Electronics The Fourteenth Fifteenth Lectures

Twelfth weekهـ 1437/ 1/ 26 -29

السلمي / سمر أ

Outline for todayField Effect Transistor

Junction field-effect transistor (JFET)

I – V Characteristic of JFET

JFET as Amplifier & Switch

Derivation of drain current & equations and calculations of JFET

Difference between field-effect transistor [FET] & Bipolar junction transistor [BJT]

Metal–Oxide–Semiconductor field-effect transistor (MOSFET)

Structure, modes , work principle & circuit symbol

What happens inside MOSFET

I – V Characteristic of MOSFET

Equations and calculations of MOSFET

MOSFET as Switch

Solving the third homework

MOSFET كمفتاح عمله

الترانزستور وحسابات MOSFETمعادالت

الثالث الواجب حل

Time of Periodic Exams The Second periodic exam in / 2 / 1437 - 1110هـ , Please everyone attend in

her group

The third homework

I put the third homework in my website in the university homework Due Tuesday

28 / 1/ 1437 H in my mailbox in Faculty of Physics Department , I will not accept

any homework after that , but if you could not come to university you should sent

it to me by email in the same day

The Forth homework

I put the forth homework in my website in the university homework Due Tuesday

5 / 2/ 1437 H in my mailbox in Faculty of Physics Department , I will not accept

any homework after that , but if you could not come to university you should sent

it to me by email in the same day

Junction field-effect transistor (JFET) I – V Characteristic of JFET (n-channel)

We notice as BJT that here also we has number of regions

in characteristic curves. The important one is Saturation

Region or Active Region which begin at the point that the curve cut with pinch –

off curve. When there are no drain current moving in circuit. We notice that before

entering Saturation Region, there is proportional relation between drain current

ID and drain voltage VDS which follow ohm

relation V=IR where called Ohmic Region.

In this region, depletion region is small and

JFET works as control of channel resistivity

by change voltage. The final region is Cut-off

Region or pinch-off which JFET works as

open switch when channel resistivity has maxim value

Junction field-effect transistor (JFET) I – V Characteristic of JFET

JFET for p-channel has the same characteristic curves of n-channel. However, the

different in applied voltage VGS between gate and channel. Because we want

reverse bias, n-channel has negative voltage of VGS and p-channel has positive

voltage of VGS duo to difference of contact circuit and battery in two n- JFET &

p- JFET

لترانزستور المختلف الدائرة توصيل بسبب JFET p- عن n- JFETوذلك

n-channel p-channel

Junction field-effect transistor (JFET)

JFET as Amplifier

The small change of gate voltage VG will obviously change in current ID between

source and drain. Like this amplification occurs .

The circuit properties and amplification of JFET at source common similar to

circuit properties of BJT at emitter common. However, the benefit of using JFET

amplifier than BJT amplifier is that in JFET input resistance is higher and gate

controls with its value.

JFET as Switch

Previously, we study how gate voltage controls with drain current passing. This

what makes JFET work as switch by changing cross-section area of channel by

rising or reducing gate voltage, therefore, rising or reducing depletion region of

reverse voltage at channel.

Junction field-effect transistor (JFET)Derivation of drain current to JFET

In diode, we find width of

depletion region

In diode p+n the gate impurities is

higher, thus to obtain

At pinch off voltage

By divide two equation

Thus, half width of channel

Junction field-effect transistor (JFET)Derivation of drain current to JFET

We want to calculate drain current

we defined density & channel area

from figure

Thus, we obtain

With, simplify the equation and integration

We find the limits of integration from figure

Junction field-effect transistor (JFET)

Derivation of drain current to JFET

Final drain current represents by following relation

At saturated state

Thus, we obtain

For n- JFET drain saturated current is negative , also are negatives.

However, is positive

Junction field-effect transistor (JFET)

Equations and calculations of JFET

From Previous equations and characteristic curve, we will find another drain

current relation of JFET (without derivation)

Where is maximum value of drain saturated current when VGS = 0

The gain for JFET is

where gm is mutual transconductance. Its unit is Ampere per Volt which is

Siemens (S) = (A)/(V)

Difference between field-effect transistor [FET] & Bipolar junction transistor [BJT]

BJT FETControl method Input current (IB or IE) input voltage (VGS)

Bias type of input circuit at active

mode

forward bias in base (B) & emitter (E) junction

reverse bias in source (S) & gate (G) junction

The gain Example voltage gain Example mutual transconductanceNoise level high Very low

Dependence in terms of carriers

and type impurities

It depends on the majority and minority carriers of two types n-

type and p-type

It depends on the majority carriers of one type n-type or p-type

Name Bipolar Unipolar

Dependence on transistor work

The minority carriers injected across the forward voltage in

junction

Controlling with depletion region width in the channel by reverse bias

Current on partsCurrent moves between emitter and base and collector (3 parts)

Current moves between source and drain

(2 parts)

Input resistance Lower duo to forward bias higher duo to reverse bias Thermal stability less best

Difference between field-effect transistor [FET] & Bipolar junction transistor [BJT] =

BJT FET

Switch work (see figure)

Slower (where it works as flow of water between the plateau)

Faster (where it works as valve control of operation of water flows)

Amplification method (in high frequencies in

circuits)

carriers moves from emitter to collector across base. Thus, it takes more time (unsuitable in

high frequency circuits)

signal on gate adjust drain current to generated a signal in drain

circuit. Thus, it not takes more time (suitable in high frequency circuits)

Field Effect Transistor

When we study BJT and JFET, we notice that their structure depend on

semiconductor of two junctions pn & np contacting in addition of metal and

oxide in manufacture but not depend on them.

