LAB DE MOSFET.pdf

7/26/2019 LAB DE MOSFET.pdf

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EXPERIMENT 6

O JECTIVE

haracteristics

eed Control

1. Unde . e structures and characteristics of JFETs .

2. PloUi

ristic

curves of a

JFET

.

3. Unde e structures and characteristics

of

MOSFETs.

4 . Plq

i

tl

n

ij transfer characteristic curve of a MOSFET.

5. l m p l ~ m e n t i n g

and

measuring a MOSFET motor speed control circuit .

DISCUSSION

Field-Effect Transistors FETs)

The field-effect transistor (FET) is a semiconductor device which depends for its operation on

the control of current by an electric field and conducts the current with a single carrier, hole

or electron. According

f r ~ c t u r e ,

there are two types of FETs, the Junction Field

Effect Transistor ( J f B t : I J t 1 ~ t ~ l 0 x i d e Semiconductor FET (MOSFET). The F T differs

from the B i p o l a p \ J J ~ c t i d ~ r l ~ i ~ t o r (BJT)

in

the following characteristics:

A It is a unipolardevice similar to the vacuum tube.

B. It is immune from radiation .

C. It has

an

extremely high input impedance, typically many megaohms.

D. It is less noisy than a BJT or a vacuum tube. _

E It has no offset voltage at zero drain current, and makes

an

e x c e i i ~ 9 1 1 ~ f c h o p p e r .

C : : : : : ; : : : : : ; : ~ . . : : .. .

F. It provides greater thermal stability than a BJT.

G. Its key disadvantage is the relatively small GBP of the device compared to the BJT.

The structure and operation of the FET .are different to those of the BJT. The current of the

BJT contains majority and minority

catri rs

which flow through two p-n junctions, while the

current of the FET is only t h ~ rnajority carrier flowing

in

drifting. For the FET, its channel

width can be controlled by a n ~ ~ ~ r n a l electric field so that the magnitude of channel current

26 1

4

f

t

t

.

t

t

t

t

t

t

t

4


2/24

l

~

is determined by the electric field . On the contrary, the magnitude of BJT collector current

is

controlled by the injected base current.

Either JFET or MOSFET, there are

n - c h a q n ~

~ n c ; l ( m ( i e l FETs according to the channel

type. Basically the operating principles ~ n d MOSFET are very similar; however,

the input resistance of the JFET is smaller than that of the MOSFET up to 10

14

0) due to a

pn-junction between the gate and channel. Furthermore, the MOSFET has many

advantages ir t

~ q q f ; ; ~ c t u r i n g

so that the MOSFET is more important than the JFET

in

m i c r o e l ~ c t

s t r y

In this experiment you will study the basic electrical

c h a t a e t . e r i s t i

b i r c u i t s of

JFET and

MOSFET

devices .

The FET is a semiconductor device which delivers the current with a single carrier. The

charge carrier in a p-channel FET

is

hole,. while

in

an n-channel FET is electron. Compared

to the BJT, the MOSFET which features smaller Gain-Bandwidth P r o d y ~ t G B P ) , higher input

~ m p e d a n c e , and lower manufacturing complication , is S l i l ~ manufacturing the

Large-Scale Integrated LSI) and Very-Large-Scale l n t e g r ; ; ~ t ~ ( ~ ; i } ~ ~ c c i i t s .

. .

- _ -: -

Junction

Field-Effect

Transistors

JFETs)

The JFET devices can be divided into p-channel and n-channel JFETs. Fig. 26-1 shows the

cross section of a typigpl. ~ ~ r t ' l e l JFET structure. The JFET is a three-terminal device

containing the s o u r ~ d i t ~ G), and the drain D) terminals . The circuit symbol shown

is the n - c h a n n e t

; r .

pchannel JFET, the arrow at the gate points in the opposite

direction.

e n

l l y >a JFET can be considered as a gate-voltage-controlled variable

resistor . Either end of the channel may be used as a source or drain .

0

t L

2b X},V x)

ID

loss

. -----VGs=O

VGs


3/24

The voltage across drain-source V

0

s, which results in a draip current 1

0

from drain to source.

This drain current passes through the channel

S Y f f O Q J l

c i ~

y

the p-type gate. Since the

gate-source voftage VGs will reverse bias the j ~ g ~ r l i j ~ ~ t i o n no gate current will result.

