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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 much lower than the collector current (o/p current). We define a current gain constant called β = IC/IB.

• Typically β ranges from 10 to 1000

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Device Equations – Cont’d

From Eq. 4.3 and 4.4 we have:

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Example

Figure 4.4

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Load line analysis – Input circuit

Fig. 4.11

The slope of the load line is -1/RB

From Kirchoff’s voltage law applied to the input circuit:

Three major steps for analyzing BJT:

1.Solve the input circuit to determine the base current2.Determine output characteristics corresponding to the base current3.Solve the output circuit to determine the VCE and IC

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Load line analysis – Output circuit

Fig. 4.11

As IB changes with input voltage VB, the output current and voltage changes according to the transistor characteristics. Almost always an amplification can be obtained.

For each value of ib, there is specific ic vs. VCE characteristic. We have to draw the load line and find the equilibrium point for that load line to find the equilibrium ic and VCE

From Kirchoff’s voltage law applied to the output circuit: