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Concepts of electrons and holes in semiconductors

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Forward and reverse bias in a p-n junction

• Under forward bias the width of the depletion region decreases. Current increases exponentially.

• Under reverse bias the width of the depletion region increases. Very low current flow (leakage current Is)

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Chapter 4: Bipolar Junction Transistor

4.1 Basic Operation of the npn Bipolar Junction Transistor

Figure 4.1 The npn BJT

Basic Operation in the Active Region:

An npn transistor (CE configuration) with variable voltage sources operating in the active region:

VBE ≈ 0.6 V to forward bias the BE junction

VCE >VBE - the base collector junction is reverse biased

We will apply the Shockley equation:

Here, the emission coefficient n = 1 (usually the case for ideal p-n junctions)

……… (4.1)

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Basic Operation in the Active Region – Cont’d

First-Order Common-Emitter Characteristics

Fig. 4.3Note:• The current flowing in a BJT is mostly due to electrons

moving from the emitter through the base to the collector

• Base current consist of two components: (i) holes crossing from the base into the emitter, and (ii) holes recombining with the electrons injected into the base

• Usually we desire the base current (i/p current) to be very low

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Factors affecting the Current Gain

For designing a BJT with high β the following points should be considered:

• Emitter doping should be higher than base doping

- this gives higher gain, since electron current constituting the collector current will be much more than the hole current constituting the base current

• The base width should be very small

- reduce base recombination so that most of the electron current from emitter flows to collector

• The geometry of the device should allow quick diffusion of electrons to the collector junction, and the electron lifetime in the base should be large

- reduce base recombination current so that most of the electron current from emitter flows to collector

Note: The BJT can be considered as a current controlled current source. Input current is Ib and output current is Ic.

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Device Equations – Cont’d

From Eq. 4.3 and 4.4 we have: