Electronic Devices

Semiconductor Diode, Application of Junction Diode as a Rectifier

  • A P-n junction can be formed during the crystal growth in a pure semi-conductor
  • At the junction, the free electrons from n-region migrate towards p-region and the holes in p-region migrate towards n-region. This process is known as DIFFUSION
  • Due to diffusion, positive ions are left over in n-region and negative ions are left over in P-region near the junction. These ions are immobile.
  • Due to the immobile ions on either side of the junction an internal electric field in formed at the junction which is directed from n-region to p-region.
  • Charge carrier region is not formed at p-n junction due to the combination of electrons and holes i.e called as DEPLETION LAYER.
  • If V is the barrier potential and D is the thickness of the depletion layer, then the electric field intensity across the junction is \tt E=\frac{v}{d} from n side to p side.
  • The direction of electric field is always from n-side to p-side.
  • Circuit symbol of p-n junction diode

  • Due to the very small size of p-n junction diodes they are used in micro circuits.
  • The movement of holes and electrons towards the junction and their recombination reduces the width of the charge depleted region.
  • A.C resistance of the diode \tt R_{a.c}=\frac{\Delta v}{\Delta I}
  • The Avalanche breakdown in reverse bias is due to the breaking of covalent bonds as a result of collision of electrons and holes with the valance electrons.
  • Zener breakdown in reverse bias is due to the breaking of covalent bonds simultaneously.
  • Light emitting diode (LED) is forward biased.
  • LED are used as photo-luminescent panels in road signs, indicator lights etc.
    Applications of Junction diode
  • Solar cells are used in calculators. Solar arrays generate electricity.
  • Zener diode is a properly doped p-n junction diode which is operated in the breakdown region in reverse bias mode.
  • Zener diode has a sharp breakdown voltage in the reverse bias condition. This voltage is called Zener Voltage (Vz).
  • Silicon is preferred over germanium while constructing Zener diodes, due to its high thermal stability and current compatibility.
  • More number of electron-hole pairs are created due to the strong electric field at the junction at Zener voltage, which increases the reverse current without change in voltage.
  • Zener diode is used as a voltage regulator.

  • \tt I_{L}=\frac{v_{o}}{R_{L}}=\frac{v_{z}}{R_{L}}=constant
  • Voltage across series resistance,
    V = input voltage – zener voltage,  V = Vi – Vz
  • Current through series resistance (R)\tt I=\frac{v}{R}=\frac{v_{i}-v_{z}}{R}
  • Current through Zener diode, IZ = I – IL

SemiConductor Diode View the Topic in this video From 00:24 To 4:54

View the Topic in this video From 00:27 To 10:31

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1. dc Current Gain
It is defined as the ratio of the collector current (IC) to the base current (IB).
\beta_{dc} = \frac{I_{C}}{I_{B}}

2. ac Current Gain
It is defined as ratio of change in collector current (ΔIC) to the change in base current (ΔIB).
\beta_{ac} = \frac{\Delta I_{C}}{\Delta I_{B}}

3. Voltage Gain
It is defined as the ratio of output voltage to the input voltage.
A_{v} = \frac{V_{0}}{V_{i}} = -\beta_{ac} \times \frac{R_{0}}{R_{i}}