Chapter : 14

What are Semiconductor Electronics: Materials, Devices And Simple Circuits ?

Term

Classification of solids on the basis of band theory

Definition / Important Points

  • Metals : The conduction band and valence band partly overlap each other and there is no forbidden energy gap. Electrons can easily move to conduction band.
  • Insulators : The conduction band is empty and valence band is completely filled and forbidden gap is quite large ( ≅ 6 eV). Electrons cannot be excited from valence band.
  • Semiconductors : There is small band gap (≅ l eV). At room temperature, some electrons can jump to conduction band.

Semiconductors

  • These are those solids which have conductivity and resistivity intermediate to metals and insulators.

Intrinsic Semiconductors

  • A doped semiconductors is known as extrinsic semiconductor. Semiconductors are doped with trivalent or pentavalent impurity to increase conductivity.
  • Depending upon the type of doping (trivalent or pentavalent), extrinsic semiconductors are of two types:
(i) n-type semiconductor : When a pure semiconductor is doped with a pentavalent impurity, we obtain a n-type semiconductor.

(ii) p-type semiconductor : When a pure semiconductor is doped with a trivalent impurity, we obtain a p-type semiconductor.

Semiconductor Diode

  • A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p-n junction connected to two electrical
    terminals.

Some Important Concepts

Term

p-n Junction Diode

Definition / Important Points

  • When a p-type crystal is placed in contact with n-type crystal so as form one piece, the assembly thus formed is known as p-n junctions diode.
  • Junction diode conducts current when diode is activated in forward biased whereas it offers high resistance when reverse biased.
  • When p-side is connected to positive terminal and n-side is connected to negative terminal of the external battery, the junction diode is said to be forward biased.
  • When n-side is connected to positive and p-side is connected to negative terminal of the external battery, the junction diode is said to be reverse biased.

I-V characteristics of a p-n junction

reversecurrent
  • The I-V characteristics of a p-n junction do not obey Ohm’s law

Knee Voltage

  • In forward biasing, the voltage at which the current starts to increases rapidly is known as cut-in or Knee voltage. For Germanium, it is 0.3 V while for Silicon it is 0.7 V.

Diode as a Rectifier

  • Rectification means conversion of alternating current into a direct current. A p-n junction can be used as rectifier because it permits current in one direction only.
  • Rectifiers are of two types:

(i) Half Wave Rectifier

(ii) Full Wave Rectifier