Classification of Semiconductor

Classification of Semiconductor

Intrinsic Semiconductor

 An intrinsic semiconductor also called an undoped semiconductor or i- type semiconductor.

It is a pure semiconductor without any significant dopant species present.

The number of charge carriers determined by the properties of the m aterial itself instead of the amount of impurities.

In intrinsic semiconductors the number of excited electrons and the number of holes are equal: n = p.

Conductivity of Intrinsic semiconductor

 The electrical conductivity of intrinsic semiconductors can be due to crystal defects or to thermal excitation.

 Both electrons and holes contribute to current flow in an intrinsic semiconductor.

       The current which will flow in an intrinsic semiconductor consists of both electron and hole current.

       That is, the electrons which have been freed from their lattice positions into the conduction band can move through the material.

       In addition, other electrons can hop between lattice positions to fill the vacancies left by the freed electrons.

       This additional mechanism is called hole conduction because it is as if the  holes  are migrating across the material in the direction opposite to the free electron movement.

 The current flow in an intrinsic semiconductor is influenced by the density of energy states which in turn influences the electron density in the conduction band.

       This current is highly temperature dependent.

Thermal excitation:

       In an intrinsic semiconductor like silicon at temperatures above absolute zero, there will be some electrons which are excited across the band gap into the conduction band and which can produce current.

       When the electron in pure silicon crosses the gap, it leaves behind an electron vacancy or "hole" in the regular silicon lattice.

       Under the influence of an external voltage, both the electron and the hole can move across the material.

       In n-type semiconductor:

The dopant contributes extra electrons, dramatically increasing the conductivity

 In p-type semiconductor:

The dopant produces extra vacancies or holes, which likewise increase the conductivity.

Extrinsic Semiconductor

 The electrical conductivity of a pure semiconductor is very small.

 To increase the conductivity, impurities are added.

 The impurity added semiconductor is called extrinsic semiconductor.

 The process of adding impurity is called doping.

 The added impurity is called dopant.

 Usually one or two atoms of impurity is added per 106 atoms of a semiconductor.

 There are two types (i) p-type and (ii) n-type semiconductors.

  When an impurity, from V group elements like arsenic (As), antimony having 5 valence electrons is added to Ge (or Si), the impurity atom donates one electron to Ge (or Si).

       The 4 electrons of the impurity atom is engaged in covalent bonding with Si atom.

       The fifth electron is free. This increases the conductivity.

       The impurities are called donors.

       The impurity added semiconductor is called n-type semiconductor, because their increased conductivity is due to the presence of the negatively charged electrons, which are called the majority carriers.

       The energy band of the electrons donated by the impurity atoms is just below the conduction band.

       The electrons absorb thermal energy and occupy the conduction band.

 Due to the breaking of covalent bond, there will be a few holes in the valence band at this temperature.

       These holes in n-type are called minority carriers.

 If a III group element, like indium (In), boron (B), aluminium (AI) etc., having three valence electrons, is added to a semiconductor say Si, the three electrons form covalent bond.

 There is a deficiency of one electron to complete the 4th covalent bond and is called a hole.The presence of the hole increases the conductivity because these holes move to the nearby atom, at the same time the electrons move in the opposite direction. 

 The impurities added semiconductor is called p-type semiconductor.

 The impurities are called acceptors as they accept electrons from the semiconductor

 Holes are the majority carriers and the electrons produced by the breaking of bonds are the minority carriers.