Transcript Operation of BJT in Saturation Section 4.5 BJT in Saturation Mode Key assumption so far: BE=Forward Biased BC=Reverse Biased What happens when these assumptions are not.

Operation of BJT in
Saturation
Section 4.5
BJT in Saturation Mode
Key assumption so far:
BE=Forward Biased
BC=Reverse Biased
What happens when these assumptions are not true?
Review: Forward Bias Diode
E
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
Review: PN Junction under
Reverse Bias
Reverse: Connect
the + terminal to the
n side.
Depletion region widens.
Therefore, stronger E.
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.
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.
A Large Signal Model of the BJT
The net collector current decreases as the collector
enter into saturation
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.
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).