EXPERIMENT 4

I/V Characteristics of MOSFET

Purpose

In this experiment, some characteristics of an N-channel enhancement

type MOSFET will be studied.

Equipment

DC power supply: (0 to 20 V)

Multimeter

Breadboard

Resistors: 1kQ

MOSFET: 2N7000

Precaution

MOSFET transistors are very susceptible to breakdown due to

electrostatic discharge. It is recommended that the MOSFE

• Always ground yourself before picking up the MOSFET chip.

. Do not touch any of the pins of the chip.

• Avoid pulling the chip in and out of the breadboard repeatedly.

• Turn the de voltages to 0 V before connecting to the chip.

Introduction

MOSFET has four terminals: gate (G), drain (D), source (S), body (B)

which represents the device substrate. In many applications, S terminal is

connected to B terminal.

Operation beyond pinch-of

Due to the high impedance between G and S terminals, it is generally

considered that i = 0 and i, -ig. Under normal circuit conditions, only

positive vs are applied to n-channel MOSFET. Fig. 4-1 shows a typical

set of i-v characteristics for the n-channel MOSFET.

stics for the n-/n18

16

bomas

14-

12

10

Triede region

in = K₂

in

5

BM

Fig. 4-1

= K. (VGS - VIN - VDS VDS

2

K₁

=

Saturation (or pinch-off) region

Pinch-off point

VOS VGS-VTR

2

- (vas - VTN)²

Cutoff region

8

9

BV

When VGS SVIN with V being the threshold voltage, the MOSFET is in

the cutoff region and i, = 0. For 0

in the triode region and

TV

6V

5V

(4-1)

where K, is the conductance parameter with unit A/V². For

VDS 2VGS -VIN 20, the MOSFET operates in the saturation region or the

pinch-off region and i, is independent of vos which is given as

AV

10

3V

<2V

(4-2) MOSFET: the

Operation beyond pinch-off is the regime that we most often use for

analog amplification.

Procedure

1. Construct the circuit as in Fig. 4-2. We will use a multimeter to

perform voltage and resistance measurements./nV1 R₁

1k

G

+

VGS

ww

D

S

Fig. 4-2

İD

+

VDS

VDD

2. Start with VDD at zero volts and use the multimeter to monitor the

voltage across R, (v₁). Slowly increase VD till v, is approximately

0.1V and measure the values of VDD and vas. Calculate the value of i

from the above measurements. Fill in the record sheet at the end of

this experiment.

3. Increase VDD by approximately 0.5V each time and repeat Step 2. Get

enough (at least 5-6) measurements.

4. Plot the square root of i, versus vas. Next, with the best of your

ability, fit a straight line to the data points.

Prelab Work

1. Prove that the MOSFET in Fig. 4-2 operates in the saturation region

and thus the transfer characteristic function is in

K₁

=.

- (VGS - VIN)².

2. Derive methods to find out two key parameters of the MOSFET: the

threshold voltage VTN and the conductance parameter K, from the plot

of the square root of i, versus VGS.

Analysis and Discussion

1. Use the graph obtained to estimate two key parameters of the

MSOFET: the threshold voltage VT and the conductance parameter

TN

K₁