Conceptual Physics

To understand the relative ease or difficulty of causing a current to flow, we need the concept of energy bands. Below we show what happens

when many atoms are brought together, say atoms that have crystallized into a solid. When atoms are brought very close together, their

energy levels interact and merge into a set of broader energy bands. Instead of the electrons of an individual atom being restricted to particular

energy levels that are so sharp and distinct they are represented with lines, the electrons now can exist with their energies in ranges of values,

or bands.

In fact, we can get more specific about these bands. We will use the diagram below to do so. As you can see, the lower energy band is the

valence band. If you have studied chemistry, you may recall that valence electrons are those most likely to participate in chemical bonds.

Although atomic electrons in the valence band can be shared with neighboring atoms in chemical reactions, they are too strongly bound to the

nucleus of their own atoms to be able to flow freely in an electric current. Electrons at lower energy levels are even more closely tied to the

nucleus, cannot flow in current, and are irrelevant to our discussion.

The highest-energy electrons are in the conduction band. It is electrons in this band that can flow in a current. They are crucial to our story:

Electrons in the valence band cannot flow in a current; electrons in the conduction band can.

It takes energy to move from one band to another, just as it takes energy for an electron to move from one energy level to another in an

isolated atom. When energy is added to an atom, its electrons can be promoted from the valence band to the conduction band, where they are

free to move and become part of a current. How much energy it takes to accomplish this determines whether the material is classified as a

conductor, a semiconductor or an insulator.

Let’s now discuss this concept with specific materials. The diagram above shows the energy band diagram of an insulator, say silicon dioxide,

for the valence and conduction bands. As in the prior illustration, the valence electrons occupy states in the lower energy band shown in the

diagram. The conduction band is the upper band. Between these two bands lies the forbidden gap, or band gap. This is the Mojave Desert of

electrons. They are not allowed to exist there. Electrons can only exist with energies in ranges like the valence or conduction bands.

As you see, the forbidden gap is comparatively large in an insulator. It takes a relatively large amount of energy (8 eV for silicon dioxide, if you

like specifics) to cause an electron to move from the valence band to the conduction band. With few electrons in the conduction band, it is

extremely difficult to produce a current.

Now we consider the other extreme: a conductor. Its energy diagram is shown above. Note how the energy bands overlap; in a conductor, this

overlapping region is only partially filled with electrons. It is “easy” for an electron to move from the valence to the conduction band when a

potential difference is introduced. This means a conductor like copper can provide a ready supply of conduction electrons, ready to rumble

when the slightest electric field from a source like a battery is applied.

Energy bands in a solid

Presence of many atoms broadens

energy levels into bands

Two outer bands of insulator

Valence band

Conduction band

Large energy gap between bands

·“Forbidden gap”

Conductor

Energy bands

·Bands overlap

(^672) Copyright 2007 Kinetic Books Co. Chapter 36