Working and Construction of Photo Transistor

Photo transistor (Photo Duo Diode)

A photo transistor is similarly to an ordinary bipolar junction transistor except that no base terminal is provided. Instead of base current the input to the transistor is provided in the form of light.

Principle  

Consider an ordinary transistor with its base terminal open circuited. The collector base leakage current I_CBO will acts as base current.

Since

           I_C = βI_B + (1 + B) I_CBO

   And  I_B = o since base is open circuited

            I_C = (1 + β) I_CBO

In photo transistor I_CBO is increased when the collector base junction is illuminated.