GATE Electrical – Analog Electronics
Page 7: Bipolar Junction Transistor (BJT) – Introduction
A Bipolar Junction Transistor (BJT) is a semiconductor device used for signal amplification and switching applications. It consists of three semiconductor layers forming two PN junctions.
1. Structure of BJT
A BJT has three regions:
- Emitter (E) – heavily doped region that emits charge carriers
- Base (B) – very thin and lightly doped region
- Collector (C) – moderately doped region that collects carriers
There are two types of BJTs:
- NPN Transistor
- PNP Transistor
2. NPN Transistor
In an NPN transistor, the emitter and collector are made of N-type semiconductor while the base is made of P-type material.
Current Flow
- Electrons are the majority carriers
- Current flows from collector to emitter
- Emitter injects electrons into the base
3. PNP Transistor
In a PNP transistor, the emitter and collector are P-type semiconductor while the base is N-type.
Current Flow
- Holes are the majority carriers
- Current flows from emitter to collector
- Base current controls collector current
4. Transistor Current Components
Three currents exist in a BJT:
- Emitter Current (IE)
- Base Current (IB)
- Collector Current (IC)
IE = IB + IC
5. Current Gain (β)
Current gain of a transistor in common emitter configuration is:
β = IC / IB
Typical values of β range from 50 to 300.
Important GATE Points
- BJT is a current controlled device.
- Base current controls collector current.
- Emitter region is heavily doped.
- Base region is very thin.
Next Page → BJT Operating Regions
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