1. Physics of Bipolar Junction Transistor

2. Overview

3. Structure and Circuit Symbol
(collects charge carriers)
(controls the carriers
that make the journey
from E to C)
(emits charge carriers)
Two PN junctions (BE and BC)
C and E cannot be swapped.

4. A Bipolar Transistor in the Active Region
VBE=0.8 V
VBC=-0.2 V
BE Junction: Forward Bias
BC Junction: Reverse Bias
Over Simplified View of BJT
In the Active Region

5. Review: Forward Bias Diode;
Depletion region shrinks due to charges from the battery.
The electric field is weaker.
Majority carrier can cross via diffusion;
Greater diffusion current.
Current flows from P side to N side

6. Review: Forward Bias Diode
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Review: Forward Bias Diode
Equilibrium
Forward Biased Diode
Majority carriers cross the junction via diffusion.
Minority carriers increased on both sides of the junction.

7. Review: PN Junction under Reverse Bias
Reverse: Connect
the + terminal to the
n side.
Depletion region widens.
Therefore, stronger E.
Minority carrier to cross
the PN junction easily
through diffusion.
Current is composed
mostly of drift current contributed
by minority carriers.
np to the left and pn to the right.
Current from n side to p side,
the current is negative. E

8. Injection of Electrons into Depletion Region
Into depletion region on the p side.
Outcome: The electron is swept
to the n side by E.

9. An Overview Electrons are injected
into the BC junction
Electrons are injected into the B; holes to the E.
Electrons
are swept across
the reversed biased BC

10. Thin Base Region
The base region is made thin in order to reduce recombination
as electrons travel from BE junction to BC junction.

11. Highly Doped Emitter In order to emphasize the current contribution
due to the electrons (which can cross the BC junction),
the emitter is heavily doped by N type materials.

12. Electrons in the Base Electrons are swept Into the collector;
low electron density at x2
Electrons injected into
the base; high electron
density at x1.
The electron gradient allows electrons to travel through diffusion.

13. BJT Current
Assumption:
BEJ: Forward Biased
BCJ: Reverse Biased

14. Emitter Area

15. Determine the Output Voltage

16. Base Current The proportional of hole current and electron current
is determined by dopants (ND and NA).
Even though the presence of holes are minimized, a small
number holes still must enter through the base.

17. Recombination Recombination
Base must supply holes that will enter the emitter and
for recombination with the electrons.

18. KCL