How Transistors Work
How transistors switch and amplify electrical signals using semiconductors — MOSFET operation, logic gates, and why billions of them fit on one chip.
A transistor is a semiconductor switch that uses a small electrical signal to control a larger current flowing through it. By applying voltage to its gate or base terminal, it turns current on or off billions of times per second, forming the basis of all digital computing and modern electronics.
The MOSFET: a voltage-controlled switch
The dominant transistor today is the MOSFET (metal-oxide-semiconductor field-effect transistor), built on silicon with three terminals: source, drain, and gate. The gate is insulated from the channel between source and drain by a thin oxide layer, so it draws almost no current — it controls by electric field alone. In an n-channel MOSFET, applying a positive voltage to the gate attracts electrons into the channel region, forming a conductive path that lets current flow from drain to source; removing the gate voltage closes the channel and stops the current. The device is therefore a switch with no moving parts, toggling between on and off states purely by voltage. Because the gate is capacitive, switching costs energy only during transitions, which is why CMOS logic — complementary pairs of n-type and p-type MOSFETs — dominates digital chips: one transistor of each pair is always off, so static power draw is near zero. The current through the channel still obeys the same circuit rules as any conductor, including Ohm's law and Kirchhoff's circuit laws.
Amplification: small signals steering large currents
Before digital logic, transistors earned their place as amplifiers. A small varying signal applied to the gate produces a large varying current through the channel, reproducing the input waveform at far greater power — the principle behind audio amplifiers, radio transmitters, and sensor circuits. Bipolar junction transistors (BJTs), the older family with base, collector, and emitter terminals, amplify current directly: a small base current controls a collector current tens to hundreds of times larger. MOSFETs instead offer voltage control with negligible input current, making them ideal where efficiency matters. Either way, the magic is gain: energy from the power supply is sculpted by a weak input into a strong output, which is also how oscillators and switching power supplies work.
From one transistor to billions on a chip
A single MOSFET is a switch; wired into complementary pairs, switches become logic gates — NOT, NAND, NOR — and gates become adders, memory cells, and processors. Manufacturing prints these structures photolithographically: patterns of light expose a photosensitive coating on a silicon wafer, and successive steps of etching, doping, and metal deposition build up transistors and the wires connecting them. Feature sizes are measured in nanometers, so a modern processor holds billions of transistors switching billions of times per second. The same semiconductor physics underlies light-to-electricity devices, as explained in how solar cells work. Heat removal, power delivery, and manufacturing precision — not transistor theory — are now the binding constraints on faster chips.
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electronics logic gates semiconductors transistors