Fet Mosfet

8/12/2019 Fet Mosfet

1/24


2/24

!here are two main types of field effect transistor, the Junction Field Effect Transistoror JFET

and the Insulated-gate Field Effect Transistoror IGFET), which is more commonly known as the

standard Metal Oxide Semiconductor Field Effect Transistoror MOSFETfor short$

The Junction Field Effect Transistor

0e saw previously that a bipolar +unction transistor is constructed using two 12+unctions in themain current carrying path between the Emitter and the Collector terminals$ !he Junction Field

Effect TransistorU%-E! or -E!# has no 12+unctions but instead has a narrow piece of high2

resistivity semiconductor material forming a "Channel" of either 2type or 12type silicon for the

ma+ority carriers to flow through with two ohmic electrical connections at either end commonly

called the 4rain and the /ource respectively$

!here are two basic configurations of +unction field effect transistor, the 2channel -E! and the 12

channel -E!$ !he 2channel -E!3s channel is doped with donor impurities meaning that the flow

of current through the channel is negative hence the term 2channel# in the form of electrons$

)ikewise, the 12channel -E!3s channel is doped with acceptor impurities meaning that the flow of

current through the channel is positive hence the term 12channel# in the form of holes$ 2channel-E!3s have a greater channel conductivity lower resistance# than their e*uivalent 12channel types,

since electrons have a higher mobility through a conductor compared to holes$ !his makes the 2

channel -E!3s a more efficient conductor compared to their 12channel counterparts$

0e have said previously that there are two ohmic electrical connections at either end of the channel

called the 4rain and the /ource$ But within this channel there is a third electrical connection which

is called the %ate terminal and this can also be a 12type or 2type material forming a 12+unction

with the main channel$ !he relationship between the connections of a +unction field effect transistor

and a bipolar +unction transistor are compared below$

omparison of onnections !et"een a JFET and a #JT

Bipolar !ransistor -ield Effect !ransistor

Emitter 2 E# 88 /ource 2 /#

Base 2 B# 88 %ate 2 %#

Collector 2 C# 88 4rain 2 4#

!he symbols and basic construction for both configurations of -E!s are shown below$

9 -et7.(/-E! !utorial


3/24

!he semiconductor "channel" of the Junction Field Effect Transistoris a resistive path through

which a voltage '4/

causes a current I4

to flow$ !he -E! can conduct current e*ually well in

either direction$ & voltage gradient is thus formed down the length of the channel with this voltage

becoming less positive as we go from the 4rain terminal to the /ource terminal$ !he 12+unction

therefore has a high reverse bias at the 4rain terminal and a lower reverse bias at the /ource

terminal$ !his bias causes a "depletion layer" to be formed within the channel and whose width

increases with the bias$

!he magnitude of the current flowing through the channel between the 4rain and the /ourceterminals is controlled by a voltage applied to the %ate terminal, which is a reverse2biased$ In an 2

channel -E! this %ate voltage is negative while for a 12channel -E! the %ate voltage is positive$

!he main difference between the -E! and a B! device is that when the -E! +unction is reverse2

biased the %ate current is practically :ero, whereas the Base current of the B! is always some

value greater than :ero$

#ias arrangement for an $-channel JFET and corresponding circuit sym!ols%

; -et7.(/-E! !utorial


4/24

!he cross sectional diagram above shows an 2type semiconductor channel with a 12type region

called the %ate diffused into the 2type channel forming a reverse biased 12+unction and it is this

+unction which forms the depletion regionaround the %ate area when no e


5/24

JFET Model

!he result is that the -E! acts more like a voltage controlled resistor which has :ero resistancewhen '

%/= > and ma


6/24

Output characteristic '-I cur(es of a typical unction FET%

!he voltage '%/

applied to the %ate controls the current flowing between the 4rain and the /ource

terminals$ '%/

refers to the voltage applied between the %ate and the /ource while '4/

refers to

the voltage applied between the 4rain and the /ource$ Because a Junction Field Effect Transistor

is a voltage controlled device, "( current flows into the gate"then the /ource current I/ #

flowing out of the device e*uals the 4rain current flowing into it and therefore I4

