Semiconductors – Learning Outcomes Interpret I-V graphs of semiconductors. Discuss conduction in semiconductors – charge carriers, intrinsic/extrinsic, p-type, n-type. Discuss current flow in a p-n junction. Demonstrate current flow in a p-n junction in forward and reverse bias. Give uses of semiconductors.

Semiconductors Semiconductors have resistivity between that of a good conductor and a good insulator. e.g. silicon, germanium, cadmium sulphide

Semiconductors At zero kelvin, semiconductors (e.g. silicon) are insulators. They have no free charges that can carry a current.

Semiconductors At higher temperatures, electrons are freed from the structure and may conduct current. They leave behind a positive hole, or hole.

Semiconductors Strictly speaking, positive holes aren’t “real” conductors, electrons are. It is much easier to consider a single hole moving than many electrons one-by-one filling and leaving holes. Holes are positively charged because they move towards the negative terminal of the power supply.

Intrinsic Conduction Pure semiconductors have an equal number of free electrons and positive holes. These are intrinsic semiconductors and exhibit intrinsic conduction. These are terrible conductors. Intrinsic conduction can be increased by increasing the temperature (releasing more free electrons and holes) or for certain semiconductors, shining light on them (likewise, more detail later).

Light-Dependent Resistor A light-dependent resistor (LDR) is a semiconductor that decreases its resistance when light shines on it. Light hitting the resistor releases electrons from the molecules, allowing them to conduct electricity.

Thermistor A thermistor is a semiconductor designed to decrease its resistance as its temperature increases. The heat energy frees electrons from the material, allowing them to be used for conduction.

Extrinsic Conduction Semiconductors with impurities may have more electrons or holes than pure semiconductors, increasing their conduction. Extrinsic conduction is increased conduction in a semiconductor due to impurities. Doping is the adding of small controlled amounts of certain impurities to a semiconductor to increase its conduction.

Doping An n-type semiconductor has more free electrons available for conduction due to its impurity. e.g. phosphorus in silicon. 4 valence electrons are used to bond to the structure, leaving 1 free electron for conduction. Electrons are the “majority” carriers.

Doping An p-type semiconductor has more positive holes available for conduction due to its impurity. e.g. boron in silicon. 3 valence electrons are used to bond to the structure, leaving 1 hole in the structure. Holes are the “majority” carriers.

P-N Junctions (Diodes) A p-n junction is a piece of semiconductor which is partly doped p-type and partly doped n-type. Where the two meet, free electrons from the n-type fill holes from the p-type, forming a neutral region called the depletion layer which acts as an insulator.

Diodes in Circuits – Forward Bias In forward bias, the p-type is connected to a positive terminal and the n-type is connected to a negative terminal. Holes move away from the positive terminal into the depletion layer. Free electrons move away from the negative terminal into the depletion layer.

Diodes in Circuits – Forward Bias For small voltages, this barely affects the resistance. Once a threshold value called the junction voltage is reached, the depletion layer is eliminated. Resistance decreases rapidly beyond this voltage.

Diodes in Circuits – Reverse Bias In reverse bias, the p-type is connected to a negative terminal and the n-type is connected to a positive terminal. Holes move towards the negative terminal away from the depletion layer. Free electrons move towards the positive terminal away from the depletion layer.

Diodes in Circuits – Reverse Bias The depletion layer widens and the diode continues to be an insulator. In reverse bias, the diode conducts minimal current (due to minority carriers). If a “breakdown voltage” is reached, the depletion region is flooded with carriers causing high current.

Uses of Semiconductors Since diodes only conduct current one way, a.c. will only be allowed to flow in one direction. This is called rectification of a.c. Semiconductors are used to make transistors, which along with other components make integrated circuits (ICs), the building blocks of computers.