Intrinsic semiconductor, Extrensic semiconductor and types
Intrinsic semiconductor
A Semiconductor, which is in its extremely pure form, is known as an intrinsic (or pure) semiconductor.
# electrons = n = # holes = p = ni
--true for pure Si, or Ge, etc.
Holes don’t move as easily (mobility of holes is always less than for electrons), but still there are so many that they will contribute at least an extra 10-20% to the intrinsic conductivity.
Analogy to metal
As a general rule, as temperature increases, scattering also increases. This decreases conductivity drastically for metals.
The mobility for an intrinsic semiconductor will also diminish with increasing temperature due to increased scattering.
Still, the extra temperature provides lots of extra electrons and holes in the conduction band for intrinsic semiconductors. This causes n to increase exponentially with Temperature.
n goes up so fast w/r to mobility that the excess electrons totally wash out the diminishing effect of extra scattering.
Thus, conductivity almost always increases with temperature for a semiconductor, the opposite of a metal.
Extrensic semiconductor
A semi-conducting material for which the electrical behavior is determined by impurities.
The process of adding other material to the crystal of intrinsic semiconductors to improves its conductivity is called doing. Doped semiconductor material is called extrinsic semiconductor.
The conductivity of silicon or germanium can be increased by a factor of up to 106 by adding a little as 0.01% of an impurity.
Types of Extrensic semiconductor
n-Type Semiconductor :- When a small amount of pentavalent impurity is added to a pure semiconductor, it is called n-type semiconductor. Such an impurity is called donor impurity.
Ex. Arsenic, bismuth, phosphorous and antimony.
p-Type Semiconductor :- When a small amount of trivalent impurity is added to a pure semiconductor, it is called p-type semiconductor. Such an impurity is called acceptor impurity.
Ex. Gallium, boron or indium.