P- TYPE AND N-TYPE SEMICONDUCTORS

P - Type and N – Type semiconductors

The conduction through a pure or intrinsic semiconductor is very low. The reason is the number of free electrons is equal to the number of holes.
To increase the conductivity through the semiconductor either free electrons or holes must be greater in number. Through the process of doping the conductivity of semiconductor is increased.

Doping

It is the process of adding impurities like the atoms of other group elements to a pure or intrinsic semiconductor. There are two methods of doping.

1.Adding the atoms of 3rd group element

2.Adding the atoms of 5th group element.

We choose 3rd and 5th group elements to dope a semiconductor because the valency of a semiconductor like germanium and silicon is 4. 

In the process of doping for every 1000 atoms of germanium or silicon one atom of 3rd group or 5th group element is added by diffusion method. This type of doping is moderate.

Note: After adding the impurities the semiconductor will be an impure semiconductor.

An impure semiconductor is called extrinsic semiconductor.

P-TYPE SEMICONDUCTOR

A “P-TYPE” SEMICONDUCTOR IS FORMED WHEN A PURE SEMICONDUCTOR LIKE GERMANIUM AND SILICON IS DOPED WITH AN ATOM OF TRIVALENT (3rd GROUP)

ELEMENT. 

Example: Boron, Aluminum, Indium etc.

Pure germanium or silicon has valency equal to 4. Hence every atom of Ge or Si form 4 covalent bonds with 4 nearest atoms in the crystal lattice. For a pure semiconductor all four bonds are 

complete for all atoms at room temperature.

A trivalent atom consists 3 valency electrons. If the pure semiconductor is doped with trivalent atom, an atom of Ge or Si is replaced by trivalent atom. Then trivalent atom can form 3 covalent bonds only with 3 nearest atoms of the crystal lattice. The fourth bond has only one electron of semiconductor and there is a vacancy semiconductor and there is a vacancy at the position of fourth electron. The fourth bond is incomplete as trivalent atom does not consist 4th electron. Hence a hole is created in the incomplete bond. The atom of trivalent element is 

called acceptor impurity as it leaves a hole.]
As many number of trivalent atoms are doped that many number of holes are created at room temperature only. If the temperature of semiconductor is now increased then a number of electrons will be free leaving that many number of holes. As the semiconductor consists excess number of holes due to doping , the concentration of holes is greater than that of free electrons.
𝑵h > 𝑵e
Hence the holes are the majority charges in p- type semiconductor and the conduction through p- type semiconductor is due to holes.

N-TYPE SEMICONDUCTOR

A “N-TYPE” SEMICONDUCTOR IS FORMED IF A PURE SEMICONDUCTOR LIKE GERMANIUM or SILICON IS DOPED WITH AN ATOM OF PENTAVALENT

(5th group) ELEMENT. Example: Arsenic, Bismuth, Antimony and phosphorous etc.
A pentavalent atom consists 5 valency electrons. If the pure semiconductor is doped with pentavalent atom, an atom of Ge or Si is replaced by 5th group atom. Then 5th group atom forms four covalent bonds with four nearest atoms of the crystal lattice. There is an extra electron in the atom of 5th group element. There will be no scope to form any bond for this extra electron. Hence it is left as free electrons which is remained within the lattice. Hence free electrons are left in the N-type semiconductor at room temperature only.
The 5th group atom is the impurity called donor atom as it donates one free electron to the lattice of semiconductor.
The number of free electrons are more than that of holes and hence electrons are the majority charge carriers in N-type semiconductor.
𝑵𝒆 > 𝑵𝒉

The conduction through N-type semiconductor is due to electrons.