Transistors

Transistor
Transistor is a semiconductor device which is used to amplify the signals as well as in switching circuits. 

Transistors

Transistor Classifications

Bipolar Junction Transistor

1. Bipolar junction transistors are classified into NPN (Negative Positive Negative) and PNP (Positive Negative Positive)  transistors.

2. Bipolar junction transistors are having both hole and electron charge carriers. In P (positive) layer majority charge carriers are holes, minority charge carriers are electrons and N (negative) layer majority charge carriers are electrons, minority charge carriers are holes.

3. Transistor having three leads, these are base (B), collector (C) and emitter (E).

4. Base lead is responsible for activating the transistor, collector is the positive lead and emitter is the negative lead, which is heavily doped with impurities.

Bipolar NPN Transistor

5. Transistor having two junctions and two diodes, which are connected back to back. 

6. Whenever supply is given to the transistor, one junction acts as forward bias and another acts as reverse bias. Due to reverse bias junction to produce a depletion layer, which is does not allow the electrons emitter to collector.

7. Here junction J1 acts as a forward bias, and junction J2 acts as a reverse bias.

8. Positive terminal of the battery is connected to the 'P' layer acts as  a forward bias and negative terminal of the battery connected to the 'P' layer acts as reverse bias.

Depletion Layer Produced Due to Reverse Biasing in J2 Junction

9. Whenever Veb voltage(0.7 volts) breaks the depletion layer, large number of  electrons (95 to 98%) move from emitter to collector and  very less number of electrons(2 to 5%) move from base to collector.

Veb Voltage Applied For Breaks the Depletion Layer

Electron Movement Occurs, Whenever Breaks the Depletion Layer

Electron Movement

Transistor Switching Operations

Switching operation is the one of the main application of the transistor.

Switch On Operation 

Both junctions are forward bias it acts like a on switch. Base current(Ib) is very less because of base is lightly doped layer.

Switch On Condition

Switch Off Pperation
Both junctions are reverse bias, then produce large depletion layer and electrons movement is zero.

Switch Off Condition