A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device made of two p-n junctions, with the terminals called emitter, base and collector. A small current at the base controls a much larger current between the collector and the emitter, so the BJT is used to amplify signals and to switch circuits.
- It is a current-controlled device, the base current (IB) controls the collector current (IC).
- It comes in two types, NPN and PNP, and works as an amplifier in the active region and as a switch in the saturation and cut-off regions.
Construction of BJT
A BJT has three doped semiconductor layers, with the base sandwiched between the emitter and the collector. The two outer layers are of one type, and the base is of the opposite type.
- Emitter (E): The heavily doped layer that supplies the charge carriers. It is n-type in an NPN transistor and p-type in a PNP transistor.
- Base (B): The thin, lightly doped middle layer. It is of the opposite type to the emitter, so it is p-type in an NPN transistor and n-type in a PNP transistor.
- Collector (C): The layer that collects the carriers, with moderate doping and the largest size. It is of the same type as the emitter.
There are two junctions in a BJT:
- Emitter-Base junction: Forward-biased during normal (active) operation.
- Collector-Base junction: Reverse-biased during normal (active) operation.
Operation of BJT
A BJT operates in three regions.
Active region
- The emitter-base junction is forward-biased and the collector-base junction is reverse-biased.
- The collector current is controlled by the base current and is almost independent of VCE.
- The BJT works as an amplifier in this region.
- IC = β · IB
Saturation region
- Both junctions are forward-biased.
- VCE is very small, a few tenths of a volt.
- The collector current no longer follows the base current. It is limited by the external circuit.
- The BJT acts as a closed (ON) switch.
Cut-off region
- Both junctions are reverse-biased.
- IB ≈ 0, so IC ≈ 0. Only a very small leakage current flows.
- The BJT acts as an open (OFF) switch.
Types of BJTs
Based on the arrangement of p-type and n-type layers, there are two types of BJT:
- NPN Bipolar Junction Transistor
- PNP Bipolar Junction Transistor
NPN Transistor
A p-type base is sandwiched between two n-type layers, which act as emitter and collector. In the active region, the emitter-base junction is forward-biased and the collector-base junction is reverse-biased. The emitter-base junction conducts only when the base-emitter voltage exceeds the barrier potential, about 0.3 V for germanium and 0.7 V for silicon.
- Electrons from the n-type emitter move into the base. This forms the emitter current (IE).
- A few electrons recombine with holes in the base. This forms the small base current (IB).
- The base is thin and lightly doped, so most electrons diffuse across to the collector. This forms the collector current (IC).
- A small reverse saturation current also flows in the collector, due to thermally generated carriers.
PNP Transistor
An n-type base is sandwiched between two p-type layers, which act as emitter and collector. The biasing is the same as in NPN, but the polarities are reversed, and the current is mainly due to the movement of holes instead of electrons.
- Holes from the p-type emitter move into the base. This forms the emitter current (IE).
- A few holes recombine with electrons in the base. This forms the small base current (IB).
- Most holes diffuse to the collector. This forms the collector current (IC).
- A small reverse saturation current also flows, due to thermally generated carriers.
Control in a BJT
A BJT is a current-controlled device. The base-emitter junction voltage decides whether the BJT is ON or OFF, and the base current controls the collector current.
- Voltage: The base-emitter voltage must exceed the barrier potential for the BJT to turn on.
- Charge carriers: Electrons carry the current in NPN transistors and holes carry it in PNP transistors. A small base current controls a much larger collector current.
- Current flow: IC = β · IB and IE = IC + IB, where β is the current gain.
Bipolar Transistor Configurations
A BJT can be connected in a circuit in three ways:
- Common Base (CB) Configuration
- Common Emitter (CE) Configuration
- Common Collector (CC) Configuration
Common Base Configuration
The base is common to both input and output. The emitter is the input and the collector is the output. The emitter-base junction is forward-biased and the collector-base junction is reverse-biased.
- Current gain: Slightly less than 1 (α = IC / IE).
- Voltage gain: High.
- Phase shift: None, the output is in phase with the input (0°).
- Impedance: Low input impedance and high output impedance.
- Uses: High-frequency circuits such as RF amplifiers, and impedance matching.
Characteristics curves
Input characteristics: The curve between emitter current (IE) and emitter-base voltage (VBE) for a fixed collector-base voltage (VCB).
Input resistance: Rin = ΔVBE / ΔIE at constant VCB.
Output characteristics: The curve between collector current (IC) and collector-base voltage (VCB) for a fixed emitter current (IE).
Output resistance: Rout = ΔVCB / ΔIC at constant IE.
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Common Emitter Configuration
The emitter is common to both input and output. The base is the input and the collector is the output. As in CB, the base-emitter junction is forward-biased and the collector-base junction is reverse-biased.
- Relations: VCE = VCB + VBE and IE = IC + IB.
- Current gain: High (β = IC / IB).
- Voltage gain: High.
- Phase shift: 180° between input and output.
- Uses: General-purpose amplifiers, audio amplifiers and voltage amplification.
Characteristics curves
Input characteristics: The curve between base current (IB) and base-emitter voltage (VBE) for a fixed collector-emitter voltage (VCE).
Output characteristics: The curve between collector current (IC) and collector-emitter voltage (VCE) for a fixed base current (IB).
Common Collector Configuration
The collector is common to both input and output. The base is the input and the emitter is the output. It is also called an emitter follower.
- Voltage gain: Slightly less than 1.
- Current gain: High (γ = IE / IB).
- Phase shift: None (0°).
- Impedance: High input impedance and low output impedance.
- Uses: Buffer and impedance matching between circuits.
Characteristics curves
Input characteristics: The curve between base current (IB) and base-collector voltage (VBC) for a fixed collector-emitter voltage (VCE).
Output characteristics: The curve between emitter current (IE) and collector-emitter voltage (VCE) for a fixed base current (IB).
Comparison of Configurations
| Feature | Common Emitter | Common Base | Common Collector |
|---|---|---|---|
| Power gain | Very high | Low | Medium |
| Voltage gain | High | High | Less than 1 |
| Current gain factor | β = IC / IB | α = IC / IE | γ = IE / IB |
| Current gain | High | Less than 1 | High |
| Phase shift | 180° | 0° | 0° |
| Input impedance | Medium | Low | High |
| Output impedance | High | Very high | Low |
Functions of BJT
- Amplification: A small base current controls a larger collector current, which boosts weak signals in devices such as audio amplifiers.
- Switching: In saturation the BJT acts as a closed switch and in cut-off as an open switch. This is the basis of logic gates and digital circuits.
- Signal modulation: BJTs are used in modulation circuits to change the amplitude, frequency or phase of a carrier signal.
Advantages
- High voltage gain: Amplifies small input signals into much larger output voltages.
- Good high-frequency response: Works well at high frequencies with a high gain-bandwidth product.
- High current handling: Carries large collector currents, so it suits power applications.
- Low forward voltage drop: Loses little voltage across it when conducting.
Disadvantages
- Lower switching speed: Switches more slowly than a MOSFET.
- More noise: Generates more electrical noise than FETs.
- Poor thermal stability: Current rises with temperature, which can lead to thermal runaway.
- Continuous base current: Needs constant base current to stay on, so it uses more control power.
Applications
- Amplifiers: Boosts weak signals in audio and RF circuits.
- Electronic switches: Turns circuits ON and OFF in digital and control circuits.
- Oscillators: Generates repeating signals in waveform and clock circuits.
- Logic circuits: Forms the basic gates in digital logic families such as TTL.