The effect of the gate-source voltage will depletion region in the channel and

therefore reduce the channel width to

i n c r e ~

t h ~ d r a i n - s o u r c e resistance resulting in less

drain current .

We first c o n s i d 4 3 r

t g ~ n c h a n n e l

JFET operation with

VGs=O

V, the drain current 1

0

increases

linearly w i t h ~ ~ ~ ~ ~ ~ ~ . ; ; s o u r c e voltage Vos . The drain current through the n-material of the

~ p t ; l u t ~ s a voltage drop along the channel , which is more positive at the

l g f ~ ~ e t i o h

t h a n at the source-gate junction. This reverse-bias potential across the

p-n junction causes a depletion region to form as shown in Fig. 26-1 . When the Vos

is

increased, the 1

0

increases, resulting a larger depletion region. As the voltage Vos

is

increased, the depletion region is fully formed across the channel. Any further increase in

V

0

s, greater than the

Vos sat)

. will result in no increase in the drain

rrtiJr itt ith'

current 1

0

then

remaining constant or saturation and designated as loss . in the VGs=O

characteristic curve of Fig. 26-1 . With the reverse-bias

~

~

~

~

:

~ : e a ~ ; e d , the depletion

region fully forms at a lower level of drain current. voltage VGs is

increased to the pinch-off value

Vp

or VGs OFF). the drain current reduces to 0, and the JFET

is

completely turned off. The pinch-off voltage

Vp

and the saturation drain-source current

loss

are two important

p a r a m ~ t ~ r ~

p

the

JFET

device and they are indicated

in

the transfer

characteristic curve ~

~ ~

i

D

lo

s

Fig. 26-2 Transfer characteristic of n channel

JFET

Vos>V

0

s

1

sat

1

)

JFET rain haracteristic

The drain characteristic is a set

of

curves for different values

of VGs

from 0 V to the pinch-off

voltage, Vp or VGs oFF the voltage at which the depletion region is formed without any drain

current and at which no drain current can occur.

26-3

4

4

4


4/24

Fig. 26-3 shows the typical n-channel JFET drain characteristic curves plotted the actual

drain current 1

at different values of d r a i n - s o u r C f t V t ~ g ~ )fos for a range of gate-source

voltage values

VGs The

drain characteristic

o r i f

6hmic region , saturation region,

avalanche region, and cutoff region.

Ohm1c reg1on or lnstauration region

10

5 .62

2.5

0 625

Con tan t

current

reg ion

or

Slltura

t

ion

region .,

1

lo

VGs=O

rnA

~

rnA

VGs=-1

r_

:

VGs=-2

rnA

rnA 1

I

4 15

0

Avalanche reg ion

I

VGs=-3

Cutoff region

Vos

Fig. 26 3 Drain characteristic curves of n-chanfielJFI:ET

/)\} ? r: .

:

:. :

For VGs=O V, the curve plotted shows that the drain u r r ~ t i f l C I

~

as Vos is increased

until a point (V

05

= 4 V) at which the current reaches saturation and loss =

10

rnA From the

previous discussion we know that the internal depletion region acts to limit the drain current.

If the gate-source voltage is set atVGs = 1 V the current increases as V

0

s is increased until

a saturation level is r e a < ; ~ f i m e at a lower level than for

VGs

=0 V , since the depletion

:

.

; c - : - . : ; : . ~ .

region, starting p a ~ r y f ~ ~ ~ ~ ~ ~ ~ to

VGs

= 1 V fully forms at a lower level of drain-source

current.

If

t h e ~

t t a g e

is

increased beyond the pinch-off value (-5 V), the drain

current

reducest

6 6: ~ d

the JFET

device is completely turned off. Since the drain current

is

a single carrier, each characteristic curve therefore passes through the origin.

JFET

Transfer haracteristic

urve

The transfer characteristic curve also called transconductance curve

p l o t

ofc:lrain

current

1

0

as a function of gate-source voltage VGs. for a constant value

of

d r a i n s o u r c e voltage Vo

5

.

The transfer curve of an n-channel JFET shown in Fig.

26-4

is

plotted from the drain

characteristic curves of Fig. 26-3 and mathematically expressed by the parabolic

approximation:

26-4


5/24

ID

Vos=15V

5.

62mA

2.

5mA

0.

625

A

~ ~ ~ + - - - + - - + - - + - - - + V s

a

lo

loss

VGs(off)

b

Fig. 26-4 Transfer curve of n-channel JFET

Metal Oxide Semiconductor FETs MOSFETs)

A field-effect

transistor

can be constructed with

. ~ ~ ~ ' . ~ i ~ ~ ~ i ~ a l

insulated from

the

channel

.

The

popular M e t a i O x i d e S e m i ~ i ~ ~ d FET (MOSFET), or

sometimes

called

the Insulated Gate Field Effect Transistor (IGFET), is constructed as

either a depletion MOSFET DMOS) or enhancement MOSFET (EMOS) . In the

depletion-mode

constru.ctioQ . a

channel

is

physically

constructed and current

between drain aqq

~

result from a voltage connected across the

drain-source

~ ~ ~ ~ ~

The enhancement MOSFET structure has no channel

formed when

~ i b e

is

constructed

.

Voltage

must be applied at the gate to

develop a channel of charge carriers so that a current results when a voltage is

applied across

the

drain-source terminals .

Since the MOSFET device has the features of low ~ o i s e and good s t a b i l i y { t i s ~ ~ u s e d

in high input impedance and high voltage amplification circuits. T@g

.v

ef Y o s u l a t i n g

; ; , : , : -

i : :

- .

..

-

layer between the gate and substrate

of

a MOSFET can easily be

d r t ~

if

n

excessive

;:;::::: ;::;:;:_.

_-

voltage

is

applied. Human body can build up extremely large electrostatic charges due to

friction. If this charge comes in contact with the terminals of a MOSFET device, an

electrostatic discharge would occur, resulting

in

a possible arc across the thin insulating layer

causing permanent damage. To avoid tf is sJamage , the MOSFET terminals are usually

shorted with a conductive ring or conductive foam in shipping and the conductive ring must

be

removed after soldering .

26-5

t

t


6/24

Fig. 26-5 shows the structure and characteristic of an n-channel MOSFET. Similar to the

JFET, it can be considered as a g a t e v o l t a g e c o o t r o J I ~ d variable resistor and it is a

three-terminal de\ltce containing the source ( S l ~

~ n d

drain

(D)

terminals. The main

difference between MOSFET and JFET is . ~ ~ ~ i g ~ ~ between the gate and p-substrate

(not a p-n junction)

of

the NMOSFET. ~

~ f ; ; th

MOSFET s input resistance is much

higher than the JFET.

D

G - 4 ~

s

lo

VGs O

: ; t : : ~ = ; ~ : : . . : : . : . . : : _ : _ _ .

Vos

8

Fig. 26-5 NMOSFET

structure, circuit symbol

and output characteristic

Fig. 26-6 shows the ~ Q ~ f y ~ ~ r a c t e r i s t i c s of depletion and enhancement NMOSFETs.

The depletion M O S

E

~ ~ ~ ~ ~ 6 a ) is shown to operate with either positive or negative

gate-source v o l t ~ s Values of VGs reducing the drain current until the pinch-off

voltage Vp ,

a f t e

f ~ t W e f i

~ 6 d r a i n

current occurs. The transfer characteristic is the same for

negative gate-source voltages, but it continues for positive values of

VGs

Since the gate

is

isolated from the channel for both negative and positive values

of

VGs

the device can be

operated with either polarity of

VGs

and no gate current resulting in either case .

Go 1

+

V S

0

~ I D

s

b)

ID

Vt

Nos?:.. Vos sat))

Fig. 26-6 NMOSFET

transfer

curves: (a) Depletion (b)

Enhancement

26-6


7/24

1 epletion

MOSFET

Fig. 26-7 shows the structure and circuit

s y l $ b ~ l

Qf

t ~ e

n: channel

depletion MOSFET.

Source and drain are made by the h i g h ~ ~ i J i h ~ c l ffi type semiconductor material and the

channel is a lower-doped n-type region.

A m ~ t ; i l ~ y e r

is deposited above then-channel on

a layer of silicon dioxide

Si0

2

) which is

an

insulating layer. This combination of a metal

gate on an oxide layer over a semiconductor substrate forms the depletion MOSFET device .

The g a t e - s o u r ~

v p ~ ~ g e

to this kind

of

MOSFET can be either positive or negative. For the

n - c h a n q e E ~ ~ ~ - ~ ~ ~ r l M

O S F E T

of Fig. 26-7, negative gate-source voltages push electrons out

of

t t l ~

b n a H ~ ~ t

f ~ ~ i o n

to deplete the channel and a large enough negative gate-source

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