In many cases, there is no electrical connection to the base. Except for the way base current is generated, the phototransistor behaves as a conventional BJT. This action produces a collector current that increases with Iλ. When light strikes the collector-base pn junction, a base current, Iλ, is produced that is directly proportional to the light intensity. When there is no incident light, there is only a small thermally generated collector-to-emitter leakage current, I CEO this dark cur- rent is typically in the nA range. The collector-base pn junction is exposed to incident light through a lens opening in the transistor package. In a phototransistor the base current is produced when light strikes the photosensitive semiconductor base region. The phototransistor effectively converts light energy to an electrical signal. A phototransistor is similar to a regular BJT except that the base current is produced and controlled by light instead of a voltage source.
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