9.2.2.1. Bipolar Junction Transistors

Table of Contents

Basic idea

A bipolar junction transistor, or BJT, is a semiconductor device used to control current. It has three terminals called the emitter, base, and collector. The word bipolar means that both electrons and holes play a role in its operation.

A BJT can act as an amplifier or as a switch. A small current entering the base can control a much larger current flowing between collector and emitter. This current control is the central idea of the device.

There are two common types of BJT, the NPN transistor and the PNP transistor. They are made from three semiconductor regions placed next to each other, with alternating doping types.

Structure of a BJT

An NPN transistor has a layer arrangement of n type, p type, n type. A PNP transistor has p type, n type, p type. The middle region is always the base, and the outer regions are the emitter and collector.

The emitter is heavily doped so it can supply many charge carriers. The base is very thin and lightly doped. The collector is designed to collect carriers that pass through the base.

The two junctions are called the emitter base junction and the collector base junction. Since a transistor contains two p n junctions, it can be viewed as two connected junctions, but its behavior is more than just two separate diodes.

Rendering scientific sketch…

NPN transistor structure

How an NPN transistor works

To understand the NPN transistor, consider its normal active operation. The emitter base junction is forward biased, and the collector base junction is reverse biased.

When the emitter base junction is forward biased, electrons are injected from the emitter into the base. Because the base is very thin, most of these electrons do not recombine there. Instead, they diffuse across the base and are swept into the collector by the electric field at the reverse biased collector base junction.

This means a small base current can control a much larger collector current. The emitter current is the total current entering the transistor through the emitter, and it splits into base current and collector current:

$$
I_E = I_B + I_C
$$

In a PNP transistor, the same general idea applies, but the current directions and voltage polarities are reversed.

Current gain

One important quantity is the current gain, often written as $\beta$ or $h_{FE}$. It relates collector current to base current:

$$
\beta = \frac{I_C}{I_B}
$$

If $\beta = 100$, then a base current of $1 \text{ mA}$ can control a collector current of about $100 \text{ mA}$ in active operation.

Another useful ratio is $\alpha$, defined by

$$
\alpha = \frac{I_C}{I_E}
$$

Since $I_E = I_B + I_C$, the values of $\alpha$ and $\beta$ are related. Usually $\alpha$ is close to 1, while $\beta$ can be tens or hundreds.

Important BJT current relations:
$$
I_E = I_B + I_C
$$
$$
\beta = \frac{I_C}{I_B}
$$
$$
\alpha = \frac{I_C}{I_E}
$$
For most BJTs in active mode, $I_C$ is much larger than $I_B$.

Operating regions

A BJT can operate in different regions depending on how its two junctions are biased.

In cutoff, both junctions are effectively not conducting in the normal transistor sense, and the transistor is off. The collector current is nearly zero.

In active region, the emitter base junction is forward biased and the collector base junction is reverse biased. This is the region used for amplification.

In saturation, both junctions are forward biased. The transistor is fully on, like a closed switch.

In reverse active region, the roles of emitter and collector are exchanged, but this mode is rarely used in basic circuits.

Region	Emitter base junction	Collector base junction	Typical use
Cutoff	Reverse biased	Reverse biased	Off switch
Active	Forward biased	Reverse biased	Amplifier
Saturation	Forward biased	Forward biased	On switch
Reverse active	Reverse biased	Forward biased	Rare

Symbol and current directions

Circuit symbols help distinguish NPN and PNP transistors. The arrow is drawn on the emitter. For an NPN transistor, the arrow points outward. For a PNP transistor, the arrow points inward.

A simple memory rule is, NPN means the emitter arrow points out, PNP means the emitter arrow points in.

Rendering scientific sketch…

BJT symbols

Voltage conditions in simple silicon BJTs

For a silicon transistor in active mode, the base emitter junction behaves roughly like a forward biased diode. For an NPN transistor, the base is typically about $0.7 \text{ V}$ above the emitter.

So a common approximation is

$$
V_{BE} \approx 0.7 \text{ V}
$$

for a conducting silicon transistor. This is only an approximate value, but it is very useful in beginner circuit analysis.

In saturation, the collector emitter voltage becomes small, often around

$$
V_{CE(\text{sat})} \approx 0.2 \text{ V}
$$

for many silicon transistors.

Common silicon BJT approximations:
$$
V_{BE} \approx 0.7 \text{ V}
$$
when the transistor is on in the usual direction, and
$$
V_{CE(\text{sat})} \approx 0.2 \text{ V}
$$
when the transistor is saturated.
These are practical approximations, not exact universal constants.

BJT as a switch

A transistor can be used as an electronic switch. In this role, the main states are cutoff and saturation.

If there is not enough base current, the transistor stays in cutoff, and little or no collector current flows. If enough base current is supplied, the transistor enters saturation, and current flows strongly from collector to emitter.

For example, in an NPN switching circuit, a small current into the base can turn on a lamp, relay, or LED circuit connected in the collector path. This allows a weak control signal to operate a larger load.

BJT as an amplifier

In the active region, a small change in base current causes a larger change in collector current. This makes amplification possible.

If the transistor is biased so that it stays in the active region, then an input signal at the base can produce a larger output signal in the collector circuit. The transistor does not create energy, but it controls the flow of energy from the power supply.

A complete study of transistor amplifier circuits involves biasing, load lines, and gain analysis, which belong to broader circuit topics. Here, the important idea is that the BJT is a current controlled device.

NPN and PNP comparison

Feature	NPN	PNP
Layer order	n p n	p n p
Main carriers from emitter	Electrons	Holes
Arrow direction	Out of emitter	Into emitter
Typical turn on condition	Base above emitter	Base below emitter

Practical picture

A good beginner picture is this. The base acts like a control terminal. The emitter supplies charge carriers. The collector gathers most of them. Because the base is thin and lightly doped, only a small fraction of carriers are lost in the base, so the base current can remain small while the collector current is much larger.

This is why transistors are so important in electronics. They let small electrical signals control larger ones, which makes switching and amplification possible in almost every electronic device.

Key ideas to remember

A bipolar junction transistor has three terminals, emitter, base, and collector.
In active operation, the emitter base junction is forward biased and the collector base junction is reverse biased.
The basic current relation is
$$
I_E = I_B + I_C
$$
The current gain is
$$
\beta = \frac{I_C}{I_B}
$$
A BJT can work as a switch or as an amplifier.

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