Advanced Higher Chemistry Unit 1 Superconductors and semiconductors

Superconductors A superconductor is a material that can conduct electricity with no resistance. When some metals are cooled to low temperatures (using liquid helium) their resistance drops to zero. The temperature at which this happens is known as the critical temperature (Tc).

This temperature is often too low to reach easily and is not cost effective (e.g. Mercury Tc = -269oC). An yttrium based compound (YBa2Cu3O7) with Tc = 92 K can be cooled with liquid nitrogen making it easier to manufacture. This 1-2-3 superconductor (molar ratio of the three metals) is known as a ceramic superconductor.

This superconductor still has problems This superconductor still has problems. As a ceramic, it is difficult to make into wires as it is less ductile than metal superconductors. p36

Meissner Effect Superconductors repel a magnetic field. When a magnet approaches a superconductor, a current is induced. The current has no resistance and this induces the superconductor to have its own magnetic field which can repel a magnet. If the magnet is sufficiently strong, but small, it will levitate above the surface of the superconductor. Meissner effect

Uses of superconductors Magnetic Resonance Imagery (MRI) – used in medicine to give images of inside the body. Has drastically reduced the need for exploratory surgery. Power transmission – superconducting underground transmission cables carry about four times more power than conventional cables.

Semiconductors The conductivity of a semiconductor increases with temperature. The conductivity of metals decreases with temperature.

Semiconductors (metalloids) are found in the area between metals and non-metals in the Periodic Table. Silicon, a covalent network, is the most widely used semiconductor.

The electrical conductivity of semiconductors also increase on exposure to light. This is called photoconductivity or the photovoltaic effect. Photoconductors only conduct in the light. They have high resistance in the dark.

Uses of photoconductors Light meters for photography Sensors for automated lights Photocopiers

Positive holes A metal is made up of a lattice of positive ions surrounded by delocalised electrons. When a current is passed through, the movement of electrons attracts the positive ions and causes the ions to move towards the electrons. When the electron has gone, the positive ions move back to their original position.

When a metal is cooled to the critical temperature, the positive ions do not move back creating a positive region (positive hole). A second electron is attracted to this positive region resulting in a pair of electrons passing through. The electrons are less scattered meaning that the resistance is lowered.

Doping Atoms with a different number of electrons to the original semiconductor can be added to increase conductivity. This is known as doping. The original element must be pure. 1 atom of dopant to 1 x 109 atoms of the parent element is all that is required.

n-type Semiconductor n-type semiconductors have negative electrons as charge carriers. n-type semiconductors have a dopant added with a higher number of electrons per atom (e.g. doping Si with As or P). The extra electrons not used in bonding to the silicon atoms act as current carriers making the silicon a better conductor.

p-type Semiconductors p-type semiconductors have positive holes as charge carriers. When an element with a lower number of electrons than the parent element is introduced, a p-type semiconductor is formed e.g. adding boron or aluminium as dopants to silicon. This causes positive holes to be formed. Semiconductor doping

p-n Junctions Crystals of silicon or germanium can be prepared with bands of n-type or p-type semiconductors. The junction between an n-type and a p-type layer is known as a p-n junction. Electrons can move across the p-n junction from n-type to p-type. This causes a separation of charges. Solar cells use this effect to convert sunlight into electricity.

When light hits the cell, electrons move towards the n-type material. They will then pass to the p-type making a current flow. Light energy is converted into electrical energy. Solar cells are also known as photovoltaic cells. The Solar Cell