Forward bias

Current increases with ǻV

Reverse bias

Current does NOT increase with |ǻV|

36.15 - Physics at work: MOSFET transistors

Transistor: A three-terminal

semiconductor device that

forms the basis of random

access memory and

microprocessors.

The physicist William Shockley proposed the first

modern-day transistors in 1952, although earlier

scientists had devised conceptual prototypes. In a

circuit, transistors can be controlled so that they work

either as strong resistors or effective conductors, which is what makes them important

in electronics applications, such as electronic on/off switches (in computers), or as part

of signal amplifiers (in your stereo).

The transistors discussed in this section are called field effect transistors, since the

conductivity (or resistance if you like) of such transistors is regulated by an electric field.

The first transistors, developed earlier, were of a type called junction transistors or

junction field effect transistors, but MOSFET transistors dominate many applications

today.

MOS stands for metal-oxide-semiconductor and FET stands for field-effect transistor.

MOSFET transistors are crucial in microelectronics, forming the basis of random access

memory (RAM) and of charge-coupled devices (CCDs). A CCD is used to record

images in digital cameras and digital video cameras.

Enough acronyms: There are many more! (Other acronyms were suggested for

MOSFET, including MISFET...) Let’s move to the design of an n channel MOSFET

transistor. The basic design is shown in Concept 1.

Silicon is lightly doped to form a p-type semiconductor, which is the bottom layer you

see in the diagrams on the right. This layer, the substrate, is deliberately very lightly

doped so that it will be a poor conductor, but the holes it contains are necessary for a

reason you will soon learn. Then two much more heavily doped islands of n-type

semiconductor, which are shown in green in the diagram, are formed on top of the

substrate. One island is the source S and the other is the drain D.

The first transistor, built at Bell Labs in 1947. The semiconductor substrate is

a germanium crystal. The three electrical leads are the source, gate and drain.

Structure of a MOSFET transistor

Substrate is lightly doped p-type

semiconductor

Two islands of n-type material sit atop

substrate

n channel connects islands

Insulator covers substrate, islands

·One island is source S, one is drain D

(^676) Copyright 2007 Kinetic Books Co. Chapter 36