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 ; 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. 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

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).