Transistor

A transistor is an active electronic device made from a semiconductor, used to amplify signals or switch electrical current. A voltage or current applied at one terminal controls current flowing between two others. Transistors can be individually packaged components or microscopic elements within an integrated circuit. Their ability to provide controllable conduction underlies electronic signal processing and the operation of computers. The principal families are bipolar junction transistors and field-effect transistors. (toshiba.semicon-storage.com)

Physical principles

Semiconductor conductivity depends on the concentration and movement of charge carriers. These include electrons and holes, which represent missing electrons in otherwise occupied electronic states. In doping, carefully controlled impurities alter carrier concentrations: n-type material has electrons as its majority carriers, while p-type material has holes. Boundaries between these regions form p–n junctions, whose behavior depends on the applied voltage. Transistors exploit such junctions, electric fields, or both to regulate conduction. (toshiba.semicon-storage.com)

A transistor does not create the energy delivered by an amplifier. Rather, its input controls energy supplied by an external power source. Appropriate circuit connections allow a small input variation to produce a larger output variation. For switching, the circuit instead uses operating conditions corresponding approximately to low and high conduction. These functions depend on both the device and its surrounding circuit, not simply on the semiconductor material. (toshiba-semicon-storage.com)

Principal types

A bipolar junction transistor (BJT) contains emitter, base, and collector regions, arranged as either NPN or PNP. During normal forward-active operation, the emitter–base junction is forward biased and the collector–base junction is reverse biased. Carriers injected from the emitter pass through the thin base and are largely collected by the collector. Both electrons and holes participate, giving the device its “bipolar” designation. (toshiba.semicon-storage.com)

For many circuit calculations, collector current is approximated by $I_C=\beta I_B$, where $I_B$ is base current and $\beta$ is current gain. This gain varies with operating conditions and between devices. A BJT used as a switch commonly operates between cutoff and saturation, rather than remaining in the forward-active region used for amplification. (toshiba-semicon-storage.com)

A field-effect transistor (FET) uses an electric field to control a conducting channel between source and drain. Its control terminal is the gate. Junction FETs regulate the channel through a reverse-biased junction. The metal–oxide–semiconductor field-effect transistor (MOSFET) instead has a gate electrically insulated from the semiconductor. A MOSFET also has a body terminal, which may be internally connected to the source in discrete devices. (toshiba-semicon-storage.com)

MOSFETs have high input impedance: their gates require little steady-state current, although switching requires charging and discharging capacitances. Enhancement-mode MOSFETs develop strong channel conduction when gate voltage passes a characteristic threshold. The insulated-gate bipolar transistor (IGBT) combines insulated-gate control with minority-carrier injection, providing different conduction and switching trade-offs from a power MOSFET. (toshiba-semicon-storage.com)

Amplification and digital circuits

An electronic amplifier establishes a transistor’s operating point through biasing. Small signals then vary current around that point. Gain, distortion, bandwidth, and noise depend on the device characteristics and circuit configuration. Transistors can amplify continuously varying signals rather than acting solely as binary switches. (toshiba-semicon-storage.com)

Digital circuits combine transistors into logic gates, whose voltage ranges represent logical states. Complementary metal–oxide–semiconductor (CMOS) circuitry pairs n-channel and p-channel MOSFETs. Ideally, a stable logic state has no direct conducting path between supply rails, although real circuits have leakage. Switching consumes energy as circuit capacitances charge and discharge. Networks of gates support arithmetic, control, and storage functions in a microprocessor. (intel.com)

Historical development

The first working point-contact transistor was developed at Bell Labs in December 1947 by John Bardeen and Walter Brattain, working in a research group led by William Shockley. It used closely spaced contacts on germanium. Subsequent junction devices offered a different structure that became important for practical semiconductor electronics. Shockley, Bardeen, and Brattain jointly received the 1956 Nobel Prize in Physics for semiconductor research and discovery of the transistor effect. (nokia.com)

The transition from individually connected devices to multiple interconnected transistors on one chip greatly increased circuit density. MOS technology and complementary circuitry subsequently became central to digital integration. This development enabled increasingly complex processors without assembling each transistor as a separate packaged component. (timeline.intel.com)

Fabrication and scaling

Integrated transistors are fabricated through repeated patterning and material-processing steps on semiconductor wafers. Photolithography transfers patterns into light-sensitive coatings; deposition, etching, and doping create insulating, conducting, and semiconductor regions. Metal interconnections link the resulting devices into circuits. A packaged chip therefore contains patterned structures made together, rather than miniature components individually installed on a board. (community.intel.com)

Miniaturization increases the number of devices that fit within a given area, a trend associated with Moore’s law. Smaller dimensions also make controlling the channel more difficult. In a FinFET, the channel occupies a narrow semiconductor fin, with the gate controlling multiple surfaces. Such three-dimensional geometry improves electrostatic control and can reduce off-state leakage compared with a conventional planar structure. (intel.com)

Performance and applications

Important transistor parameters include maximum voltage and current, gain or transconductance, on-state resistance or voltage drop, switching speed, and permissible power dissipation. Ratings depend on specified operating and thermal conditions. Power devices also have a safe operating area defining allowable combinations of voltage, current, and operating duration. (toshiba-semicon-storage.com)

Applications extend beyond computation to signal amplification, power supplies, converters, and motor drives. Power MOSFETs are well suited to rapid switching, while IGBTs are widely used in high-voltage, high-current conversion. Their different carrier mechanisms create trade-offs between conduction loss and switching loss, so the same transistor structure is not optimal for every application. (toshiba.semicon-storage.com)