= I/#$

!he characteristics curves e


7/24

Breakdown Region 2 !he voltage between the 4rain and the /ource, '4/

# is high

enough to causes the -E!3s resistive channel to break down and pass uncontrolled

ma


8/24

#iasing of JFET *mplifier

!his common source C/# amplifier circuit is biased in class "&" mode by the voltage divider

network formed by resistors R6 and R9$ !he voltage across the /ource resistor R/is generally set to

be about one *uarter of '44

, '44

7? #$ !he re*uired %ate voltage can then be calculated using this

R/value$ /ince the %ate current is :ero, I

%= > # we can set the re*uired 4C *uiescent voltage by

the proper selection of resistors R6 and R9$

!he control of the 4rain current by a negative %ate potential makes the Junction Field Effect

Transistoruseful as a switch and it is essential that the %ate voltage is never positive for an 2channel -E! as the channel current will flow to the %ate and not the 4rain resulting in damage to

the -E!$ !he principals of operation for a 12channel -E! are the same as for the 2channel -E!,

e


9/24

gate" and +ust like the -E!, the MOSFETalso acts like a voltage controlled resistor were the

current flowing through the main channel between the 4rain and /ource is proportional to the input

voltage$ &lso like the -E!, this very high input resistance can easily accumulate large amounts of

static charge resulting in the MOSFET becoming easily damaged unless carefully handled or

protected$

)ike the previous -E! tutorial, .(/-E!s are three terminal devices with a %ate, 4rain and

/ource and both 12channel 1.(/# and 2channel .(/# .(/-E!s are available$ !he maindifference this time is that .(/-E!s are available in two basic forms@

6$ 4epletion !ype 2 the transistor re*uires the %ate2/ource voltage, '%/

# to switch the

device "(--"$ !he depletion mode .(/-E! is e*uivalent to a "ormally Closed" switch$

9$ Enhancement !ype 2 the transistor re*uires a %ate2/ource voltage, '%/

# to switch the

device "("$ !he enhancement mode .(/-E! is e*uivalent to a "ormally (pen" switch$

!he symbols and basic construction for both configurations of .(/-E!s are shown below$

!he four .(/-E! symbols above show an additional terminal called the /ubstrate and is not

normally used as either an input or an output connection but instead it is used for grounding the

substrate$ It connects to the main semiconductive channel through a diode +unction to the body or

H -et7.(/-E! !utorial


10/24

metal tab of the .(/-E!$ Usually in discrete type .(/-E!s, this substrate lead is connected

internally to the source terminal$ 0hen this is the case, as in enhancement types it is omitted from

the symbol for clarification$

!he line between the drain and source connections represents the semiconductive channel$ If this is

a solid unbroken line then this represents a "4epletion" normally closed# type .(/-E! and if the

channel line is shown dotted or broken it is an "Enhancement" normally open# type .(/-E!$ !he

direction of the arrow indicates either a 12channel or an 2channel device$

#asic MOSFET Structure and Sym!ol

!he construction of the .etal (


11/24

drain current and decreasing the drain current as the gate voltage goes more negative$ In other

words, for an 2channel depletion mode .(/-E!@ '%/

means more electrons and more current$

0hile a 2'%/

means less electrons and less current$ !he opposite is also true for the 12channel

types$ !hen the depletion mode .(/-E! is e*uivalent to a "normally2closed" switch$

+epletion-mode $-hannel MOSFET and circuit Sym!ols

!he depletion2mode .(/-E! is constructed in a similar way to their -E! transistor counterparts

were the drain2source channel is inherently conductive with the electrons and holes already presentwithin the 2type or 12type channel$ !his doping of the channel produces a conducting path of low

resistance between the 4rain and /ource with :ero %ate bias$

Enhancement-mode MOSFET

!he more common Enhancement-mode MOSFETis the reverse of the depletion2mode type$ 5ere

the conducting channel is lightly doped or even undoped making it non2conductive$ !his results in

the device being normally "(--" when the gate bias voltage is e*ual to :ero$

& drain current will only flow when a gate voltage '%/

# is applied to the gate terminal greater



12/24

than the threshold voltage '!5

# level in which conductance takes place making it a

transconductance device$ !his positive ve gate voltage pushes away the holes within the channel

attracting electrons towards the o


13/24

The MOSFET *mplifier

ust like the previous unction -ield Effect transistor, .(/-E!s can be used to make single stage

class "&" amplifier circuits with the Enhancement mode 2channel .(/-E! common source

amplifier being the most popular circuit$ !he depletion mode .(/-E! amplifiers are very similar

