1. Electronics The Sixteenth and Seventh Lectures
eleventh week
/ 7/ 1438 هـ
أ / سمر السلمي

2. Outline for today Solving the fourth homework
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 fourth homework

3. To improve your grads (optional)
Time of Periodic Exams
The Second periodic exam in / 8/ 1438 هـ11, Please everyone attend in her group
The Forth homework
I put the forth homework in my website in the university homework Due Tuesday 20 / 7/ 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
To improve your grads (optional)
Look in the news in my website (

4. 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

5. 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)

6. 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

7. 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 =

8. 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

9. 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.

10. 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

11. 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 )

12. 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

13. 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

14. 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

15. 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?) =

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

17. 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)

18. 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

20. Solving the fourth homework