Concepts of electrons and holes in semiconductors

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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
We will apply the Shockley equation

• 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 gtVBE - the base collector junction is
  reverse biased

(4.1)
Here, the emission coefficient n 1 (usually the
case for ideal p-n junctions)
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Basic Operation in the Active Region Contd

• Note
• 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 ß IC/IB.
• Typically ß ranges from 10 to 1000

Fig. 4.3
First-Order Common-Emitter Characteristics
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Device Equations Contd
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
The slope of the load line is -1/RB
Fig. 4.11
From Kirchoffs voltage law applied to the
input circuit

• Three major steps for analyzing BJT
• Solve the input circuit to determine the base
  current
• Determine output characteristics corresponding
  to the base current
• Solve the output circuit to determine the VCE
  and IC

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Load line analysis Output circuit
From Kirchoffs voltage law applied to the
output circuit
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.
Fig. 4.11
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