Basic Operating Principle of a Bipolar Junction Transistor
Bipolar junction transistor under different biasing condition:
- The terminals of a transistor are called Emitter (E), Base (B) & Collector (C).
- If no external biasing voltages are applied (i.e.; VBB and VCC are open circuited) all transistor currents must be zero. The transistor will be in thermal equilibrium condition with potential barriers and at the base emitter and the base collector functions respectively.
- Corresponding depletion layer widths will be and. It is clear from the diagram that p type carriers in the base region of an n-p-n transistor are trapped in a “potential well” and cannot escape. Similarly, in a p-n-p transistor p type carriers in the emitter and collector regions are separated by a “potential hill”.
- When biasing voltages are applied as shown in the figure, the base emitter junction (JBE) becomes forward biased whereas the base collector junction is reverse biased. Potential barrier and depletion layer width at JBE reduces to and respectively. Both these quantities increase at JCB .As the potential barrier at J(AA CBCB,Wφ BE is reduceda large number of minority carriers are introduced in to Base and the Emitter regions.
- A portion of the minority carriers reaching the base recombines with majority carriers. The rest, defuse to the edge of the depletion region at JCB where they are swept away to the collector region by the large electric field.
- Under this condition the transistor is said to be in the Active region.
- As VBE is increased injected minority charge into the base region increases and so does the base current and the collector current. The total voltage drop between collector and emitter will be the difference between the forward bias voltage drops at JBE and JCB. Under this condition the transistor is said to be in the saturation region.