Semiconductors Flashcards
Semiconductor
Is a substance whose resistivity is between that of a good conductor and a good insulator. The resistivity of a semiconductor decreases as its temperature increases.
- examples; silicon, germanium, cadmium sulphide
- semiconductors called light emitting diodes (LEDs) can produce light
Valence electrons
Four outer electrons on a silicon atom
Silicon is an insulator near zero kelvin
Conduction electrons
The outer election arrangement in silicon at room temperature. At this temperature some of the valence electrons have enough thermal energy to break their covalent bonds and are free to wander from atom to atom. Such electrons are called conduction electrons
Holes
When an electron breaks free form a covalent bond, it leaves behind a gap in the atom from which it came. This gap is called a positive hole or simply a hole
Intrinsic conduction
Conduction in a pure semiconductor due to electrons moving from negative to positive (- to +) and an equal number of holes moving in the opposite direction is called intrinsic conduction. The semiconductor is called an intrinsic semiconductor.
To increase the conductivity (reduce the resistivity) of a semiconductor you must increase the number of mobile charge carriers present in it. How can this be done?
- increasing the temperature of a semiconductor increases its conductivity
- shining light on some semiconductors increases their conductivity
- the conductivity of a semiconductor can be increased by the presence of certain impurities
Doping
The adding of small controlled amounts of certain impurities to a pure semiconductor to increase its conductivity is called doping
N-type semiconductor
An n-type semiconductor is one in which the impurity added produces more free electrons available for conduction, e.g phosphorus in silicon
The majority charge carriers are negatively charged electrons and the material is called an n-type semiconductor
There is also some intrinsic conduction taking place with some electrons and an equal number of holes moving in the material. The holes are the minority charge carriers. Note that a piece of n-type semiconductor does not contain excess negative charge. It is still neutral.
P-type semiconductor
A p-type semiconductor is a semiconductor in which the impurity added produces extra holes which are available for conduction, e.g boron in silicon
The majority charge carriers are positively charged holes and the material is called a p-type semiconductor
There is also some intrinsic conduction taking place with some electrons and an equal number of holes moving in the material. Obviously the electrons are the minority charge carriers
Extrinsic conduction
Increased conduction in a semiconductor due to the addition of impurities is called extrinsic conduction. The semiconductor formed is called an extrinsic semiconductor
P-N junction
Also called a p-n diode or a semiconductor diode
Is a piece of semiconductor with part of it doped p-type and the rest doped n-type
Depletion later
Is the region at both sides of a p-n junction that contains no free majority charge carriers. It thus behaves as an insulator
Junction voltage
The p.d. that exists across a p-n junction caused by holes and electrons moving across the junction when it was formed is called the junction voltage
Reverse biased p-n junction
If the positive terminal of a battery is connected to the n-type and the negative terminal is connected to the p-type, the diode is said to be reverse biased
A reverse biased p-n junction does not conduct current
Forward biased p-n junction
If the battery is connected with its positive terminal to the p-type and the negative terminal to the n-type the diode is said to be forward biased
A forward biased p-n junction conducts current.
