Test: Transistors

For a proper functioning of the transistor, the emitter-base region must be forward-biased and collector-base region must be reverse-biased. In semiconductor circuits, the source voltage is called the bias voltage. In order to function, bipolar transistors must have both junctions biased.

In a transistor, base bring interaction between emitter and collector. In a transistor, the base region is made very thin and lightly doped as compared to that of an emitter so that there may be less recombination of electrons and holes in this region. Due to which the base current is quite weak and the collector current is nearly equal to the emitter current.
If the base region of a transistor is doped equally to that of the emitter region, then a large electron hole recombination will take place in base, due to it, the base current would increase but collector current would decrease. Then the very purpose of a transistor would be defeated.
 

For a proper functioning of the transistor, the emitter-base region must be forward-biased and collector-base region must be reverse-biased.
In semiconductor circuits, the source voltage is called as the bias voltage. In order to function, bipolar transistors must have both junctions biased. This condition causes a current to flow through the circuit. The depletion region of the device is reduced and majority current carriers are injected towards the junction. One of the junctions of a transistor must be forward biased and other must be reverse biased when it operates.

When a transistor is used in the common emitter mode as an amplifier, the base-emitter junction acts as input junction and collector-emitter junction acts as output junction. The input junction is made forward biased by applying forward voltage and output junction is made reverse biased by applying reverse voltage. The input signal is connected in series with the voltage applied to forward bias the base-emitter junction.

There are two depletion regions in a transistor, since there are two PN junctions.

In the transistor circuit, the current flows from emitter to collector via base.
I_e​=I_b​+I_c​ where variables have their usual meanings.
Hence, emitter current is slightly more than the collector current.

As we can see from the diagram, the emitter in the npn transistor is of n-type. Therefore majority charge carriers in the emitter of an npn transistor are called electrons.