---
title: "Difference between BJT and FET"
subject: "Electronics Devices and Circuit"
chapter: 3
key: "semester_3/electronics_devices_and_circuit:bjt-vs-fet-difference"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/bjt-vs-fet-difference
site: BCT Engineering
---

# Difference between BJT and FET

Electronics Devices and Circuit · Chapter 3: Field-Effect Transistor

- Asked in: 2074 Ashwin, 2 marks; 2069 Chaitra, 4 marks (asked 2 times)
- Video: [What is Field Effect Transistor (FET)? || Differences between BJT and FET || Types of FET](https://www.youtube.com/watch?v=cOICDYuY-gA) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/bjt-vs-fet-difference
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-3

## Question

State the difference between BJT and FET.

## Short answer

BJT is current-controlled with low input resistance and uses both electrons and holes; FET is voltage-controlled with very high input resistance and uses majority carriers. FETs have lower noise, better thermal stability, and higher switching speed.

## Answer

| Point | BJT | FET |
|---|---|---|
| Type of device | Bipolar: both electrons and holes carry current | Unipolar: only majority carriers carry current |
| Control | Current controlled (IB controls IC) | Voltage controlled (VGS controls ID) |
| Input resistance | Low (about 1 to 10 kΩ) | Very high (10⁷ to 10¹⁴ Ω) |
| Terminals | Emitter, base, collector | Source, gate, drain |
| Noise | Higher | Lower |
| Thermal stability | Poor (thermal runaway possible) | Better (negative temperature coefficient of ID) |
| Transconductance (gain) | Higher gm | Lower gm, so lower gain at the same current |
| Size and fabrication | Larger, more complex | Smaller, simpler, used in VLSI |
| Switching speed | Slower (minority carrier storage) | Faster |
| Application | Analogue amplifiers, power circuits | Digital ICs (CMOS), high-impedance input stages, switches |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does control differ between BJT and FET?
   A. BJT is current controlled; FET is voltage controlled
   B. BJT is voltage controlled; FET is current controlled
   C. Both are current controlled
   D. Both are voltage controlled
   Answer: A. IB controls IC in a BJT; VGS controls ID in an FET. (Key point: Control)

2. Which has higher input resistance?
   A. BJT
   B. FET, because the gate junction is reverse-biased or insulated
   C. Both are equal
   D. Neither
   Answer: B. FET input resistance is 10^7 to 10^14 Ω versus a few kΩ for BJT. (Key point: Input resistance)

3. What does bipolar mean in BJT?
   A. Both electrons and holes carry current
   B. Two terminals
   C. Two supplies
   D. Two junctions only
   Answer: A. FETs are unipolar, with only majority carriers conducting. (Key point: Carriers)

4. Why is the FET more thermally stable?
   A. ID has a negative temperature coefficient
   B. IC rises with temperature
   C. It has more terminals
   D. It is larger
   Answer: A. Rising temperature reduces ID, so there is no thermal runaway. (Key point: Thermal stability)

## Related in Electronics Devices and Circuit

- [BJT bipolar vs FET unipolar; MOSFET transconductance](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Abjt-bipolar-fet-unipolar-gm) — Electronics Devices and Circuit, chapter 3