A transistor is a device that amplifies and controls the flow of electricity. The word “transistor” comes from combining “transfer” and “resistor” — literally, a device that transfers resistance. Invented in the 1950s, it became a revolutionary electronic component that replaced the vacuum tube.

Today’s smartphones and PCs contain billions of transistors integrated onto a single chip. In the world of semiconductors, the transistor is the smallest unit underlying all logic and computation.

There are two main types of transistors:

・Bipolar Junction Transistor (BJT)

・Field-Effect Transistor (FET)

While each operates on a different principle, both serve the same purpose: controlling a large current using a small signal.

A BJT is built from two PN junctions combined together.

There are two types:

・NPN type (N-P-N)

・PNP type (P-N-P)

Taking the NPN type as an example, it consists of three regions:

・Emitter

・Base

・Collector

How current flows: when a small current is applied to the base, a much larger current flows from the collector to the emitter.

This makes it possible to turn a small input into a large output — in other words, amplification.

1.When a small current flows into the base, the PN junction between the base and emitter becomes forward biased.

2.Electrons are injected from the emitter into the base.

3.Because the base is extremely thin, most of these electrons pass through without recombining and continue on to the collector.

4.As a result, the collector current is amplified to roughly 100 times the base current.

This is known as the current amplification effect.

An FET is a transistor that controls current using voltage. Whereas a BJT controls current with current, an FET controls current with voltage.

The most representative type of FET is the MOSFET (Metal-Oxide-Semiconductor FET). Its components are:

・Source

・Drain

・Gate

・Channel

When voltage is applied to the gate, an electric field is generated across the oxide layer. This electric field alters the state of the channel — the pathway through which electrons travel.

・With no voltage applied, the channel remains closed and no current flows.

・With voltage applied, the channel opens, allowing electrons to flow from the source to the drain.

In other words, the gate voltage determines whether the switch is ON or OFF.

Because MOSFETs consume little power and are well suited to integration, they are the mainstream device used in today’s semiconductor integrated circuits.

1.Switch: ON/OFF control (digital circuits)

2.Amplifier: Boosting a small signal (analog circuits)

The “0”s and “1”s used in computers are represented by the ON and OFF states of these transistors.

Transistors have been shrinking year after year and have now entered the 3nm generation. Tens of billions of transistors are integrated onto a single chip, enabling high-speed computation while keeping power consumption low.

In recent years, new structures have also emerged, including:

・FinFET (fin-shaped transistor)

・GAAFET (Gate-All-Around structure)

・Nanosheet transistors

・3D stacked structures (adopted by companies such as TSMC and Samsung)

・The limits of miniaturization (increasing quantum tunneling effects)

・Issues with leakage current and heat generation

・Rapidly rising manufacturing costs

・Practical implementation of new materials (SiC, GaN, carbon nanotubes, 2D materials)

Going forward, the industry is expected to shift from purely physical miniaturization toward new materials, new structures, and AI-assisted design.

・A transistor is a semiconductor device that controls and amplifies current or voltage.

・BJTs are controlled by current; FETs are controlled by voltage.

・Transistors are the fundamental building block of all electronic devices, ICs, and CPUs.

・Going forward, design is expected to evolve from the nanoscale toward the atomic scale.

1.What is the main role of a transistor?

2.What determines the operation of a MOSFET?

3.Which structures are used in today’s mainstream designs?

 

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.