1. Electronics The Fourteenth and Fifteenth Lecture
Tenth week
14- 12/ 7/ 1438 هـ
أ / سمر السلمي

2. Outline for today Chapter 4: Field Effect Transistor FET parts
Types FET
JFET junction field-effect transistor
Structure
What happens inside 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]
Solving the third homework

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4. Field Effect Transistor
The field effect transistor uses electric field to control the conduction shape of channel for type extrinsic semiconductor either n-type or p-type . Therefore, it called n-channel or p-channel, respectively. Also, we will focus at majority carriers of channel. because of this, it called also unpolar junction transistor
FET parts
As any transistor, it has three parts. Their name are
Source: a majority carrier enters from it which is the start point of channel.
Drain : a majority carrier comes out from it which is the end point of channel
Gates: the third part which controls of conduction of a majority carrier of channel which is above the channel

5. Types FET famous types in field-effect transistor are
1- (JFET (junction field-effect transistor
2- (MESFET) metal–semiconductor field-effect transistor
3- (MISFET) metal– Insulator –semiconductor field-effect transistor
4- (MOSFET) metal–oxide–semiconductor field-effect transistor
We will focus on the first type JFET and last type MOSFET in our study

6. JFET junction field-effect transistor
JFET Structure
As BJT structure, it distribute three extrinsic types of n-type & p-type in respectively way. Here the channel is in the center
of 3 three types either n-channel or p-channel.
Source (S) and Drain (D) are metal bars at start
& end of the channel. Gates (G) is metal bar
contact with other extrinsic type (which has
more impurities than in channel),and places
above and blow the channel. P+

7. JFET junction field-effect transistor
What happens inside JFET (n-channel)
We will focus at n-channel. At the beginning, we contact transistor with two voltage sources . One of them contacts with circle between source (S) and drain (D), the other contacts as voltage bias to gates (G). We must notice that source contacts with grounded.
When we look at figure below, we notice that battery voltage VDS is the reason to current exit which come out from positive battery to negative battery in n-channel (which here is drain current direction ID ). therefore, the direction of electronic current (majority carrier in n-channel) invert ID direction and move from source to drain.
However, applied voltage VGS at gates p+ and
n-channel is reverse bias or reverse voltage.
Thus, length of the depletion region
between p+n and n p+ junctions is big
which controls of moving electronic current
which move from source S to drain D.

8. JFET junction field-effect transistor
What happens inside JFET n-channel
Notice that the depletion region between p+n and n p+ junctions depend on dimension x . the width of depletion region different in place x =0 from place x =L
Where is wide in L duo to applied voltage change VD.
The voltage at source is
less than voltage at drain.
Thus, it controls with the
cross-section area of
conduction n-channel and
narrows from drain side.
The depletion region works
as side door or gate which
open and close the channel.

9. JFET junction field-effect transistor
What happens inside JFET n-channel
Notice from below figure The depletion region cause closing gate in the drain. Therefore, there no electronic current get out, also drain current (real current in circuit) ID = 0 (correct to be constant). This condition is called saturated state to current. At the beginning of saturated state , we called pinch–off or pinch–off voltage Vp. Channel resistivity is constant duo to impurities constant. it controls with channel resistivity by change the cross-section area of it.

10. 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

11. 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

12. 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.

13. 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

14. 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

15. 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

16. 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)

17. 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

18. 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)

19. Solving the third homework