The photosensitive element is a semiconductor device whose resistance changes in response to variations in light intensity. As an example, we will explore the structure and characteristics of a phototransistor. A phototransistor operates in the active region, with its base terminal left open. In an NPN-type phototransistor, the collector must be connected to the positive supply voltage, while in a PNP-type, the connection is reversed. When light strikes the base region of the phototransistor, it generates a large number of electron-hole pairs. Under the influence of the internal electric field, these charge carriers move, causing electrons to flow from the collector and base to the emitter, resulting in an emitter current.
Phototransistors are known for their high gain, which is a result of their internal amplification mechanism. This makes the material quality crucial—homogeneous structures can achieve gains ranging from 50 to several hundred, making them cost-effective and widely used. On the other hand, heterogeneous structures, although offering higher performance, are more expensive and less commonly utilized. Another key advantage of phototransistors is their low noise level, which makes them ideal for sensitive applications.
The characteristic curve of a phototransistor closely resembles that of a regular bipolar junction transistor (BJT), except that instead of base current, the amount of incident light controls the operation. Due to their high gain and low noise, phototransistors find extensive use in various applications such as automotive systems, home lighting, and entertainment electronics. Their ability to convert light into electrical signals efficiently makes them a valuable component in modern electronic circuits.
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