Transistors: the switch that changed everything
A transistor is a switch with no moving parts, turned on and off by a voltage. Start with a water tap, then wire a battery, a resistor and a bulb through an n-channel MOSFET and watch the gate voltage open and close the switch. Finish with its p-channel twin, why on and off mean 1 and 0, and the 16 billion transistors on one chip.
5 quick questions
Pick an answer to see why. Your score saves with your progress.
01In the water-tap analogy, which part of the transistor is the handle?
The gate is the control. A small voltage on it decides whether a much bigger current can flow from drain to source, just as a small turn of the handle controls a big flow of water.
02An n-channel MOSFET has a threshold voltage of 0.43 V. Its gate is at 1.0 V. What happens?
1.0 V is above the threshold, so the gate pulls electrons into a channel that joins source and drain. The switch is on.
03Why does almost no current flow into the gate?
The gate is a plate on a thin oxide layer (an insulator). The oxide blocks current, so the gate controls the channel with its voltage alone. That is why the gate trace in the figure takes a tint but never shows moving current dashes.
04When is a p-channel MOSFET on?
A p-channel MOSFET is the opposite of an n-channel one: it turns on with its gate low and off with its gate high. Chips pair the two kinds; that pairing is called CMOS.
05Roughly how many transistors are on Apple's M1 chip (2020)?
Apple says the M1 holds 16 billion transistors. Each one is a tiny switch like the one in the figure.
Glossary
- Transistor
- A switch with no moving parts. A small voltage on one terminal controls a bigger current through the other two.
- MOSFET
- Metal-oxide-semiconductor field-effect transistor: the kind of transistor that modern chips are built from.
- Gate
- The control terminal of a MOSFET: a plate on a thin insulator whose voltage opens or closes the channel.
- Source and drain
- The two terminals the switched current flows between. In an n-channel MOSFET, current enters at the drain and leaves at the source.
- Channel
- A thin conducting layer that forms under the gate when the gate voltage is high enough, joining source and drain.
- Threshold voltage (Vth)
- The gate voltage at which the channel forms and the transistor starts to turn on; about 0.4 V in a typical chip process.
- Ground
- The 0 V reference point in a circuit, where currents return and from which voltages are measured.
- CMOS
- Complementary MOS: circuits that pair n-channel and p-channel MOSFETs, so one is on while the other is off.
Notes and sources
- The board circuit is a teaching model: the 1.5 V cell, the bulb and the 0 V / 1.0 V gate settings show the idea and are not taken from one datasheet.
- Threshold values: NMOS 0.43 V and PMOS -0.4 V for the generic 0.25 µm CMOS process in MIT 6.374 (after Rabaey, Chandrakasan and Nikolić, Digital Integrated Circuits). The exact value depends on the process and the device.
- Below the threshold a real transistor is not perfectly off: a small leakage current still flows. This lesson treats it as zero.
- Current is drawn in the conventional direction (from + to -); the electrons in the wires and in the n-channel drift the opposite way.
- The first MOS transistors were demonstrated by John Atalla and Dawon Kahng at Bell Labs (first working device 1959, demonstrated 1960). Apple's M1 (2020) holds 16 billion.
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