Question

Explain the minority carrier distribution / charge storage phenomenon in the base region of a bipolar transistor when it is in the forward-active region and how it is related to both carrier transport and diffusion capacitance.

Answer 1

A bipolar transistor (BJT) has 2 junctions viz. B-E and B-C junctions. Depending on bias voltages applied to junctions, the operation mode of BJT will change.

If we consider an npn bipolar transistor, then in the forward active mode, B-E junction is forward biased (viz. Vbe > 0) while the B-C junction is inverse biased (Viz. Vbc < 0).

To understand the phenomenon of charge storage, we need to look at the state transition of the bipolar transistor.

In cut-off mode, the transistor’s B-E junction is inverse biased.

To take it into forward mode, we apply a forward voltage to the B-E junction, creating a potential difference there. This potential difference results in creation of a diffusion capacitance across the junctions of transistor. To go from reverse bias to forward active mode, the B-E junction capacitance will take some time to charge up [Refer to basic theory of initial conditions of a capacitor]. Essentially diffusion capacitance builds up because of transport of carriers between junctions of the transistor at different potential levels.

The minority carriers will also some time to traverse the length of B-C and enter the collector junction. As such, all the minority carriers do not recombine in the quasi-neutral region, resulting in excess minority carriers in the Base region. Depending on the time the minority carriers spend in the base junction (prior to recombination with majority carriers), an emitter current is generated.

Hope this clarifies the phenomenon of carrier transport leading to diffusion capacitance in the base region of a BJT.