Semiconductors materials such as Silicon (Si), Germanium (Ge) and Gallium Arsenide (GaAs), have electrical properties somewhere between those of a “conductor” and an “insulator”. They neither are not good conductors nor are they good insulators (hence their name “semi”-conductors). They have very few “fee electrons” in their valence shell because their atoms are closely grouped together in a tight crystalline pattern called a “crystal lattice”.
However, their ability to conduct electricity can be greatly improved by adding certain “impurities” to this crystalline structure thereby, producing more free electrons than holes or vice versa. By controlling the amount of impurities added to the semiconductor material it is possible to control its conductivity.
“Semiconductor materials made from silicon are used to produce diodes, mosfet's, bipolar transistors and all types of integrated circuits”
What is Doping?
These impurities are called donors or acceptors depending on whether they produce electrons or holes respectively. This process of adding impurity atoms to semiconductor atoms (the order of 1 impurity atom per 10 million (or more) atoms of the semiconductor) is called Doping.
In order for a silicon crystal to conduct electricity, we need to introduce an impurity atom that has five outer electrons in its outermost valence shell to share with its neighbouring atoms. The most common type of “pentavalent” (5-electron) impurities used to dope silicon are Antimony (symbol Sb) or Phosphorus (symbol P), because they have 51 electrons arranged in five shells around their nucleus with the outermost orbital having five electrons.
The resulting semiconductor basics material has an excess of currentcarrying electrons each with a negative charge, and is therefore referred to as an “N-type” material. Then N-type Semiconductors are materials which have pentavalent impurity atoms (Donors) added and conduct by “electron” movement. In these types of materials the donors are positively charged and there are a large number of free electrons. If we go the other way, and introduce a “trivalent” (3-electron) impurity into the crystalline structure, such as Boron (symbol B) or Indium (symbol In), which have only three valence electrons available in their outermost orbital, the fourth closed bond cannot be formed. Therefore, a complete connection is not possible, giving the semiconductor material an abundance of positively charged carriers known as “holes” in the structure of the crystal where electrons are effectively missing.
The doping of Boron atoms causes conduction to consist mainly of positive charge carriers resulting in what is called a “P-type” material. Then P-type Semiconductors are a material which have trivalent impurity atoms (Acceptors) added and conducts by the movement of “holes”. In these types of materials the acceptors are negatively charged and there are a large number of holes for free electrons to fill.
So by using different doping agents to a base material of either Silicon (S) or Germanium (Ge), it is possible to produce different types of basic semiconductor materials, either N-type or P-type for use in electronic semiconductor components, microprocessor and solar cell applications.
The most basic semiconductor device is the diode formed from the fusing together of both N-type and P-type semiconductor materials. The semiconductor diode is a device that allows current to pass through it in only one direction. This characteristic of a diode has many useful applications in electronics such as rectification of AC voltages and currents to DC.