---
title: "BJT as switch; MOSFET versus BJT"
subject: "Basic Electronics Engineering"
chapter: 3
key: "semester_2/basic_electronics_engineering:bjt-as-switch-mosfet-vs-bjt"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/bjt-as-switch-mosfet-vs-bjt
site: BCT Engineering
---

# BJT as switch; MOSFET versus BJT

Basic Electronics Engineering · Chapter 3: Transistor

- Asked in: 2070 Magh, 4+2 marks
- Video: [Difference Between MOSFET & BJT | MOSFET Basics | How does a MOSFET work?](https://www.youtube.com/watch?v=nwmqYoBBVbQ) — Foolish Engineer
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/bjt-as-switch-mosfet-vs-bjt
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-3

## Question

Explain how BJT can be used as a switch. What are the differences between MOSFET and BJT?

## Short answer

A BJT acts as a switch by operating in cut-off (OFF, VCE ≈ VCC) when base current is zero, and in saturation (ON, VCE(sat) ≈ 0.2 V) with sufficient base drive. MOSFET is voltage-controlled, unipolar, high input impedance, and fast switching; BJT is current-controlled, bipolar, slower, and prone to thermal runaway.

## Answer

#### BJT as a switch
A transistor used as a switch operates only in two regions: **cut-off** (switch OFF) and **saturation** (switch ON), and not in the active region.

```text
              +VCC
                |
              [RC]  (load)
                |
                +----o Vout
                |C
 Vin o--[RB]--B-|  NPN
                |E
                |
               GND
```

- **OFF state (cut-off):** Vin = 0 (or below 0.7 V). The base-emitter junction is not forward biased, so IB = 0, IC ≈ 0 and VCE ≈ VCC. The transistor acts like an open switch.
- **ON state (saturation):** Vin is high (for example 5 V). IB = (Vin − 0.7)/RB is made large enough that IB > IC(sat)/β, where IC(sat) = (VCC − VCE(sat))/RC. Then VCE = VCE(sat) ≈ 0.2 V and the transistor acts like a closed switch.

The output is the inverse of the input, so the circuit also works as a NOT gate (inverter). It is used in digital circuits, LED and relay drivers, and logic families (TTL, RTL).

**Example:** VCC = 5 V, RC = 1 kΩ, β = 100. IC(sat) = 4.8 mA, so IB(min) = 48 µA. For Vin = 5 V, RB ≤ (5 − 0.7)/48 µA ≈ 90 kΩ; choose RB = 47 kΩ for firm saturation.

#### MOSFET versus BJT
| Point | MOSFET | BJT |
|---|---|---|
| Carriers | unipolar (one carrier type) | bipolar (electrons and holes) |
| Control | voltage controlled (VGS) | current controlled (IB) |
| Input impedance | very high (about 10¹² Ω) | low (kΩ) |
| Gate/base current | almost zero | not zero |
| Size and packing | small, easy to integrate | larger |
| Power consumption | very low (CMOS) | higher |
| Switching speed | fast, good for logic | slower (storage time) |
| Noise and thermal stability | low noise, no thermal runaway | thermal runaway possible |
| Static damage | gate oxide can be damaged by static | more robust |

## Check yourself

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

1. Which two regions does a BJT use as a switch?
   A. Active and saturation
   B. Cut-off and active
   C. Cut-off and saturation
   D. Active and breakdown
   Answer: C. OFF is cut-off (open switch), ON is saturation (closed switch); the active region is avoided. (Key point: Regions used as a switch)

2. What base current is needed to ensure saturation?
   A. IB < IC(sat)/β
   B. IB > IC(sat)/β
   C. IB = 0
   D. IB = IC(sat) × β
   Answer: B. IB must exceed IC(sat)/β so the transistor tries to carry more than the load allows, forcing saturation with VCE ≈ 0.2 V. (Key point: Saturation condition)

3. What is the logic function of the BJT switch circuit?
   A. Buffer
   B. NOT gate (inverter)
   C. AND gate
   D. OR gate
   Answer: B. High input saturates the BJT and pulls the output low, and low input gives output near VCC. (Key point: Switch output)

4. Which statement correctly compares MOSFET with BJT?
   A. MOSFET is current controlled with low input impedance
   B. MOSFET is voltage controlled with very high input impedance and almost zero gate current
   C. MOSFET has thermal runaway while BJT does not
   D. MOSFET is bipolar and BJT is unipolar
   Answer: B. MOSFET is controlled by VGS through an insulated gate, giving very high input impedance; the BJT is current controlled by IB. (Key point: MOSFET vs BJT)

## Related in Basic Electronics Engineering

- [BJT bipolar, MOSFET unipolar; BJT diff amp](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Abjt-bipolar-mosfet-unipolar-diff-amp) — Basic Electronics Engineering, chapter 3, asked 2×
- [BJT bipolar, MOSFET unipolar; CMOS NOT](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Abjt-bipolar-mosfet-unipolar-cmos-not) — Basic Electronics Engineering, chapter 3

## Related in other subjects

- [BJT as a switch in power circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Abjt-as-power-switch) — Power Electronics, chapter 1, asked 2×
- [Short note: π-models of BJT and MOSFET](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Api-model-bjt-mosfet-note) — Electronics Devices and Circuit, chapter 2
- [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
- [Transconductance of BJT, JFET and MOSFET](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Atransconductance-bjt-jfet-mosfet) — Electronics Devices and Circuit, chapter 3