Semiconductor Technology from A to Z

A transistor is an electronic semiconductor device used to switch or amplify current. In a field-effect transistor (FET), a gate electrode controls the current flow between the source and drain terminals without being part of the current path itself. The most common design is the MOSFET: with no voltage applied to the gate, the p-doped substrate separates source and drain like a blocked p-n junction; the transistor is off. Only a positive gate voltage attracts electrons to the surface and forms a conducting n-channel there – the same mechanism already described under doping, here made electrically controllable. How the MOSFET is actually fabricated is covered in the chapter Construction of a n-channel FET.

The name MOSFET (metal oxide semiconductor field-effect transistor) dates from a time when the gate really was made of aluminium. Highly doped polysilicon has been used instead for decades, which strictly speaking makes IGFET (insulated gate FET) the more accurate term; with high-k metal gate technology MOSFET is correct again, provided an oxide is still used as the insulator. The channel does not form gradually but only above the threshold voltage – the most important parameter of the transistor, which can be set deliberately through the substrate doping and the choice of gate material.

The second fundamental transistor type is the bipolar transistor. It consists of two p-n junctions connected back to back, with the layer sequence n-p-n or p-n-p; its terminals are called emitter, base and collector. Unlike the MOSFET, both types of charge carriers – electrons and holes – contribute to the current flow here, hence "bipolar". A small voltage at the thin base layer opens the junction to the emitter and allows a much larger current to flow from emitter to collector. Bipolar transistors are faster than MOSFETs but require more area. Details are covered in the chapter Construction of an NPN bipolar transistor.

As feature sizes shrink, a flat, planar gate eventually can no longer reliably shut off the channel. In a FinFET, the channel is instead built as a thin, upright silicon "fin" that the gate wraps around on three sides – source, drain and the basic switching function remain identical to the MOSFET. This multi-sided access reduces leakage currents and allows the threshold voltage to be defined more sharply. How a FinFET is built and fabricated in detail is described in the chapter Construction of a FinFET.

Transistors are used not only for switching but also for storing data. The DRAM cell combines a single transistor with a capacitor for this purpose (1T1C cell): the capacitor stores one bit as an electric charge, and the transistor connects it to the bitline via the wordline when reading or writing. Because the charge slowly leaks away, it must be refreshed every few milliseconds – hence "dynamic". How the capacitor and access transistor are fabricated is shown in the chapter DRAM Fabrication: The Trench Capacitor Process.