Q.19 Compare BJT and FET. (asked in MU Dec 2025; also in June 2025 KT)
Best answer
Answer: A BJT is a bipolar, current-controlled device with low input impedance; a FET is a unipolar, voltage-controlled device with very high input impedance.
| Point | BJT | FET |
|---|---|---|
| Carriers | Bipolar: both electrons and holes | Unipolar: only majority carriers |
| Control | Current controlled (\(I_B\) controls \(I_C\)) | Voltage controlled (\(V_{GS}\) controls \(I_D\)) |
| Terminals | Emitter, base, collector | Source, gate, drain |
| Input impedance | Low (kΩ) | Very high (MΩ and above) |
| Noise | More | Less |
| Thermal stability | Less stable (thermal runaway possible) | More stable |
| Gain | Higher voltage gain | Lower voltage gain |
| Size / fabrication | Larger | Smaller; preferred in ICs |
| Types | NPN, PNP | JFET (n/p-channel), MOSFET |
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