However, there are another transistor as MESFET & MISFET & MOSFET

depend on structure of MES contact metal and semiconductor or enter between

them insulator as MIS contact or oxide as MOS

Therefore, to study MOSFET, we should review MOS contact that we study in

second chapter

metal–oxide–semiconductor field-effect transistor (MOSFET)

Structure

(n-channel of enhancement

mode) notice

as JFET: source (S), drain (D)

& gates (G). also metal contact

with them.

source & drain contains of extrinsic

semiconductor n-type has more impurities

oxide layer (SiO2) between

metal & semiconductor

there is semiconductor

substrate of p-type

=

metal–oxide–semiconductor field-effect transistor (MOSFET)

Modes MOSFET (Structure & work principle)Duo to different of modes or bias of MOS contact

1- enhancement -mode :

Its structure explain in previous slide. The work

principle of this mode depend on inversion layer

between source & drain .

2- depletion - mode

Its structure similar to previous mode but there

cannel between source & drain of the same

semiconductor type of source & drain. This

channel has not have more impurities.

The work principle of this mode depend on

depletion layer

metal–oxide–semiconductor field-effect transistor (MOSFET)

MOSFET Modes (circuit symbol)

its circuit symbol similar to JFET

from source & drain but the

different is semiconductor substrate

also the place of the arrow: its not

in gate but in substrate. Finally,

there are symbol difference

between two modes.

At depletion mode substrate

connected as one line.

metal–oxide–semiconductor field-effect transistor (MOSFET)

What happens inside MOSFET (n-channel of enhancement mode)

battery contact with circuit similar to JFET . One contacts with circle between

source & drain VDS, the other contacts ,as voltage bias, gate with substrate &

source VGS .When positive voltage applied at gate above threshold Voltage

[VG > VT ]. Inversion layer of electrons form below gate, which called n-channel,

contacts between source & drain which have n-type and more impurities. Thus

source & drain can support with more electron to

inversion layer and electronic current moves from

source to drain but real current moves opposite

direction. Increasing bias voltage on gate makes

more carriers flow in inversion layer. Thus, current

increase between source & drain. Because of this,

it called enhancement mode

metal–oxide–semiconductor field-effect transistor (MOSFET) What happens inside MOSFET (n-channel of enhancement mode)

Since there are n-type at source, drain & inversion layer but p-type at substrate,

there will be depletion region between n-type &

p-type. While source contacts with grounded,

and voltage difference between gate & channel,

and having VG at end source & VG - VD at end

drain, in this case drain voltage will be

VD <(VG – VT) . At increasing voltage to pinch

off point or saturation state start

(VG – VT) = VD(sat.) . For more increasing to

strong saturation state VD(strg. sat.) > (VG – VT )

metal–oxide–semiconductor field-effect transistor (MOSFET)

What happens inside MOSFET (n-channel of depletion mode)

Almost similar to previous mode with some

differences. Because of conduction channel

down gate, without voltage applied meaning

electronic current not passing between

source & drain in channel. However, when

negative bias voltage applied at gate less

than threshold voltage, the contact between

source & drain cut off. As in previous mode,

at increasing voltage to pinch off point than

saturation state

metal–oxide–semiconductor field-effect transistor (MOSFET) I – V Characteristic of MOSFET (n- channel of enhancement mode)

Almost similar to JFET characteristic curves, there is Ohmic Region duo to

applying Ohm Relation V=IR . After pinch off curve or points, there is Saturation

Region and here drain saturated

current almost constant with

drain saturated voltage.

Final region is Cut off Region

metal–oxide–semiconductor field-effect transistor (MOSFET)

I – V Characteristic of MOSFET

(n-channel of enhancement mode) Details of the operation in the first two regions

Saturation Region pinch off piont Ohmic Region

metal–oxide–semiconductor field-effect transistor (MOSFET)

I – V Characteristic of MOSFET (n-channel of depletion mode)

similar to JFET characteristic curves of enhancement mode of three regions :

Ohmic Region, Saturation Region & Cut off Region.

Also, pinch off curve or points.

The different is the change

of gate voltage value (why?)

=

metal–oxide–semiconductor field-effect transistor (MOSFET)

I – V Characteristic of MOSFET

metal–oxide–semiconductor field-effect transistor (MOSFET)Equations and calculations of MOSFET

We find drain current at Saturated of MOSFET (without derivation) (n- channel of

enhancement mode)

Where is the surface mobility of electrons

(for channel) & Cox capacity of oxide layer

& z channel thickness & L channel length.

The gain of MOSFET similar to JFET

where gm is mutual transconductance. Its unit is Ampere per Volt which is

Siemens

(S) = (A)/(V)

metal–oxide–semiconductor field-effect transistor (MOSFET)MOSFET as Switch

Previously, we knew what happens inside MOSFET in two modes. From those

information, MOSFET is close switch Fully-ON (electronic current passing

between source & drain) when voltage or electronic field not applied in depletion

mode (before battery contact with circuit) oppose to enhancement mode when

voltage or electronic field applied (at battery contact with circuit)

MOSFET is open switch Fully-OFF (electronic current not passing between

source & drain) when voltage or electronic field applied in depletion mode (at

battery contact with circuit) oppose to enhancement mode when voltage or

electronic field not applied (before battery contact with circuit)

In addition, working switch effect by Saturation & Cut off Region & pinch off

points [we explained it previously in what happened inside MOSFET ]

Also, this transistor works as Amplifier similar to JFET

Solving the third homework