A Phototransistor is similar to an ordinary BJT, except that its collector-base junction is constructed like a photodiode. Along with a base current, the input to the transistor is in the form of illumination at the junction. For a given light source illumination level, the output of a phototransistor is defined by the area of the exposed collector-base junction and the dc current gain of the transistor. The collector-base junction of the phototransistor functions as a photodiode generating a photocurrent which is fed into the base of the transistor section. Thus, like the case for a photodiode, doubling the size of the base region doubles the amount of generated base photocurrent. This photocurrent (IP) then gets amplified by the dc current gain of the transistor. As is the case with signal transistors, hFE is not a constant but varies with base drive, bias voltage and temperature. At low light levels the gain starts out small but increases with increasing light (or base drive) until a peak is reached. As the light level is further increased the gain of the phototransistor starts to decrease. HFE will also increase with increasing values for VCE. The current -voltage characteristics of a typical transistor will demonstrate this effect.