Transistors and Memory

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.