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.
When I_CBO is increased the collector current is also increased therefore for a given amount of illumination on a very small area of the photo transistor provides much largest output current. Therefore the photo transistor is a light detector which combines a photo diode and a transistor amplifier

Construction and working: 

The construction of a photo transistor is just like a conventional NPN transistor with a little hole made on the surface mean to collector base junction. A small lens is fixed on this hole for allowing a focused light beam to concentrate on the collector base junction. In the modern methods of fabrication highly light effective materials are used instead of making a hole and fixing of lens on it.

From the above fig it is clear that the emitter bas junction J_E is forward based, whereas the collector base junction J_C is reverse based. When the transistor is kept in darkness there will be very few minority change carriers (thermally generated) which will cause the flow of reverse saturation collector current. This current for obvious reasons will be negligible small. On light being found at the collector base junction additional photo generated minority charge carriers will be available which will add to the reverse saturation current. Thus as soon as the light source is applied the transistor starts conducting and amplified current starts flowing though the reverse biased junction.

Characteristics of Photo Transistor

The above fig show the characteristics of photo transistor shown for I_C verses V_CE as a function of illumination H. the current in a photo transistor is dependent mainly on the intensity of light entering the lens and is less affected by the voltage applied to the external circuit.