EEE1012Introduction to Electrical &

Electronics EngineeringChapter 7: Field Effect Transistor

by Muhazam Mustapha, October 2010

Learning Outcome

• Be able to explain some basic physical theory and operation of FET

• Be able to do calculation on DC and AC analysis on FET circuit

By the end of this chapter students are expected to:

Chapter Content

• Theory of FET

• FET Operation

• DC Analysis

• AC Analysis

Field Effect Transistor

Field Effect Transistor

• FET is a piece of electronic device that conducts electricity by the control of a gate

• It can be considered as a voltage controlled resistor or voltage controlled current source

Gate

• Current flows through the center body of channel from terminals called drain to source

• Gate is a plate not touching the substrate

Drain

Source

Channel

FET Types

• There are many types of FET– MOSFET – Metal Oxide Semiconductor FET– JFET – Junction FET– NMOS – n-channel MOSFET– PMOS – p-channel MOSFET

• We will cover mostly NMOS

Channel Types

• FET is also characterized by its channel

• n-channel– The majority carrier in the channel is electron

• p-channel– The majority carrier in the channel is hole

Modes

• Enhancement mode– FET is normally NOT conducting current even

when given voltage at drain and source– Gate is to increase the current

• Depletion mode– FET is normally conducting current when

given voltage at drain and source– Gate is to decrease the current

Depletion Mode

Drain

Source

Gate

• p-channel– Current flow is reduced by

putting a positive voltage at gate to repel holes flow and finally block the current

– The more positive the gate, the less current flow

+ve

−ve

p-channel

Hole Flow

Gate’s electric field repelling holes

+ve

Depletion Mode

Drain

Source

Gate

• n-channel– Current flow is reduced by

putting a negative voltage at gate to repel electrons flow and finally block the current

– The more negative the gate, the less current flow

+ve

−ve

n-channel

Electron Flow

Gate’s electric field repelling electrons

−ve

Enhancement Mode (PMOS)

Drain

Source

Gate

• p-channel

– When negative voltage is put to drain that is made of highly p dopant (p+), reverse bias junction is formed at drain – hence no current flows

– Negative voltage is put to gate to attract holes and effectively compensate the reverse biases – until current can flow

−ve

+ve

n-substrate

Gate’s electric field attracting holes

p-channel formation

p+

−ve

p+

Hole Flow

Enhancement Mode (NMOS)

Drain

Source

Gate

• n-channel

– When positive voltage is put to drain that is made of highly n dopant (n+), reverse bias junction is formed at drain – hence no current flows

– Positive voltage is put to gate to attract electrons and effectively compensate the reverse biases – until current can flow

+ve

−ve

p-substrate

Gate’s electric field attracting electrons

n-channel formation

n+

+ve

n+

Electron Flow

Circuit Symbol and Notations

n-channel

p-channel

Depletion Enhancement JFET

Operation Region

I-V Characteristic

Ohmic Region

VGS = 5.0V

VGS = 4.5V

VGS = 4.0V

VGS = 3.5V

VGS = 3.0V

VGS = 2.5V

ID, mA

Saturation Region

Cutoff Region

VDS, V0 1 2 3 4 5 6 7 8 9

15

10

5

0

Operation Region

• Cutoff– VGS < VT and VGD < VT

– No current flow

• Ohmic / Triode– VGS > VT and VGD > VT

– Linear I-V characteristic

Operation Region

• Saturation– VGS > VT and VGD < VT

– ID is controlled by VGS (Saturation Region Formula):

2)( TGSD VVKI

Conductance parameter

Threshold voltage – minimum voltage to form a conducting channel

FET In Digital Circuit

NAND, NOR and NOT Gates

DC Biasing

FET Biasing

• Biasing an FET means putting its VDS and ID into a desired position in the ID-VDS graph

• This is done normally if we want the FET to operate in saturation region

• The biasing process is a little tricky since ID is controlled by VG – not directly by VDS

