1. Electronics The Thirteenth Lecture
eigth week
29/ 6/ 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

3. The first periodic exam in / 8 / 1438 هـ 11 The third homework
Time of Periodic Exams
The first periodic exam in / 8 / 1438 هـ 11
The third homework
I put the third homework in my website in the university homework Due Monday 13 / 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. 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