Chap. 5 Field-effect transistors (FET)

•Importance for LSI/VLSI

–Low fabrication cost

–Small size

–Low power consumption

•Applications

–Microprocessors

–Memories

–Power Devices

•Basic Properties

–Unipolar device

–Very high input impedance

–Capable of power gain

–3/4 terminal device, G, S, D, B

–Two possible device types: enhancement mode; depletion mode

–Two possible channel types: n-channel; p-channel

5-1

MOSFET Structure

Physical structure of a n-channel device:

Typically L = 0.35 to 10 m, W = 2 to 500 m, and the thickness of the oxide layer is in the range of 0.02 to 0.1 m.

Gate (G) insulated by thin layer of silicon dioxide

Source (S) and Drain (D) regions are heavily doped n+ regions in the substrate (B) (also called body)

5-2

MOSFETs

•MOS - metal oxide semiconductor structure (original devices had metal gates, now they are silicon)

•NMOS - n-channel MOSFET

•PMOS - p-channel MOSFET

•CMOS - complementary MOS, both n-channel and p-channel devices are used in conjunction with each other (most popular in IC’s)

•MESFET - metal semiconductor structure, used in high-speed GaAs devices

•JFET - junction FET, early type of FET

5-3

With VGS = 0 there are two pn junctions between drain and source. Current cannot flow in either direction because one or the other of the junctions would be reverse-biased.

However, if VGS > VT (threshold voltage), electrons are attracted to the region below the gate, and an induced, conducting n-channel forms between the drain and source.

NOTE: iS = iD and iG = 0

A p-channel enhancement-type MOSFET is similar in construction but has an n-type substrate with p+ regions for the drain and source.

5-4

Cross section of a CMOS integrated circuit. Note that the PMOS transistor is formed in a separate n-type region, known as an n well.

The two devices are isolated from each other by a thick region of oxide.

CMOS

5-5

Symbols

G

D

S

B G

D

S

B

p Channel MOSFET(enhancement-type)-simplified symbolshown below

n Channel MOSFET(enhancement-type)-simplified symbol shown below

+VDS

-+ VGS

-

drain

source

gate

source

drain

gate

iS

iDiG = 0

(Substrate is connected to source)5-6

An n-channel MOSFET with vGS and vDS applied and with

the normal directions of current flow indicated. The characteristics for p-channel devices are exactly the same except that voltage polarities and current directions are inverted. (Operates in triode and cutoff regions as a switch.)

Output characteristics (n-channel)

+VDS

-

5-7

Input characteristics (n-channel)

+VDS

-

ID = K(VGS-VT)2

5-8

Summary of MOSFET behavior

•VGS > VT (threshold voltage) for the device to be on

•VDS > VGS - VT for device to be in saturation region

•ID = K(VGS-VT)2

•Enhancement mode device, VT > 0

(we will be dealing with enhancement mode devices in Chapter 13– MOS Digital Circuits)

•Depletion mode device, VT < 0 (conducts with VGS = 0)

5-9

Comparison of BJT and FET

FET•voltage controlled

•VGS > VT

•for device to be on

•operates in saturation region (amplifier);VDS > VGS - VT

•ID = K(VGS-VT)2

BJT•current controlled

•VBE 0.7 V•for device to be on

•operates in linear region (amplifier); BE junction forward biased, CB junction reversed biased

•IC = IB

5-10

ID = K(VGS-VT)2

K = transconductance parameter

K = k' (W/L)where k’ is the process transconductance parameter

k' = n Cox, where n is the mobility of

electrons, and Cox is the capacitance of the oxide(It’s value is determined by fabrication technology.)

W/L is the aspect ratio, W is the width of the gate, L is the length of the gate.

ID W/L

MOSFET aspect ratio

5-11