A Bit More Atomic Structure

To understand more about the conductivity of materials, we should look a little bit more into the structure of the atoms (see here if you missed the introduction). As we mentioned before, atoms have layers of electrons, the amount of energy electrons have on the first layer is larger than those electrons further away from the nucleus. Hence they are closely bound to the nucleus are require a large amount of energy to pull them away from the atom (this required energy is called the Binding Energy and it’s measured in Electronvolts, eV).

Conductors vs. Insulators

The reason why materials like copper are such good conductors is because there are so many free electrons. Electrons can be made to move easily by applying electrical ‘pressure’ (or voltage). Insulators have some free electrons but most are bound to the nucleus so cannot carry current.

Introduction to Semiconductors

The electrons in both conductors and insulators aren’t particularly dynamic and so little can be said about them; semiconductors are the main interest of today. The concept was first invented in in the beginning of the 20th Century and became more popular with the invention of the transistor in the 1940s and 05. Semiconducting material is material that can alter its conductivity based on external conditions such as light, heat, electric fields or magnetic fields. There is so much to talk about with Semiconductors that the rest of the tutorials in the Physic category will be devoted to Semiconductors. We will start by looking at the atom structure and behaviour of semiconductors and the difference between n-type and p-type materials and later move on to look at Diodes, MOSFETs and Transistors.

For a material to be a semiconductor it has to be doped: adding impurities into a pure substance to alter its electrical properties. A pure semiconductor is called an Intrinsic Semiconductor. We are going to start off by looking at Intrinsic Semiconductors to see how their atoms are arranged and how doping changes them. Silicon is the most common semiconductor material.

As we said in the beginning of this tutorial, electrons in an atom have varying degrees of energy: these energies can be categorised into different bands.  The outer electrons are called Valence Electrons and these have their own group of energies called the Valence Band. These electrons can be excited and move from the Valence Band into the Conduction Band where they are ‘free’.

Energy Gap Comparison

Energy Gap Comparison

It’s the number of free mobile electrons that determines the conductivity of a material. In an insulator, the Energy Gap (the difference between the valence band and the conduction band) is very large and hence there are few conduction electrons. In a conductor the valence and conduction bands overlap and hence there are a huge number of conduction electrons. In a semiconductor, the energy gap is small.

Next we’ll look more at the atom structure of intrinsic semiconductors, including electrons and holes in n-type and p-type materials. Next Tutorial.