Phototransistor

The phototransistor has a light sensitive, collector-base p-n junction which works on the principle of photodiode. The photodiodes have small transparent windows that allow light to fall on the p-n junction and electron-hole pairs are generated by the absorption of light energy in the form of photons.

The phototransistor is exposed to incident light through a lens opening in the transistor package. When there is no incident light, there is a small thermally generated collector-to-emitter leakage current, I_CE0; this is called the dark current and is typically in the nanoampere range. When light strikes the collector-base p-n junction, a base current lx is produced that is directly proportional to the light intensity. This action produces a collector current which increases with according to the relation, I_c = hf_el λ= Poch- Except for the way the base current is generated, the phototransistor behaves as a conventional bipolar junction transistor. In many cases, there is no electrical connection to the base.

Since the actual photogeneration of base current occurs in the collector-base region, the larger the physical area of this region, the more the base current is generated. Thus, a typical phototransistor is designed to offer a large area to the incident light.

A phototransistor can be either a two-lead or a three-lead device. In the three-lead device, the base lead is brought out so that the device can be used as a conventional bipolar transistor with or without the light sensitivity feature. In the two-lead configuration, the base is not electrically connected, and the device can be used only with light as the input. Note that each individual curve on the graph corresponds to a certain value of light intensity (in this case mW/cm²) and that the collector current increases with light intensity. Also note their similarity with the collector curves of an ordinary BJT.

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• What is phototransistor?