- Transistor Definition: A transistor is defined as a semiconductor device used to amplify or switch electronic signals.
- Transistor Structure: An NPN transistor consists of three regions: emitter, base, and collector, each with different doping levels.
- Biasing and Voltage Drops: The emitter-base junction is forward biased and the base-collector junction is reverse biased, with a 0.7-volt drop in silicon transistors.
- Current Flow and Control: When a positive voltage is applied to the base, it allows electrons to flow from the emitter to the collector, controlling the current.
- Transistor Working Principle: The working principle of a transistor involves controlling a large collector current with a small base current, enabling amplification and switching.
This explanation covers a silicon NPN transistor in common-emitter forward-active operation. The emitter is heavily doped to inject electrons, the base is thin and lightly doped, and the collector is more lightly doped than the emitter. The collector is often made physically larger to withstand voltage and dissipate heat. These are functional design trends, not fixed dimensions for every transistor.
An NPN transistor has an n-type emitter, p-type base and n-type collector. Bias must be defined between the three terminals; one emitter-to-collector battery does not by itself forward-bias the base-emitter junction. In forward-active operation, the base-emitter junction is forward biased and the base-collector junction is reverse biased. In cutoff, the base-emitter junction is not appreciably forward biased, so only small leakage currents remain. In saturation, both junctions are forward biased and the simple active-region gain model no longer applies.
The heavily doped emitter supplies many majority-carrier electrons. The thin, lightly doped base limits recombination. The collector structure supports the reverse-biased junction and collects carriers. Actual feature sizes vary greatly with transistor technology, voltage rating and current rating.
A conducting silicon base-emitter junction is often approximated as a 0.7 V drop for a first circuit estimate. That value is not a fixed barrier voltage. Base-emitter voltage follows an exponential current-voltage relationship and changes with collector current, temperature and device construction. The collector-base voltage is set by the external bias and operating region, not merely by subtracting 0.7 V from one source.
The base-emitter voltage therefore does not always remain at 0.7 V. Values near 0.6 to 0.8 V are common for conducting silicon BJTs at ordinary currents, but a datasheet model or measured characteristic is needed for accurate design.
Collector voltage does not need to overcome the base-emitter barrier. Cutoff occurs when the base-emitter junction lacks enough forward bias to inject more than leakage-level carrier current. A collector supply can still reverse-bias the collector-base junction, and small leakage current can flow.
Note: An ideal cutoff model sets collector current to zero, but a real transistor has leakage. If external collector current is negligible, almost the entire collector supply can appear across the transistor, depending on the circuit.
Raising the base above the emitter forward-biases the base-emitter junction. Electrons are injected from the n-type emitter into the p-type base. A small fraction recombines with holes in the base and contributes to base current. Most injected electrons diffuse across the thin base before recombining.
Electrons that reach the reverse-biased base-collector depletion region are swept into the collector by its electric field. Their motion through the neutral base is primarily diffusion, not acceleration from extra kinetic energy. Electron flow is from emitter to collector, while conventional collector current is defined from collector to emitter.
Base current includes the charge needed to replace carriers lost through recombination and other junction-current components. In forward-active operation, collector current is often approximated as β times base current, but β varies between devices and with current, temperature and collector voltage.
Kirchhoff’s current law gives emitter current as the sum of collector current and base current: IE = IC + IB under the usual current-direction convention. Most emitter-injected electrons reach the collector in a well-designed forward-active NPN transistor.
Increasing base-emitter voltage raises collector current approximately exponentially while the transistor remains forward active. Designers often use the simpler IC ≈ βIB relationship for hand calculations, but it is an operating-point approximation rather than the physical cause of collector current. An amplifier biases the transistor in an active region and uses a small input change to produce a larger output change. A switch drives it between cutoff and saturation while external components limit current.
