The Transistor: The Tiny Switch Behind the Digital World

Look at the device you are reading this on. Inside it, billions of tiny switches are flipping on and off billions of times a second, and every one of them is a transistor. There are more transistors on Earth than any other manufactured object in history. This guide explains why the transistor replaced the vacuum tube, how a handful of physicists at Bell Labs created it, how it actually works, and how it grew from one device into the engine of the information age.

The problem: vacuum tubes

In the early 20th century, electronics ran on vacuum tubes: glass bulbs with a glowing filament inside. Tubes could amplify a weak signal or act as a switch, which made radio, telephone networks and the first computers possible. They also had serious drawbacks. They were big, fragile, hot and power-hungry, and they burned out.

The ENIAC computer, completed in 1945 in the United States, used about 18,000 vacuum tubes, weighed some 27 tons and filled a large room. Operators spent much of their time hunting for tubes that had failed. Telephone companies faced the same problem across their networks. The call for a small, solid, reliable replacement was loud.

Bell Labs, 1947

At Bell Telephone Laboratories, the research arm of the American telephone company, William Shockley led a group studying semiconductors, materials that conduct electricity only under some conditions. Two members of the group, John Bardeen, a theoretical physicist, and Walter Brattain, an experimentalist, found that a small amount of current flowing into a carefully prepared crystal of germanium could control a much larger current. In December 1947 their device, the point-contact transistor, amplified an audio signal.

The name came from “transfer resistor”. Bell Labs announced it publicly in mid-1948. Shockley, who felt left out of the breakthrough, soon worked out a more robust design, the junction transistor. In 1956 all three men shared the Nobel Prize in Physics.

How a transistor works

A transistor has three connections. In a modern MOSFET they are called the source, the drain and the gate. Think of it as a tap: current flows from source to drain, and the voltage on the gate decides how much gets through.

Try it: build logic from switches

A transistor acts as a switch, and switches can be wired into logic gates. The playground below shows what two input bits do under different gates. A half adder, which adds two bits, needs only an XOR gate for the sum and an AND gate for the carry.

Logic from NAND in Python

Every logic function can be built from a single gate type, NAND, which is why chip designers love it. Here are NOT, AND, OR and XOR built from NAND alone.

def nand(a, b): return 0 if (a and b) else 1
def not_(a): return nand(a, a)
def and_(a, b): return not_(nand(a, b))
def or_(a, b): return nand(not_(a), not_(b))
def xor(a, b): return and_(or_(a, b), nand(a, b))

print(" A B | AND OR XOR")
for a in (0, 1):
    for b in (0, 1):
        print(f" {a} {b} |  {and_(a,b)}   {or_(a,b)}   {xor(a,b)}")

Output:

 A B | AND OR XOR
 0 0 |  0   0   0
 0 1 |  0   1   1
 1 0 |  0   1   1
 1 1 |  1   1   0

Billions of such gates, arranged into adders, memory cells and processors, make up a computer. The binary logic of 0 and 1 also depends on the idea of zero as a number.

Shrinking the switch

The key was the integrated circuit. Instead of wiring individual transistors by hand, manufacturers could print many of them, along with the connections, on a flat piece of silicon by a photographic process. Each improvement in printing let them fit more transistors on the same chip. In 1965 Gordon Moore observed that the number of components on a chip had been doubling regularly, and the trend he described has become known as Moore’s law.

India and the chip

India has a long record in chip design, and many Indian engineers work for global semiconductor companies. On the manufacturing side, the Semiconductor Complex Limited was set up at Mohali in Punjab in 1976 and made some of the country’s earliest chips. In 2021 the government launched the India Semiconductor Mission to attract chip fabrication and assembly plants. Since then, several projects have been approved, including a chip assembly and test plant by Micron in Sanand, Gujarat, and a planned semiconductor fabrication plant by Tata Electronics in Dholera, Gujarat. Because plans and timelines change, check the official India Semiconductor Mission site for the latest status.

Mind map: the transistor at a glance

Common myths

• Myth: The transistor was a single person’s invention. Three physicists shared the Nobel Prize, and many engineers improved it afterwards.
• Myth: Moore’s law is a law of physics. It is an observation about the industry’s progress.
• Myth: The first transistors were made of silicon. The first were germanium; silicon took over in the 1950s.
• Myth: Transistors only store data. They switch and amplify signals; memory is built from groups of them.

Frequently asked questions

Keep exploring

Sources

• Transistor, Encyclopaedia Britannica
• Transistor: innovation at Bell Labs, Encyclopaedia Britannica
• Integrated circuit, Encyclopaedia Britannica
• 1947: Invention of the point-contact transistor, Computer History Museum
• The Nobel Prize in Physics 1956, Nobel Prize Outreach
• India Semiconductor Mission, Government of India

Featured image: Image: Unidentified U.S. Army photographer, via Wikimedia Commons, Public domain. Dates and counts are rounded.