3 mm thickness which are cut from a crystalline silicon rod. The thin wafer is doped on one side to produce p-material (e.g., boron doped silicon) and on the other side to produce n-material (e.g., phosphorous doped silicon). The p material is typically connected to a metal base and the top side of the wafer (n-type) has a grid of electrical contacts. When light strikes the top of the wafer, it can penetrate through the p-n junctions and free electrons which can cross the junction into the n-type region. Electrons are then held in the n-region, being unable to recross the junction into the p-region. This establishes a voltage of about 0.5 volts and a cell as described above can produce about 1 watt of electrical output.

Phototransistors

A standard transistor is a three-lead device, and although the phototransistor has three regions, typically NPN, the base region is enlarged and generally does not have a lead attached to it. The collector-base junction is sensitive to light falling on it; a base current proportional to the light intensity is produced. This initiates a collector current proportional to the light intensity.

Phototransistor

The two terminal phototransistor at left allows a current to flow which is proportional to the illuminance on the surface of the unit. It is typically more sensitive than the photodiode, but also slower. It can be used to make a simple light meter.