Semiconductors

Semiconductors are of two types : 1. Intrinsic semiconductors   2.Extrinsic semiconductor 

Intrinsic Semiconductor : Pure form of semiconductor is known as Intrinsic ( Pure ) semiconductor. The silicon and germanium are two most widely used intrinsic semiconductors. The below figure represents the intrinsic structure of silicon atom. It has 4 valence electrons which makes it a tetravalent.

Silicon or Germanium Atom representation

The circle with +4 designation represents the central portion of the atom, including the nucleus and the electron except those in the outermost orbit. Red dot represents the valence electrons. These atoms are bonded together by a force which results from sharing of the neighboring atoms. This bonding force is known as Covalent bond.

The intrinsic semiconductor behaves as an insulator at absolute zero temperature ( 0 K ). As the temperature rises to room temperature(300K) some of the covalent bond breaks.

 

Intrinsic semiconductor with a broken covalent bond 

 

The energy required to break such a covalent bond is equal to the band gap energy (  Eg ). The vacancy of an incomplete covalent bond left behind the dislodged electron is known as a hole. A hole can serve as a carrier of electricity like the free electron. This combination of free electron and a hole is called as electron-hole pair.

Hole Conductivity Mechanism:

Let us imagine that the hole is created by an electron breaking away from the covalent bond. At this instant,  it is possible that an electron from neighbouring atom leaves its covalent bond to fill up the hole. However, this electron generates  a new hole in its initial position.When this occurs, it appears that a hole has moved in a direction opposite to that of the electron. This hole, in its new position may be filled by an electron from another covalent bond. Let consider above figure for more illustration,

Here red dot represents the electron and red empty circle represents hole. Consider the hole at point A, electron at this position had broken away from a covalent bond and moved anywhere in structure and is not shown in this figure. Now due to the thermal agitation, an electron at B may jump to position A.

This reestablishes the covalent bond at A. But creates a new hole at B. We can say that hole at A has moved to new position at B. Similarly, the hole may continue to move from B to C, C to D and D to E. This net displacement of hole from A to E constitutes the electric current.

The motion of hole, in one direction, actually means a transport of a negative charge by equal distance in opposite direction.   

Extrinsic Semiconductors: To increase the conductivity of intrinsic semiconductor at room temperature, a certain amount of impurity atoms can be added to it, this resulting semiconductor is called as impure or extrinsic semiconductor. The purpose of adding impurity is to increase either the number of free electrons or holes in a semiconductor. Depending on the type of  impurity added there are two types: 1. N- Type Semiconductor 2. P- Type Semiconductor

N- Type Semiconductor 

N Type Semiconductor : 

The N type semiconductor are obtained by adding pentavalent impurity atoms. Pentavalent impurity added to the pure semiconductor displaces some of its atom. As Shown in N  type semiconductor figure , there is structure of silicon lattice containing an Antimony atom at is center.
Out of 5 valence electrons of Antimony , 4 electrons will form a covalent bond with 4 valence electrons of silicon atom. The 5th electron is an extra electron, and is loosely bound with antimony atom. This extra electron can be donated to  available as a carrier of the current. 

After donation , the impurity atom becomes a positively charged ion and is known as  a donor ion. Bounded by four covalent bonds in the crystal lattice and therefore it is immobile. Thus in N type semiconductor , the current flows due to the movement of electrons and holes but major part of conduction is due to electrons. That is why electrons are majority carriers and holes are minority carriers in N type semiconductor.

P type Semiconductor

P Type Semiconductor :

The P type semiconductor are obtained by adding trivalent impurity atoms by replacing few pure semiconductor atoms. The 3 valence electrons of Indium atom forms 3 covalent bonds by sharing one electron with the electrons of neighboring atoms. However , the fourth covalent bond is incomplete leading to form a hole. Now due to thermal motion, indium atom seeks its surrounding atoms to acquire the 4th electron to complete its covalent bond. After accepting , impurity atoms become negatively charged immobile ion. Thus in P type semiconductor , majority part of the current flows due to holes and electrons as minority carriers.