Concepts of electrons and holes in semiconductors

1
Concepts of electrons and holes in semiconductors
2
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)

3
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)
4
Basic Operation in the Active Region Contd
Fig. 4.3

• 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 very low

First-Order Common-Emitter Characteristics
5
Factors affecting the Current Gain
Note The BJT can be considered as a current
controlled current source. Input current is Ib
and output current is Ic.

• For designing a BJT with high ß 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

6
Device Equations Contd
From Eq. 4.3 and 4.4 we have