Semiconductor Basics

Semiconductor Basics

If Resistors are the most basic passive component in electrical or electronic circuits, then the Signal Diode can be classed as the most basic "Active" component. It is a very simple non-linear electronic device that will only allow current to flow through it in one direction only, acting more like a one way electrical valve. But, before we have a look at how signal diodes work we first need to understand their basic construction and concept.

Diodes are made from a single piece of Semiconductor material which has a positive "P-region" at one end and a negative "N-region" at the other, and has a resistivity somewhere between that of a conductor and an insulator. But what is a "Semiconductor" material?, firstly let's look at what makes something either a Conductor or an Insulator.

Resistivity

The electrical Resistance of an electrical or electronic component or device is generally defined as being the ratio of the Voltage difference across it to the Current flowing through it, basic Ohm´s Law principals. The problem with using resistance as a measurement is that it depends very much on the physical size of the material being measured as well as the material out of which it is made. For example, If we were to increase the length of the material (making it longer) its resistance would also increase. Likewise, if we increased its diameter (making it fatter) its resistance would then decrease. So we want to be able to define the material in such a way as to indicate its ability to either conduct or oppose the flow of electrical current through it no matter what its size or shape happens to be. The quantity that is used to indicate this is called Resistivity and is generally given the Greek symbol of ρ, (Rho) and is measured in Ohms per metre, (Ω's/m).

If the resistivity of various materials is compared, they can be classified into three main groups, Conductors, Insulators and Semi-conductors as shown below.

Resistivity Chart

Notice also that there is a small margin between the resistivity of conductors, eg. Silver, Gold etc, compared to the large margin for the resistivity of insulators, eg. Glass and Quartz. The resistivity of all the materials also depends upon their temperature.

Conductors

From above we now know that Conductors are materials that have a low value of resistivity allowing them to easily pass an electrical current due to there being plenty of free electrons floating about within their basic atom structure. When a positive voltage potential is applied to the material these "free electrons" leave their parent atom and travel together through the material forming an electron or current flow. Examples of good conductors are generally metals such as Copper, Aluminium, Silver or non metals such as Carbon because these materials have very few electrons in their outer "Valence Shell" or ring, resulting in them being easily knocked out of the atom's orbit. This allows them to flow freely through the material until they join up with other atoms, producing a "Domino Effect" through the material thereby creating an electrical current.

Generally speaking, most metals are good conductors of electricity, as they have very small resistance values, usually in the region of micro-ohms per metre with the resistivity of conductors increasing with temperature because metals are also generally good conductors of heat.

Insulators

Insulators on the other hand are the exact opposite of conductors. They are made of materials, generally non-metals, that have very few or no "free electrons" floating about within their basic atom structure because the electrons in the outer valence shell are strongly attracted by the positively charged inner nucleus. So if a potential voltage is applied to the material no current will flow as there are no electrons to move and which gives these materials their insulating properties. Insulators also have very high resistances, millions of ohms per metre, and are generally not affected by normal temperature changes (although at very high temperatures wood becomes charcoal and changes from an insulator to a conductor). Examples of good insulators are marble, fused quartz, p.v.c. plastics, rubber etc.

Insulators play a very important role within electrical and electronic circuits, because without them electrical circuits would short together and not work. For example, insulators made of glass or porcelain are used for insulating and supporting overhead transmission cables while epoxy-glass resin materials are used to make printed circuit boards, PCB's etc.

Semi-conductors

Semi-conductors materials such as Silicon and Germanium, have electrical properties somewhere in the middle, between those of a "Conductor" and an "Insulator". They are not good conductors nor good insulators (hence their name semi-conductors). They have very few "fee electrons" because their atoms are closely grouped together in a 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. 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.

The most commonly used semiconductor material is Silicon. It has four valence electrons in its outer most shell which it shares with its adjacent atoms in forming covalent bonds. The structure of the bond between two silicon atoms is such that each atom shares one electron with its neighbour making the bond very stable. As there are very few free electrons available to move from place to place producing an electrical current, crystals of pure silicon (or germanium) are therefore good insulators, or at the very least very high value resistors. Silicon atoms are arranged in a definite symmetrical pattern making them a crystalline solid structure. A crystal of pure silicon (silicon dioxide or glass) is generally said to be an intrinsic crystal.

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

N-type Semiconductor.

In order for our silicon crystal to conduct electricity, we need to introduce an impurity atom such as Arsenic, Antimony or Phosphorus into the crystalline structure. These atoms have five outer electrons in their outermost co-valent bond to share with other atoms and are commonly called "Pentavalent" impurities. This allows four of the five electrons to bond with its neighbouring silicon atoms leaving one "free electron" to move about when an electrical voltage is applied (electron flow). As each impurity atom "donates" one electron, pentavalent atoms are generally known as "Donors". Antimony (symbol Sb) is frequently used as a pentavalent additive as it has 51 electrons arranged in 5 shells around the nucleus. The resulting semiconductor material has an excess of current-carrying electrons, each with a negative charge, and is therefore referred to as "N-type" material with the electrons called "Majority Carriers" and the resultant holes "Minority Carriers".

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

P-Type Semiconductor.

If we go the other way, and introduce a "Trivalent" (3-electron) impurity into the crystal structure, such as Aluminium, Boron or Indium, only three valence electrons are available in the outermost covalent bond meaning that the fourth 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. As there is a hole an adjoining free 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, and so forth giving the appearance that the holes are moving as a positive charge through the crystal structure (conventional current flow). As each impurity atom generates a hole, trivalent impurities are generally known as "Acceptors" as they are continually "accepting" extra electrons. Boron (symbol B) is frequently used as a trivalent additive as it has only 5 electrons arranged in 3 shells around the nucleus. Addition of Boron causes conduction to consist mainly of positive charge carriers results in a "P-type" material and the positive holes are called "Majority Carriers" while the free electrons are called "Minority Carriers".

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

Summary

N-type (e.g. add Antimony)

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 are:

• 1. The Donors are positively charged.
• 2. There are a large number of free electrons.
• 3. A small number of holes in relation to the number of free electrons.
• 4. Doping gives:
  •   positively charged donors.
  •   negatively charged free electrons.
• 5. Supply of energy gives:
  •   negatively charged free electrons.
  •   positively charged holes.

P-type (e.g. add Boron)

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

In these types of materials are:

• 1. The Acceptors 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:
  •   negatively charged acceptors.
  •   positively charged holes.
• 5. Supply of energy gives:
  •   positively charged holes.
  •   negatively charged free electrons.

and both P and N-types as a whole, are electrically neutral.