Bipolar Junction Transistors
A bipolar junction transistor uses two closely spaced PN junctions. It is called “bipolar” because both electrons and holes participate in its operation. The two basic forms are NPN and PNP.
Why a BJT is not two separate diodes
The base is extremely thin compared with ordinary separate diode structures. When the base-emitter junction is forward biased, carriers are injected into the base. Only a small fraction recombine there; most cross the base and reach the reverse-biased base-collector junction, whose electric field sweeps them into the collector.
This shared carrier transport is the transistor action that two independent diodes do not reproduce.
NPN operation
For ordinary forward-active operation:
- The base-emitter junction is forward biased.
- The base-collector junction is reverse biased.
- Electrons are injected from the N-type emitter into the P-type base.
- Most of those electrons cross the thin base and are collected.
A relatively small base current is associated with a much larger collector current.
PNP operation
The polarities and main carrier type are reversed. A PNP transistor conducts with its emitter at the more positive potential in the common low-voltage convention, and holes are the dominant injected carriers.
Three common operating regions
| Region | Junction condition | Typical use |
|---|---|---|
| Cutoff | Base-emitter junction not sufficiently forward biased. | Switch “off.” |
| Forward active | Base-emitter forward biased; collector-base reverse biased. | Linear amplification. |
| Saturation | Both junctions forward biased. | Switch “on” in many low-speed circuits. |
Common-emitter amplifier
The common-emitter circuit uses the emitter as a reference node, applies the input to the base, and takes output from the collector. It provides voltage gain and inverts the signal: increasing base drive generally lowers collector voltage when a collector resistor is used.
Emitter follower
A common-collector or emitter-follower stage takes output from the emitter. Its voltage gain is near one, but it provides current gain and lower output impedance. It is useful as a buffer.
Common-base stage
The common-base circuit has low input impedance and good high-frequency behavior. It appears in RF, cascode, and specialized analog circuits.
Bias stability
BJT current changes strongly with temperature and base-emitter voltage. Practical amplifier circuits use emitter resistors, feedback, current sources, or other bias methods to make operating current less dependent on transistor gain and temperature.
Thermal runaway
As a silicon BJT warms, the base-emitter voltage needed for a given current falls. Poorly controlled circuits can therefore draw more current as they heat, creating still more heat. Emitter degeneration and proper thermal design help stabilize the operating point.