Revisiting BJTs

My notes on bipolar junction transistors

Working Principle

N type doping: Elements with extra electrons are injected. These free electrons improve conductivity.
P type doping: Elements lacking an electron are injected. The lack of electrons (holes) are used to improve conductivity.

Emitter
- Heavily doped, more than B and C. This region is contains a large amount of electrons which are required to enable current flow. Additionally, these large amounts of electrons flowing from the Emitter to Collector give the property of amplification of current.
- Bigger than Base but smaller than collector.
Base:
- Small region - this to ensure recombination between holes and electrons are limited. Also, ensures only a few electrons are able to go from Base to Emitter, this forces the majority to migrate to the Collector.
Collector:
- Largest region -> to facilitate the migration of as many electrons as possible from the Emitter.
  - Also recombination releases heat which needs to be dissipated to prevent burn out.
- Moderately doped: ensures that enough electrons are attracted to the +ve terminal.

When voltage Electrons are repelled into breaking the NP junction at the Emitter Side (fwd bias). The electrons are pulled away on the Collector side (rev bias).
When a second supply is added between Base and Emitter, more electrons are repelled which enable a large flow (amplification) of electrons. Also, since the region on the top is very narrow, only a small amount of electrons will flow through this route. The majority of the electrons will go from Base to Collector.
This flow of electrons is enough to overcome the reverse biased junction while also the positive terminal connected to the Collecter attracts all the recently migrating electrons to flow into the path between CE.

The more base current is supplied, the easier it is for electrons to flow from Emitter to Collector (Amplification)

Note: Current flows from the Collector to Emitter (opposite to electron flow)

Base Emitter	Base Collector	Region of Operation
Fwd	Rev	Active (amplification)
Fwd	Fwd	Saturation (ON)
Rev	Rev	Cut off (OFF)
Rev	Fwd	Inverse active. Similar to active region only difference is current flows from Emitter to Collector.

Cut off: This region is used as SWITCH OFF. Base current = 0. No current flow through the Collector to Emitter.

Vce is approx Vcc. In an ideal scenario, there is no current flow through the Collector, however, there are small leakage currents that flow regardless. This is when the transistor is fully ON.
Saturation: This region is used as SWITCH ON The Base Emitter and Base Collector are both FWD biased. Across BE, N is connected to GND and P is connected to +Vbb. Across BC, P is connected to Vbb (+) and N is connected to Vcc (-ve because Vbb>Vcc).

This way, both BE and BC are forward biased. Vce = Vcc - IcRc where Rc is the resistance between Collector and Vc.
Active: This region is used for amplification This is possible when BE is forward biased and BC is reverse biased. Ic increases proportionally as Ib increases. In between cut off and saturation region.