to the -E! amplifiers, e


14/24

;$ /aturation Region 2 with '%/

8 'threshold

the transistor is in its constant current region

and is switched "fully2("$ !he current I4/

= ma$'

for an 2channel device and 2>$A' to 2>$G' for a 12channel device$

In the ne


15/24

enable us to switch high currents or high voltage loads, doing so becomes e


16/24

% ut-off .egion

5ere the operating conditions of the transistor are :ero input gate voltage 'I

#, :ero drain current

I4

and output voltage '4/

= '44

!herefore the .(/-E! is switched "-ully2(--"$

ut-off haracteristics

!he input and %ate are

grounded >v #

%ate2source voltage less than

threshold voltage '%/

J '!5

.(/-E! is "fully2(--" Cut2

off region #

o 4rain current flows I4

= >

#

'(U!

= '4/

= '44

= "6"

.(/-E! operates as an "open

switch"

!hen we can define the "cut2off region" or "(-- mode" of a .(/-E! switch as being, gate

voltage, '%/

J '!5

and I4

= >$ -or a 12channel Enhancement .(/-E!, the %ate potential must be

more positive with respect to the /ource$

/% Saturation .egion

In the saturation or linear region, the transistor will be biased so that the ma


17/24

'4/

= >' ideal saturation#

.in channel resistance

R4/on#

J >$6F

'(U!

= '4/

= >$9' R4/

$I4

#

.(/-E! operates as a "closed

switch"

!hen we can define the "saturation region" or "( mode" of a .(/-E! switch as gate2source

voltage, '%/

8 '!5

and I4

= .a


18/24

Example $o

)ets assume that the lamp is rated at Dv, 9?0 and is fully "(", the standard .(/-E! has a

channel "(2resistance" R4/on#

# value of >$6ohms$ Calculate the power dissipated in the

.(/-E! switch$

!he current flowing through the lamp is calculated as@

!hen the power dissipated in the .(/-E! will be given as@

Mou may think, well so what, but when using the .(/-E! as a switch to control 4C motors or

high inrush current devices the "(" channel resistance R4/on#

# is very important$ -or e


19/24

&o"er MOSFET Motor ontrol

Because of the e


20/24

&-channel MOSFET S"itch

the load and the ground$ !his also allows the gate drive or switching signal to be referenced to

ground low2side switching#$

But in some applications we re*uire the use of 12channel enhancement2mode .(/-E! were the

load is connected directly to ground$ In this instance the .(/-E! switch is connected between the

load and the positive supply rail high2side switching# as we do with 11 transistors$

In a 12channel device the conventional flow of drain current is in the negative direction so a

negative gate2source voltage is applied to switch the transistor "("$ !his is achieved because the

12channel .(/-E! is "upside down" with its source terminal tied to the positive supply ' 44$

!hen when the switch goes )(0, the .(/-E! turns "(" and when the switch goes 5I%5 the

.(/-E! turns "(--"$

!his upside down connection of a 12channel enhancement mode .(/-E! switch allows us to

connect it in series with a 2channel enhancement mode .(/-E! to produce a complementary or

C.(/ switching device as shown across a dual supply$

omplementary MOSFET Motor ontroller

!he two .(/-E!s are configured to produce a bi2directional switch from a dual supply with the

motor connected between the common drain connection and ground reference$ 0hen the input is

)(0 the 12channel .(/-E! is switched2( as its gate2source +unction is negatively biased so the

motor rotates in one direction$ (nly the positive '44

supply rail is used to drive the motor$

0hen the input is 5I%5, the 12channel device switches2(-- and the 2channel device switches2

( as its gate2source +unction is positively biased$ !he motor now rotates in the opposite direction

9> -et7.(/-E! !utorial


21/24

because the motors terminal voltage has been reversed as it is now supplied by the negative 2'44

supply rail$ !hen the 12channel .(/-E! is used to switch the positive supply to the motor for

forward direction high2side switching# while the 2channel .(/-E! is used to switch the

negative supply to the motor for reverse direction low2side switching#$

!here are a variety of configurations for driving the two .(/-E!s with many different

applications$ Both the 12channel and the 2channel devices can be driven by a single gate drive ICas shown$ 5owever, to avoid cross conduction with both .(/-E!s conducting at the same time

across the two polarities of the dual supply, fast switching devices are re*uired to provide some

time difference between them turning "(--" and the other turning "("$ (ne way to overcome this

problem is to drive both .(/-E!s gates separately$ !his then produces a third option of "/!(1" to

the motor when both .(/-E!s are "(--"$

omplementary MOSFET Motor ontrol Ta!le

.(/-E! 6 .(/-E! 9 .otor -unction

(-- (-- .otor /topped (--#

( (-- .otor Rotates -orward

(-- ( .otor Rotates Reverse

( ( (! &))(0E4

1lease note that it is important that no other combination of inputs are allowed at the same time as

this may cause the power supply to be shorted out, as both .(/-E!s, -E!6and -E!

