1. Operation of BJT in Saturation
Section 4.5

2. BJT in Saturation Mode Key assumption so far: BE=Forward Biased
BC=Reverse Biased
What happens when these assumptions are not true?

3. 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

4. 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

5. Hole Current into the Collector
A reverse biased BCJ keeps
holes in the base.
But as BCJ becomes forward
biased, the strong electric field
which opposes of the movement
of holes into the collector is weakened.
There is now a hole current into the collector.
Net Result: heavy saturation leads to a sharp rise in the base current and a rapid
fall in β.

6. A Large Signal Model of the BJT
The net collector current decreases as the collector enter into saturation

7. Example 4.15
A bipolar transistor is biased with VBE =700 mV and has a nominal β of 100. How much B-C forward bias can the device tolerate if β must not degrade by more than 10%? For simplicity, assume base-collector and base-emitter junctions have identical structures and doping levels.

8. General Rules
As a rule of thumb, we permit soft saturation with VBC <400 mV because the current in the B-C junction is negligible, provided that various tolerances in the component values do not drive the device into deep saturation.
For a device in soft saturation or active region, we approximate IC as Isexp(VBE/VT)
In the deep saturation region, the collector-emitter voltage approaches a constant value called VCE, SAT (about 200 mV).