DOPING

Doping is the process of adding small amounts of impurities into pure semi conductors to boost their electrical conductivity. Doped semiconductors are called extrinsic semiconductors.

Small numbers of dopant atoms can change the ability of a semiconductor to conduct electricity. When the order of one dopant atom is added per 100 million atoms, the doping is said to be light or low. However when many more dopant atoms are added, on the order of one per ten thousand atoms, the doping is referred to as heavy or high.

Dopant Elements

Generally, semiconductors are the group four (older notation) elements such as silicon and germanium. In this article, we will use silicon for explanation.

For doping purposes, group III and group V elements are used as dopant elements. When a group III such as Boron and Aluminum, element is used in doping, it is referred to as acceptor atom and gives the p-type semiconductor. On the other hand, a group V such as phosphorous and arsenic, dopant is called a donor atom and gives the n-type semiconductor.

Boron is the p-type commonly used dopant because it diffuses at a rate that makes junction depths easily controllable. Phosphorous is used for bulk-doping of silicon while arsenic is used to diffuse junctions because it diffuses more slowly than phosphorous and thus controllable.

A Silicon Atom Structure

The diagram above shows the structure and lattice of a 'normal' pure crystal of Silicon.

N-type Semiconductor

An n-type semiconductor is an extrinsic semiconductor formed when a semi conductor is doped with a group V element.  Consider a semiconductor like silicon, in order for silicon crystal to conduct electricity, we need to introduce an impurity atom from group V elements such as Arsenic, Antimony or Phosphorus into the crystalline structure. These atoms have five outer electrons in their outermost orbital to share with neighboring atoms and are commonly called "Pentavalent" impurities.

This allows four out of the five orbital electrons to bond with its neighbouring silicon atoms leaving one "free electron" to become mobile. Extra valence electron are  added that become unbounded from individual atoms and allow silicon to be electrically conductive. Each impurity atom "donates" one electron, pentavalent atoms are generally known as "donors".

The resulting semiconductor material has an excess of current-carrying electrons, each with a negative charge, and is therefore referred to as an "N-type" material. The extra electrons are the "Majority Carriers" while the resulting holes are called "Minority Carriers".

Then a semiconductor material is classed as N-type when its donor density is greater than its acceptor density, in other words, it has more electrons than holes thereby creating a negative pole as shown.

Antimony Atom in  Doping

 The diagram above shows the structure and lattice of the donor impurity atom Antimony.

P-Type Semiconductor

A p-type semiconductor is an extrinsic semiconductor formed when a semiconductor is doped with a group III element like Boron or Aluminium. The group III elements are "Trivalent" (3-electron). They lack the fourth valence electron and hence creates broken bond (holes)-the fourth closed bond cannot be formed. Therefore, a complete connection is not possible, giving the semiconductor material an excess of positively charged carriers known as "holes" in the structure of the crystal where electrons are effectively missing.

As there is now a hole in the silicon crystal, a neighbouring electron is attracted to it and will try to move into the hole to fill it. However, the electron filling the hole leaves another hole behind it as it moves. This in turn attracts another electron which in turn creates another hole behind it, and so forth giving the appearance that the holes are moving as a positive charge through the crystal structure (conventional current flow).

The introduced hole is a positive charge carrier and this becomes a "P-type" semiconductor. The positive holes are called "Majority Carriers" while the free electrons are called "Minority Carriers".   As each impurity atom generates a hole, trivalent impurities are generally known as "Acceptors" as they are continually "accepting" extra or free electrons.

Boron Atom in Doping

The diagram above shows the structure and lattice of the acceptor impurity atom Boron.

Semiconductor Summary

N-type (doping with a group V element e.g Phosphorous)

These are materials which have Pentavalent impurity atoms (Donors) added and conduct by "electron" movement and are called, N-type Semiconductors.

In these types of materials;

            1.        The Donor (pentavalent) atoms are positively charged.   

           2.         There are a large number of free electrons- introduced by the donor atoms.

           3.      Fewer holes in relation to the number of free electrons.

           4.      Doping gives:

1.   positively charged donors (the Pentavalent element). 
2.   negatively charged free electrons. 

          5.      Supply of energy gives:  

•   negatively charged free electrons. 
•   positively charged holes.

P-type (doped with trivalent element e.g. Boron)

These are materials which have Trivalent impurity atoms (Acceptors) added and conduct by "hole" movement and are called, P-type Semiconductors.

In these materials:

1. The Acceptor (trivalent) atoms are negatively charged. 
2. There are a large number of holes. 
3. A small number of free electrons in relation to the number of holes. 

4.       Doping gives:

1.   negatively charged acceptors. 
2.   positively charged holes.
4.   Supply of energy gives:
1.    positively charged holes.
2.    negatively charged free electrons.