How does a Thermoelectric Cooler (TEC) work?

Thermoelectric Cooler (TEC) is a semiconductor-based electronic component working on Peltier Effect, that works as a small refrigerator or heat pump depending upon application. When DC current flows through it, one side becomes cold and the other side hot, but to continuously pump heat from the cold side and keep it cooler, a sink is required at the hot side of TEC.

TEC1

To distinguish between the electrical properties of materials we can group them into three sections:

(a) conductors
(b) semiconductors
(c) insulators.

The classification of these materials as solids, metals and insulators is according to the positioning of the conduction as well as the valence bands.

Energy band diagrams show the energy levels of the electrons in the material. We are only interested in two of the bands, the conduction band and the valence band. The valence band is occupied by the electrons with the highest energy level of those which are still attached to their parent atoms, these are the outer most (or valence) electrons. The conduction band is occupied by electrons which are free from their parent atoms. These electrons are free to move through the material. (When a voltage is applied these electrons will drift to produce an electrical current.) In semiconductors there is an gap between the valence and conduction bands. This energy gap reflects the amount of energy that would be needed to remove an electron from it’s parent atom (i.e. to transfer it from the valence to the conduction band).

Images below show the energy band gaps in all three.

band 1

band 2

band 3

Semiconductors are mainly classified into two categories: Intrinsic and Extrinsic.

An intrinsic semiconductor material is chemically very pure and possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes).

Where as an extrinsic semiconductor is an improved intrinsic semiconductor with a small amount of impurities added by a process, known as doping, which alters the electrical properties of the semiconductor and improves its conductivity. Introducing impurities into the semiconductor materials (doping process) can control their conductivity.

Doping process produces two groups of semiconductors: the negative charge conductor (n-type) and the positive charge conductor (p-type).

Pentavalent impurities

(5 valence electrons) produce n-type semiconductors by contributing extra electrons.

Trivalent impurities

(3 valence electrons) produce p-type semiconductors by producing a “hole” or electron deficiency.

How does a TEC work ?

The idea is to absorb energy at one end and emit at the other. For this, the semiconductor and metal combination (metal-semiconductor junctions) is made in such a way that the passing electrons in the circuit, fall or rise from conduction to valence bands or viceversa to  release or absorb heat at respective ends.

couple

When DC current is applied across a couple, electron moving from the metal to the p-type semiconductor  releases energy in the form of heat to enter the valence band.This released energy heats the metal. Conversely the electron absorbs energy as it passes back to the central metal region and again as it is promoted into the conduction band of the n-type semiconductor. The heat absorption results in active cooling in this metal region. Finally, the electron leaves the conduction band of the n-type material releasing heat into the last metal region.

Advertisements