bipolar junction transistor
**Bipolar Junction Transistor (BJT)** is a **three-terminal current-controlled semiconductor device consisting of two p-n junctions** — historically the dominant switching device before CMOS, now primarily used in analog, RF, and BiCMOS applications requiring high current drive and speed.
**BJT Structure and Operation**
- Three regions: Emitter (E), Base (B), Collector (C).
- **NPN**: Thin p-type base sandwiched between n-type emitter and collector.
- **PNP**: Thin n-type base between p-type emitter and collector.
- Current control: Small base current $I_B$ controls large collector current $I_C$.
- $I_C = \beta \cdot I_B$ where $\beta$ (current gain) = 50–500 for silicon BJTs.
**Operating Regions**
- **Active**: $V_{BE}$ forward biased, $V_{BC}$ reverse biased. Amplification region.
- **Saturation**: Both junctions forward biased. Both junctions conducting → used for digital "on".
- **Cutoff**: Both junctions reverse biased. Device off.
**BJT vs. MOSFET**
| Parameter | BJT | MOSFET |
|-----------|-----|--------|
| Control | Current ($I_B$) | Voltage ($V_{GS}$) |
| Input impedance | Low (~kΩ) | Very high (~TΩ) |
| Speed (fT) | Higher | Lower (but closing gap) |
| Noise | Lower 1/f | Higher 1/f |
| Power consumption | Higher | Lower |
**HBT (Heterojunction Bipolar Transistor)**
- Emitter uses wider bandgap material (SiGe, InGaP, GaN).
- Suppresses reverse injection: Higher $\beta$, lower noise, higher fT.
- SiGe HBT: fT > 300 GHz — used in 5G PA, automotive radar.
- InP HBT: fT > 700 GHz — used in extreme millimeter-wave circuits.
**BiCMOS Process**
- Combines CMOS logic with SiGe:C HBT on same chip.
- Used in: RF transceivers, D/A converters, precision analog, automotive radar SoCs.
BJTs and HBTs remain **indispensable in high-speed, high-frequency, and precision analog applications** — where MOSFET limitations in noise, gain, and frequency response make bipolar transistors the only viable choice.