---
title: "BJT meaning and npn active mode"
subject: "Basic Electrical and Electronics Engineering"
chapter: 3
key: "semester_1/basic_electrical_and_electronics_engineering:bjt-why-and-npn-operation"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/bjt-why-and-npn-operation
site: BCT Engineering
---

# BJT meaning and npn active mode

Basic Electrical and Electronics Engineering · Chapter 3: Introduction to Electronics Engineering

- Asked in: 2081 Chaitra (new course), 2+4 marks
- Video: [Starter Guide to BJT Transistors (ElectroBOOM101 - 011)](https://www.youtube.com/watch?v=2uowMENwiHQ) — ElectroBOOM
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/bjt-why-and-npn-operation
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_and_electronics_engineering/chapter-3

## Question

Why is transistor called Bipolar Junction Transistor? Explain the operation of the npn transistor in active mode.

## Short answer

It is called bipolar because both electrons and holes conduct current. In active mode, the forward-biased emitter-base junction injects electrons into the thin base; most diffuse to the reverse-biased collector-base junction and are swept into the collector, giving IC ≈ β IB.

## Answer

#### Why "bipolar"

A transistor is called a Bipolar Junction Transistor because its current is carried by **both** types of charge carriers, electrons and holes, and it has two p-n junctions (emitter-base and collector-base). In a field-effect transistor, only one carrier type conducts (unipolar).

#### npn transistor in active mode

In the active region, the emitter-base junction is **forward biased** and the collector-base junction is **reverse biased**.

```text
     E (n)        B (p)        C (n)
   +--------+  +--------+  +----------+
   | e- ---->  thin  ---->  collected |
   +--------+  +--------+  +----------+
     VEE (+ forward)     VCC (reverse)
         IE = IB + IC
```

**Operation:**
1. The heavily doped emitter injects a large number of electrons into the base, because the forward bias lowers the barrier at the emitter-base junction.
2. The base is very thin and lightly doped, so only about 1-5% of the electrons recombine with holes there. This forms the small base current $I_B$.
3. The rest (95-99%) diffuse across the base and reach the collector-base junction. The reverse bias there creates a strong field that sweeps them into the collector, giving collector current $I_C$.
4. Currents obey

$$
I_E=I_B+I_C, \qquad I_C=\alpha I_E+I_{CBO}, \qquad I_C=\beta I_B
$$

with $\alpha=0.95$-$0.99$ and $\beta=\alpha/(1-\alpha)$ (typically 50-300).

A small change in base current causes a large change in collector current, so the transistor acts as a current amplifier.

## Check yourself

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

1. Why is the transistor called a Bipolar Junction Transistor?
   A. Its current is carried by both electrons and holes
   B. It has two terminals of opposite polarity
   C. It works only with positive and negative supplies together
   D. Its current is carried by only one type of carrier
   Answer: A. Both electrons and holes take part in conduction, and it has two p-n junctions. A unipolar device such as an FET uses one carrier type. (Key point: Meaning of bipolar)

2. In the active region of an npn transistor, how are the junctions biased?
   A. Both forward biased
   B. Emitter-base reverse, collector-base forward
   C. Both reverse biased
   D. Emitter-base forward, collector-base reverse
   Answer: D. Active mode needs the emitter-base junction forward biased to inject carriers and the collector-base junction reverse biased to collect them. (Key point: Bias in active mode)

3. Why is the base current only a small part of the emitter current?
   A. The emitter is lightly doped and injects few electrons
   B. The base is thin and lightly doped, so few electrons recombine there
   C. The collector repels most electrons back to the emitter
   D. The base is thick so most electrons are lost as heat
   Answer: B. Only about 1-5% of the injected electrons recombine in the thin, lightly doped base; the rest reach the collector. (Key point: Why base current is small)

4. Which set of relations is correct for the transistor in active mode?
   A. I_E = I_B - I_C and I_C = beta I_B
   B. I_C = I_B + I_E and beta = alpha(1 - alpha)
   C. I_E = I_B + I_C and beta = alpha/(1 - alpha)
   D. I_B = I_E + I_C and beta = 1 - alpha
   Answer: C. The emitter current is the sum of base and collector currents, and beta = alpha/(1 - alpha), giving typical values of 50-300. (Key point: Current relations)

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