Semiconductor Electronics Materials Devices And Simple Circuits

Current gain in common emitter mode of transistor

or
Given,

For a transistor, emitter current is the sum of base current and collector current.
i.e.,

The term LED is abbreviated as Light Emitting Diode. It is forward- biased junction which emits spontaneous radiation.
Current in the circuit and voltage in the circuit
From Ohms law,

To switch on the transistor, the emitter-base junction of a transistor is forward biased while collector-base junction is reverse biased. The cut-off voltage for silicon is , so to switch on a silicon transistor a potential difference of approximately is required between the base and emitter.

To detect the intensity of light photodiode is used in reverse bias.