Electronics 105: P-Type and N-Type Semiconductors

Understand the difference between 'N-Type' and 'P-Type' semiconductors.

Guide

Semiconductors

Semiconductors do not conduct currentwell and are not much value for electronics in their normal state. The reason for this there normal state of semiconductive materials have a limited number of free electrons in the conduction band and so by implication, a limited number of holes in the valence band of the atoms.

The semiconductive material must first be modified by either increasing the number of free holes or free electrons to increase its electrical conductivity capacity. This is done by adding impurities to the semiconductive material to make the semiconductive material impure (extrinisic).

Doping

The conductivity of silicon and germanium can be drastically increased by the controlled adding of impurities to the semiconductive material. This process is called Doping and will increase the number of current carriers (either electrons or holes). There are two types of impurities that can be used, named n-type and p-type.

N-Type Semiconductor

To increase the number of electrons in the conduction band of silicon, "pentative" impurities are added. These are atoms with five valence electrons such as Arsenic (As), Phosphorus )P), bismuth (Bi) and Antimony (Sb). Each pentavalent atom forms covalent bonds with four adjacent silicon atoms, leaving one extra electron free to move into the conduction band and is no longer attached to any specific atom. The doping of N-Type semiconductors does not leave a hole in any valence bands because it is in excess of the number required to be filled.

P-Type Semiconductors

To increase the number of holes in pure Silicon, "trivalent" impurities are added. Trivalent atoms contain only 3 valence electrons such as the material Boron (B), Indium (In) and Gallium (Ga). Each trivalent atom forms covelant bonds with four adjacent Silicon atoms. All three electrons of the trivalent valence atoms are used in the covalent bond but since Silicon requires 4 electron in its valence shell, a hole results for every trivalent atom added.

Majority and Minority Carriers

In a semiconductive material, the type of movement, either Electron Current or Hole Current, determines what is refered to as the "Majority Carrier".

In N-Type semiconductors, the Electrons are referred to as the Majority carrier and the Hole refferred to as the Minority carrier. This is because there are more free Electrons in a N-Type semiconductor than there are free Holes.

In P-Type semiconductors, the Holes are referred to as the Majority carrier and the Electron as the Minority carrier. This is because there are more free Holes in a N-Type semiconductor than there are free Electrons.