---
title: "BJT bipolar, MOSFET unipolar; CMOS NOT"
subject: "Basic Electronics Engineering"
chapter: 3
key: "semester_2/basic_electronics_engineering:bjt-bipolar-mosfet-unipolar-cmos-not"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/bjt-bipolar-mosfet-unipolar-cmos-not
site: BCT Engineering
---

# BJT bipolar, MOSFET unipolar; CMOS NOT

Basic Electronics Engineering · Chapter 3: Transistor

- Asked in: 2068 Magh, 2+2 marks
- Video: [Transistors - Field Effect and Bipolar Transistors:  MOSFETS and BJTs](https://www.youtube.com/watch?v=Bine_PbyFSQ) — Physics Videos by Eugene Khutoryansky
- 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-bipolar-mosfet-unipolar-cmos-not
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-3

## Question

Why BJT is bipolar and MOSFET is a unipolar device? Draw the circuit diagram of NOT gate using CMOS.

## Short answer

BJT is bipolar because its current involves both electrons and holes; MOSFET is unipolar because conduction uses only one carrier type (electrons or holes). A CMOS NOT gate connects a PMOS and NMOS in series, gates tied as input, drains as output; it inverts with almost zero static power draw.

## Answer

#### Why BJT is bipolar
In a BJT, current is carried by **both electrons and holes**. In an NPN transistor, the emitter injects majority electrons into the p-type base, where they become minority carriers, and some of them recombine with holes in the base. Both majority and minority carriers take part in conduction, so the device is **bipolar**.

#### Why MOSFET is unipolar
In a MOSFET, the current flows through the induced (or built-in) channel using **only one type of carrier**: electrons in an n-channel device or holes in a p-channel device. There is no minority carrier injection and no gate current, since the gate is insulated by SiO₂. It is therefore a **unipolar** device.

#### CMOS NOT gate
```text
                VDD
                 |
               S |
          +--G-o|   PMOS
          |    D |
 A o------+      +------o Y = A'
          |    D |
          +--G--|   NMOS
               S |
                 |
                GND
```
The PMOS source is at VDD and the NMOS source is at ground. The two gates are joined to form the input A, and the two drains are joined to form the output Y.
- A = 0: PMOS is ON, NMOS is OFF, so Y = VDD (logic 1).
- A = 1: PMOS is OFF, NMOS is ON, so Y = 0 (logic 0).

So Y = A' (inversion), with almost zero static power, since one transistor is always OFF.

## Check yourself

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

1. Why is a BJT bipolar?
   A. Electrons and holes (majority and minority carriers) both take part in conduction
   B. It has two supplies
   C. It has an insulated gate
   D. It uses only holes
   Answer: A. Electrons injected into the p-base become minority carriers and recombine with holes, so both carriers matter. (Key point: Why BJT is bipolar)

2. Why does a MOSFET have no gate current?
   A. The gate is insulated from the channel by SiO2
   B. The gate is heavily doped
   C. The gate is grounded
   D. The drain blocks it
   Answer: A. The oxide insulates the gate; only one carrier type flows in the channel, so the device is unipolar. (Key point: Why MOSFET is unipolar)

3. In the CMOS inverter, what happens when A = 0?
   A. PMOS ON, NMOS OFF, Y = VDD (1)
   B. PMOS OFF, NMOS ON, Y = 0
   C. Both ON, Y undefined
   D. Both OFF, Y floats
   Answer: A. A low gate turns the PMOS on and the NMOS off, pulling Y to VDD. (Key point: CMOS NOT when A = 0)

4. Why does the CMOS NOT gate have almost zero static power?
   A. Both transistors are always on
   B. One transistor is always off, so there is no dc path from VDD to ground
   C. The supply is zero
   D. It uses BJTs
   Answer: B. For each steady input one of the two series transistors is off, blocking current from VDD to GND. (Key point: CMOS static power)

## 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×
- [CMOS inverter / NOT gate operation](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acmos-inverter-operation) — Basic Electronics Engineering, chapter 3, asked 4×
- [BJT as switch; MOSFET versus BJT](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Abjt-as-switch-mosfet-vs-bjt) — Basic Electronics Engineering, chapter 3

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