What Are Transistors?

Transistors are three-terminal semiconductor devices that control the flow of electrical current between two terminals (collector/emitter in bipolar devices, or drain/source in field-effect devices) in response to a signal applied to a third control terminal (base or gate). This controlled amplification and switching capability makes the transistor the fundamental active element of virtually all modern electronics. Since its invention at Bell Laboratories in 1947, the transistor has displaced vacuum tubes in nearly every application, enabling the miniaturization of electronic systems from room-sized computers to single-chip processors containing tens of billions of devices.

Transistors draw on solid-state physics, semiconductor materials science, and fabrication technology. Their behavior arises from the controlled movement of charge carriers (electrons and holes) through doped semiconductor regions, governed by electric fields applied through the control terminal.

Bipolar Transistors

Bipolar junction transistors (BJTs) operate through the injection and transport of both types of carriers across narrow semiconductor junctions. An NPN transistor consists of a thin p-type base region sandwiched between n-type emitter and collector regions; a small base current modulates a much larger collector current, giving the device its current gain (beta or h_FE). The bipolar transistor's high transconductance and well-characterized noise behavior make it the preferred device in low-noise amplifiers, precision analog circuits, and high-speed logic families such as emitter-coupled logic (ECL).

Bipolar technology has evolved from discrete germanium devices of the 1950s through silicon planar devices to modern heterojunction bipolar transistors (HBTs). HBTs replace the silicon base with a narrower-bandgap alloy, reducing base resistance and increasing the maximum oscillation frequency f_max, enabling operation at millimeter-wave frequencies for 5G and radar front ends. IEEE review of bipolar transistor technology past and future directions traces this evolution and identifies further scaling opportunities in SiGe and III-V HBT processes.

CMOS Technology

Complementary metal-oxide-semiconductor (CMOS) technology uses n-channel and p-channel MOSFETs together on the same substrate. In a MOSFET, an electric field applied to the gate electrode through a thin oxide layer induces or depletes a conducting channel between source and drain, controlling current without consuming steady-state gate current. CMOS logic gates draw power only during switching transitions, giving them orders-of-magnitude lower static power than bipolar logic, which drove the near-universal adoption of CMOS for digital integrated circuits starting in the 1980s.

Transistor scaling in CMOS has followed a trajectory described by Moore's Law, with minimum feature sizes reaching below 3 nm in production. At these dimensions, maintaining electrostatic control over the channel requires three-dimensional gate architectures. The FinFET, introduced commercially around 2011, wraps the gate around a vertical silicon fin on three sides. IEEE review of FinFET to gate-all-around multibridge-channel FET architectures documents the subsequent transition to gate-all-around (GAA) devices, where the gate surrounds the channel on all four sides in stacked nanosheet or nanowire geometries, improving drive current and leakage control at the most advanced nodes.

Wide-bandgap materials extend transistor capabilities beyond the limits of silicon. Gallium nitride high-electron-mobility transistors (GaN HEMTs) exploit the high electron mobility in a two-dimensional electron gas formed at the aluminum gallium nitride (AlGaN)/GaN heterointerface. The resulting devices combine high breakdown voltage with fast switching, as documented in IEEE overview of GaN and SiC power devices, making them well suited for power conversion, RF power amplifiers, and millimeter-wave circuits.

Applications

Transistors have applications in a wide range of disciplines, including:

• Digital logic and microprocessors (billions of CMOS transistors per die)
• Analog and RF amplification, from audio preamplifiers to millimeter-wave front ends
• Power conversion in dc-dc converters, motor drives, and solar inverters
• Sensing and signal conditioning in biomedical, automotive, and industrial systems
• Memory storage in DRAM, SRAM, and Flash cells