The transistor is the active component at the core of modern electronic devices. Among the various transistor types, the Bipolar Junction Transistor (BJT) developed as an amplifying device capable of controlling a large current using a small one.

 

BJTs remain widely used today in analog applications such as audio amplifiers, power control, and signal processing. The term “bipolar” refers to the fact that these devices rely on both electrons (negative) and holes (positive) as carriers.

BJTs come in two main configurations:

・NPN type: Layers arranged in the order N–P–N.

・PNP type: Layers arranged in the order P–N–P.

Each has three terminals:

・Emitter: Supplies carriers.

・Base: Controls the current.

・Collector: Receives the current.

Current flows from the emitter to the collector, with the base acting like a faucet controlling that flow.

1.A small current is applied to the base (the base current).

2.The PN junction between the base and emitter becomes forward biased and conducts.

3.A large number of electrons are injected from the emitter into the base.

4.Most of these electrons pass through the base and reach the collector.

 

As a result, the collector current ends up roughly 100 times larger than the base current. This ability to control a large current using a small one is the basic principle behind amplification.

Depending on the applied voltage, a BJT operates in one of three regions:

・Cutoff region: No base current flows, and the transistor is OFF (no current flows).

・Active region: The base current can be controlled, and the transistor performs amplification.

・Saturation region: The base current is excessive, and the collector-emitter path conducts fully (the switch is ON).

Analog circuits primarily make use of the active region, while digital circuits primarily use the cutoff and saturation regions.

The key metric describing a BJT’s performance is its current gain (β), defined by the following formula:

>β = Collector current (Ic) ÷ Base current (Ib)

For example, if β = 100, a base current of 1mA can control a collector current of 100mA.

BJTs are also used to control signals in an ON/OFF manner:

・No current flowing into the base → cutoff (OFF)

・Current flowing into the base → saturation (ON)

This behavior is used to drive relays, control microcontroller outputs, and serve as an electronic switch in general.

A BJT’s collector current tends to increase as temperature rises. If left unchecked, this can lead to further heating and eventually to a destructive condition known as thermal runaway.

 

To prevent this, circuit designs typically include measures such as:

・Inserting an emitter resistor to apply negative feedback and stabilize the circuit.

・Using a heat sink to limit temperature rise.

・Adding a temperature compensation circuit to suppress current increases.

The NPN and PNP types operate on essentially the same principle, but with reversed voltage polarity and carrier direction.

・NPN type: Electrons serve as the primary carriers, operating relative to a positive power supply.

・PNP type: Holes serve as the primary carriers, operating relative to a negative power supply.

In modern electronic circuits, the NPN type is the dominant choice.

・Amplifier circuits for audio and radio signals

・Motor and relay drive control

・Switching power supplies and inverters

・Amplification of sensor signals

・Transistor-transistor logic (TTL) circuits

Although a classic component, the BJT remains active today in precision analog control and high-current applications.

・The BJT is a representative amplifying device that controls a large current using a small one.

・The base current determines the collector current, enabling current amplification.

・The three main operating regions are cutoff, active, and saturation.

・Useful for both switching and amplification, the BJT remains a fundamental building block of electronic circuits.

1.What does “bipolar” mean in the context of a BJT?

2.What does current gain (β) measure?

3.Name the three main operating regions of a BJT.

 

Column Supervisor: Koji Kakumoto (Otis Group Co., Ltd.)

After studying abroad and working in planning and development at a trading company, he joined Otis Group Co., Ltd. in 2011. While primarily working in the Corporate Planning Department, he has also served concurrently in manufacturing and technical divisions, and since 2018 has served as Representative Director, working to drive business growth and strengthen the organization.

 

This article is a general technical explanation intended for educational purposes and does not refer to any specific company, product, or technology.