Phototransistor
The phototransistor has a light sensitive collector base PN junction. It 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, this is called dark current. When light strikes the collector base PN junction a base current (Iλ) is produced that is directly proportional to the light intensity. This action produces a collector current that increases with base current (Iλ). The relationship between the collector current and the base current schematic symbol is known in figure.
![Phototransistor Symbol Phototransistor Symbol](/sites/default/files/article-images/photo-transistor-symbol.png)
Since, the actual photo generation of base current occurs in the collector, base region, the larger the physical area of this region, the more base current is generated. A photo transistor with a biasing circuit and typical collector characteristic curve is shown in figure.
![Phototransistor Biasing Phototransistor Biasing](/sites/default/files/article-images/photo-transistor-biasing.png)
![Phototransistor Biasing Graph Phototransistor Biasing Graph](/sites/default/files/article-images/photo-transistor-biasing-graph.png)
Notice that each individual curve in the graph corresponds to a certain value of light intensity and that the collector current increases with light intensity.
Phototransistors are not sensitive to all light but only to light within a certain range of wavelengths. They are most sensitive to particular wavelengths.
![Phototransistor Response Graph Phototransistor Response Graph](/sites/default/files/article-images/photo-transistor-responce-graph.png)