What Are Transistors?

Transistors are three-terminal semiconductor devices that control the flow of electrical current or voltage between two terminals using a signal applied to a third. They serve as the elementary building blocks of all modern electronic circuits, providing the amplification and switching functions on which analog and digital systems depend. Invented at Bell Laboratories in 1947, the transistor replaced the vacuum tube in nearly all applications and enabled the subsequent integration of billions of devices onto a single silicon chip through complementary metal-oxide-semiconductor (CMOS) technology. Understanding transistor physics and design requires knowledge of semiconductor band theory, carrier transport, and device fabrication.

Bipolar Transistors

Bipolar junction transistors (BJTs) consist of three alternating semiconductor regions: emitter, base, and collector, configured as either npn or pnp junctions. Current flow involves both electrons and holes (hence "bipolar"). A small base current controls a much larger collector-emitter current, providing current gain. BJTs offer high transconductance and fast switching in discrete form, making them the preferred choice for RF power amplifiers and precision analog circuits where their predictable exponential current-voltage relationship is exploited for logarithmic and bandgap reference functions. Heterojunction bipolar transistors (HBTs) replace the base-emitter homojunction with a wider-bandgap emitter material, improving injection efficiency and allowing base doping levels that reduce base resistance while maintaining high current gain. HBTs in indium phosphide and gallium arsenide material systems achieve transition frequencies above 300 GHz, making them the primary technology for millimeter-wave power amplifiers in 5G base stations and imaging systems. IEEE Transactions on Electron Devices publishes foundational device physics and measurement results for both silicon and compound semiconductor bipolar devices.

Field Effect Transistors and MOSFETs

Field effect transistors (FETs) control current between source and drain through an electric field applied by a gate electrode, with negligible gate current in steady state. The metal-oxide-semiconductor FET (MOSFET) uses a thin gate oxide to capacitively couple the gate voltage to the semiconductor surface, modulating the conductance of a channel between source and drain. CMOS circuits pair n-type and p-type MOSFETs so that only one device conducts in each logic state, minimizing static power dissipation. This property allowed transistor densities to scale from thousands to billions of devices per chip over five decades, following the trajectory described by Moore's Law.

Scaling has now reached gate lengths of a few nanometers, where quantum mechanical tunneling through the gate oxide and carrier scattering at atomically thin channel regions limit further performance gains. NIST's semiconductor metrology program develops measurement techniques for these nanoscale device dimensions, supporting process development at the technology frontier.

Power Transistors

Power transistors handle high voltages and currents in applications including motor drives, power supplies, and inverters. Insulated gate bipolar transistors (IGBTs) combine the high input impedance of a MOSFET gate with the low on-state voltage drop of a bipolar collector, making them the dominant device in industrial motor drives and grid-connected inverters for voltages from a few hundred volts to several kilovolts.

Wide-Bandgap Transistors

Wide-bandgap semiconductor transistors based on silicon carbide (SiC) and gallium nitride (GaN) exploit the higher electric breakdown fields of these materials to achieve blocking voltages and on-resistance combinations that silicon cannot reach. SiC MOSFETs are displacing silicon IGBTs in electric vehicle traction inverters and renewable energy converters, reducing switching losses and enabling higher operating temperatures. GaN high-electron-mobility transistors (HEMTs) operate at frequencies into the tens of gigahertz with high power density, finding application in 5G massive MIMO base station amplifiers and satellite communications. Research from Sandia National Laboratories and published through the Journal of Applied Physics documents GaN and SiC device reliability, defect physics, and high-temperature performance.

Applications

• CMOS digital logic and memory in microprocessors and system-on-chip devices
• GaN HEMT amplifiers in 5G base station radio units and phased array radar systems
• SiC MOSFETs in electric vehicle inverters for reduced switching loss and higher efficiency
• HBT power amplifiers in smartphone RF front ends for cellular and WiFi transmission
• IGBT modules in industrial variable-speed motor drives and grid-tied inverters
• Power MOSFET switches in dc-dc converters for data center server power supplies