1. Electronics The Fifteenth and Sixteenth Lectures
eleventh week
28/ 2/ 1438 هـ - 2/ 3/ 1438 هـ
أ / سمر السلمي

2. Outline for today Field 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
MOSFET عمله كمفتاح
معادلات وحسابات الترانزستور MOSFET
حل الواجب الثالث

3. Time of Periodic Exams The Forth homework
The Second periodic exam in / 3 / 1438 هـ , Please everyone attend in her group
The Forth homework
I put the forth homework in my website in the university homework Due Tuesday 7 / 3/ 1438 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 in the same day

4. 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

5. 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
وذلك بسبب توصيل الدائرة المختلف لترانزستور n- JFET عن - JFET p
n-channel p-channel

6. JFET as Switch 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.

7. 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

8. 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

9. 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

10. 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)

11. Difference between field-effect transistor [FET] & Bipolar junction transistor [BJT]
BJT
FET
Control 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 transconductance
Noise 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 parts
Current 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

12. 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)

13. 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

14. Structure metal–oxide–semiconductor field-effect transistor (MOSFET)
(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 =

15. 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

16. 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.

17. 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

18. What happens inside MOSFET (n-channel of 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 )

19. 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

20. 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

21. 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

22. 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?) =

23. metal–oxide–semiconductor field-effect transistor (MOSFET)
I – V Characteristic of MOSFET

24. 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)

25. 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) opposite 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) opposite 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