Refer to Giorgio Rizzoni’s Fundamentals of Electrical Engineering: Chapter 11.3, Example 11.4

FET Biasing

• The biasing formula will also be a little different from BJT since in FET saturation region we have formula:

ID = K(VGS-VT)2

• Another difference is that IG is zero (whereas IB is not zero in BJT) since the gate is not in contact with the channel

Refer to Giorgio Rizzoni’s Fundamentals of Electrical Engineering: Chapter 11.3, Example 11.4

FET Biasing

• The position of the biased FET’s VDS and ID is called Q point

• The value of VGS is also required for the biasing

• There are a few biasing configuration exist, but for the purpose of non-EE class, we will only study the most popular configuration called self-bias common source configuration– Refer to Giorgio Rizzoni’s Fundamentals of Electrical

Engineering Figure 11.8(a)

Refer to Giorgio Rizzoni’s Fundamentals of Electrical Engineering: Chapter 11.3, Example 11.4

FET Biasing

R2

R1

VDD

RD

RS

IG

ID

VDS

+

−VGS

+

−

Refer to Giorgio Rizzoni’s Fundamentals of Electrical Engineering: Chapter 11.3, Example 11.4

RG

RD

RS

IG

ID

VDS

+

−VGS

+

−VGG

Thevenin’s EquivalentVGG = (VDD)(R2)/(R1+R2)

RG = R1 || R2

VDD

FET Biasing

• The target of biasing process is to find the value of the resistors so that Q point is position at around VDD/2 in the ID-VDS characteristic graph

• R1 and R2 will determine VGS

• VGS will determine ID

• ID = K(VGS−VT)2

• Then from KVL, VDS = VDD−ID(RD+RS)– This equation is what called load-line equation

Refer to Giorgio Rizzoni’s Fundamentals of Electrical Engineering: Chapter 11.3, Example 11.4

FET Biasing

Steps:

• R1 and R2 will be combined using Thevenin’s theorem to form RG

• Use KVL on GS loop to get VGS from RS and ID

• Use saturation region formula to get a quadratic equation on VGS or ID, then solve the other one

• Use KVL on DS loop (load-line equation) with the required VDS for the Q point to get RD

Refer to Giorgio Rizzoni’s Fundamentals of Electrical Engineering: Chapter 11.3, Example 11.4

FET Biasing

• Class discussion: Giorgio Rizzoni’s Fundamentals of Electrical Engineering: pages 500 – 501, Example 11.4

AC Analysis

AC Analysis• AC analysis is done to determine the

performance of FET amplifier circuit• There are a few parameters of interest, like

input and output resistance, but for the purpose of non-EE class, we will do only voltage gain (no current gain, why?)

• AC analysis is done after biasing is completed and assuming there is some AC signal being introduced into the circuit as superimpose on top of the DC values (biasing)

AC Analysis• The oscillation of the input and output signals

will be denoted by Δ (delta)• For this class we will consider the I-V

characteristic of the sinusoidal input and output signals will be the same as the DC relationship– next slide

AC Analysis

R2

R1

VDD

RD

RS

ΔVG

ΔVO

AC Analysis

DDDSO IRVV

Voltage Gain

G

O

V

V

2)( TGSMinDMin VVKI

2)( TGSMaxDMax VVKI

DMinDMaxD III

AC Analysis

In the formula for ΔVO, it only depends on ΔID even though from the KVL at the output it should also depends on ΔIS.

The reason for this is in real circuit we put a capacitor across RS which effectively SHORTS circuit RS when AC current flows – means we can disregard RS in AC analysis formula.

R2

R1

VDD

RD

RS

ΔVG

ΔVO

AC Analysis

• Self exercise: Based on Giorgio Rizzoni’s Fundamentals of Electrical Engineering Example 11.4, get the Vp-p of output if the Vp-p of input is 5mV