9could be

switched "(" at the same time, fuse = bang #$

#ipolar Junction Transistor Tutorial

!he #ipolar Junction TransistorB!# is a three layer device constructed form two

semiconductor diode +unctions +oined together, one forward biased and one reverse biased$

!here are two main types of bipolar +unction transistors, the 1 and the 11 transistor$



22/24

!ransistors are "urrent Operated +e(ices" where a much smaller Base current causes a

larger Emitter to Collector current, which themselves are nearly e*ual, to flow$

!he arrow in a transistor symbol represents conventional current flow$

!he most common transistor connection is the Common2emitter configuration$

Re*uires a Biasing voltage for &C amplifier operation$

!he Base2Emitter +unction is always forward biased whereas the Collector2Base +unction is

always reverse biased$

!he standard e*uation for currents flowing in a transistor is given as@ IE= I

B I

C

!he Collector or output characteristics curves can be used to find either Ib, Ic or N to which

a load line can be constructed to determine a suitable operating point, L with variations in

base current determining the operating range$

& transistor can also be used as an electronic switch to control devices such as lamps,

motors and solenoids etc$

Inductive loads such as 4C motors, relays and solenoids re*uire a reverse biased

"-lywheel" diode placed across the load$ !his helps prevent any induced back emf3s

generated when the load is switched "(--" from damaging the transistor$

!he 1 transistor re*uires the Base to be more positive than the Emitter while the 11

type re*uires that the Emitter is more positive than the Base$

Field Effect Transistor Tutorial

Field Effect Transistors, or -E!3s are "'oltage Operated +e(ices" and can be divided

into two main types@ unction2gate devices called -E!3s and Insulated2gate devices called

I%-E!Os or more commonly known as .(/-E!s$

Insulated2gate devices can also be sub2divided into Enhancement types and 4epletion

types$ &ll forms are available in both 2channel and 12channel versions$

-E!3s have very high input resistances so very little or no current .(/-E! types# flows

into the input terminal making them ideal for use as electronic switches$

!he input impedance of the .(/-E! is even higher than that of the -E! due to the

insulating o


23/24

!o turn the 2channel -E! transistor "(--", a negative voltage must be applied to the

gate$

!o turn the 12channel -E! transistor "(--", a positive voltage must be applied to the

gate$

2channel depletion .(/-E!s are in the "(--" state when a negative voltage is applied

to the gate to create the depletion region$

12channel depletion .(/-E!s, are in the "(--" state when a positive voltage is applied to

the gate to create the depletion region$

2channel enhancement .(/-E!s are in the "(" state when a "ve" positive# voltage

is applied to the gate$

12channel enhancement .(/-E!s are in the "(" state when "2ve" negative# voltage is

applied to the gate$

The Field Effect Transistor Family-tree

Biasing of the %ate for both the +unction field effect transistor, -E!# and the metal ov 2ve ve >v

12channel >v ve >v ve 2ve >v

9; -et7.(/-E! !utorial


24/24

+ifferences !et"een a FET and a #ipolar Transistor

-ield Effect !ransistors can be used to replace normal Bipolar unction !ransistors in electronic

circuits and a simple comparison between -E!3s and transistors stating both their advantages and

their disadvantages is given below$

-ield Effect !ransistor -E!# Bipolar unction !ransistor B!#

6 )ow voltage gain 5igh voltage gain

9 5igh current gain )ow current gain

; 'ery input impedance )ow input impedance

? 5igh output impedance )ow output impedance

A )ow noise generation .edium noise generation

D -ast switching time .edium switching time

Easily damaged by static Robust

G/ome re*uire an input to turn it

"(--"Re*uires :ero input to turn it "(--"

H 'oltage controlled device Current controlled device

6> E

Fet Mosfet

Documents

Transcript of Fet Mosfet