1. Electronics The Thirteenth and Fourteenth Lecture
Tenth week
21- 24/ 2/ 1438 هـ
أ / سمر السلمي

2. Outline for today Chapter 3: Bipolar junction transistor
Finding minority carrier distributions & terminal currents in BJT
First: the solution of diffusion equation in the base region in BJT
Second: Evaluation Values of Terminal Currents in BJT
minority carrier distributions for modes of operation for BJT for npn
Chapter 4: Field Effect Transistor
FET parts
Types FET
JFET junction field-effect transistor
Structure
What happens inside JFET
Solving the third homework

3. Time of Periodic Exams The third homework
The Second periodic exam in / 3 / 1438 هـ , Please everyone attend in her group
The third homework
I put the third homework in my website in the university homework Due Tuesday 22 / 2/ 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)
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4. Finding minority carrier distributions & terminal currents in BJT
we deal with active mode and base common configuration to npn . At the beginning and to make calculation ease, we assume the following :
1- the thickness of base WB is small, therefore, the diffusion here for electronic current from emitter to collector . In addition, we neglects drift current at base.
2- emitter efficiency γ ≈ 1 because emitter current is only electronic current
3- reverse leakage current or reverse saturation current is neglected
4- the action part in base and two
junctions have regular section area
and electronic current move in one
direction or one dimension which is x
5- all currents and voltages are stable A
VEB
VCB WB xp n p
ΔnE
ΔnC

5. First: the solution of diffusion equation in the base region in BJT
we deal with active mode and base common configuration to npn . Previously, we study about a excess of minority -carriers of electrons concentration in p-type in base region.
Electronic current enter to base from emitter, also come out from base to collector.
To calculate excess of electrons to two end-sides of base at two depletion regions from emitter side and collector side to obtain :
excess of electrons concentration from
end-side depletion regions of emitter
end-side depletion regions of collector A
VEB
VCB WB xp n p
ΔnE
ΔnC

6. First: the solution of diffusion equation in the base region in BJT
If we assume that emitter & base junction has strong forward bias which means
and base & collector junction has strong reverse bias which means
, we will obtain
We mentioned before of minority carrier diffusion equations which is quadratic equation, hence the solution are
Where Ln the diffusion length of minority
carrier electron . We must remember that
thickness of base is small which means that
WB ≤ Ln to move all electronic current from
emitter to collector. A
VEB
VCB WB xp n p
ΔnE
ΔnC

7. First: the solution of diffusion equation in the base region in BJT
Finding constants C with boundary conditions at the two end-sides of base
When multiply the first equation with , we obtain
And collected with second equation to obtain C1
by substituting in the first equation to obtain C2

8. First: the solution of diffusion equation in the base region in BJT
by substituting with constants C in
we mentioned that high reverse voltage to obtain
We can write equation in this from
where

9. First: the solution of diffusion equation in the base region in BJT
the figure shows the distribution of minority carriers in the base also in the emitter and collector
where δn
ΔnE
M1ΔnE
M2ΔnE

10. Second: Evaluation Values of Terminal Currents in BJT
We will find current by known current density to two end-sides of base
At driven excess of electron in base reign in p-type respect of xp
This driven at borders of depletion region from emitter
Therefore, electronic emitter current
by substituting in constants
since the
So emitter current

11. Second: Evaluation Values of Terminal Currents in BJT
This derivation at the borders of the depletion region from collector side
Therefore, collector current is
By substituting in constants

12. Second: Evaluation Values of Terminal Currents in BJT
Finally, we can find base current from emitter and collector currents
When taking into consideration , we obtain threse approximate value

13. minority carrier distributions for modes of operation for BJT for npn
Active mode: the figure shows the distribution of minority carriers in the base also in the emitter and collector in active mode

14. minority carrier distributions for modes of operation for BJT for npn
Saturation mode: the figure shows the distribution of minority carriers in the base also in the emitter and collector in saturation mode

15. minority carrier distributions for modes of operation for BJT for npn
Cut - off mode the figure shows the distribution of minority carriers in the base also in the emitter and collector in cut off mode

16. minority carrier distributions for modes of operation for BJT for npn
Inverted mode :the figure shows the distribution of minority carriers in the base also in the emitter and collector in inverted mode

17. 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

18. 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

19. 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+

20. 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.

21. 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.

22. 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.

23. Solving the third homework