A transistor has three terminals: the base, the collector and the emitter. Its useful feature is that the small current fed into the base controls the much larger current flowing from collector to emitter.
When no current reaches the base, the transistor blocks the main current, it is off. When even a small base current flows, the transistor switches on and allows a large current through. So a tiny input decides whether a big current is on or off, with no moving parts, and it can switch very quickly.
This makes it ideal for automatic circuits. Paired with a light-dependent resistor or a thermistor, a transistor can switch a street lamp on when it gets dark, or sound an alarm when a room gets too hot, because the sensor changes the base current.
In SPM you should explain how a small base current controls the output current and how a transistor is used as a switch in sensor circuits.
Common misconceptions
- A transistor amplifies by creating extra energy -> The large current comes from the supply; the base current only controls it, energy is not created.
- The base current is larger than the collector current -> The base current is small; it controls a much larger collector current.
- A transistor switch has moving contacts like a normal switch -> It switches electronically with no moving parts, so it can operate very fast.
The physics behind it
A transistor is a current-operated switch with three terminals: base, collector and emitter. The physical quantity that matters is electric current, measured in amperes (A).
A tiny base current controls a much larger collector current, and their ratio is the current gain.
Current gain has no unit: gain = collector current ÷ base current. Suppose the base current is 0.02 mA and the gain is 100.
Then the collector current = 100 × 0.02 mA = 2 mA. A base signal of only 0.02 mA has switched a current one hundred times larger.
For a common npn transistor to turn on, the base-emitter junction must be forward biased past about 0.7 V; below this the base current is essentially zero and the transistor sits in cut-off, blocking the collector current (off). Once the base current is large enough the transistor saturates and passes the full collector current (on).
The switch has no moving contacts, so it changes state in microseconds.
See it in daily life
Look at an automatic soap or hand-sanitiser dispenser in a shopping-mall washroom in Kuala Lumpur. An infrared sensor beneath the nozzle produces only a feeble current when your hand reflects its beam back.
That current is far too small to drive the little pump motor directly.
The feeble sensor current is fed into the base of a transistor. It is enough to switch the transistor on, which then lets a much larger current from the battery flow through the pump, and the soap squirts out.
Take your hand away, the sensor current stops, the base current disappears, and the transistor switches the pump off again.
The same trick runs automatic taps, the reverse-parking beeper in a car, and a plant-pot gadget that waters when it senses dry soil. In every case a weak signal from a sensor cannot power the output device itself; the transistor lets that weak signal control a strong current, acting as a fast, silent switch with no metal contacts to spark or wear out.
How this comes up in SPM
In Paper 2 this idea is examined with the command words explain, describe and state. You may be asked to state the function of the base, collector and emitter, to describe how a transistor is connected as a switch, or to explain how the base current controls the collector current.
Within the same Electronics chapter, the transistor sits next to several neighbouring content standards. It builds on the semiconductor diode and on n-type and p-type materials, since the transistor is a sandwich of doped layers.
It connects forward to the transistor used as a current amplifier, and to sensor circuits that combine it with a light-dependent resistor or a thermistor in a potential divider. It also links to logic gates, where transistor switching produces the on and off states.
A common requirement is to relate the base current to the collector current using the current gain, and to explain, step by step, why removing the base current turns the output off. Keep your reasoning in terms of current control, not energy creation.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)