A Semi Conductor is a material that has a Resistance that is greater than a conductor but less than an insulator. Silicon and Germanium are Semiconductors, while they dont have any free electrons in their wafers they have 4 covalently shared electrons. These electrons (especially at lower temperatures) find themselves tightly bound yet are not 'missed' if they get caught up in some electron movement (current).
Semiconductors are important because ....
Conduction in semiconductors:
the distinction between intrinsic and extrinsic conduction; p-type and n-type semiconductors.
This example is called an intrinsic semiconductor a pure wafer, if you like, of Silicon. If we imagine a potential difference to be set up across the wafer we will see electrons flowing towards the positive terminal. These electrons pop up out of the covalent bonds between the silicon atoms. If the temperature is greater then the freedom of movement is also great. Thus we have temperature sensitive semiconductor devices within which the resistance drops as the temperature increases.
This diagram is of a Silicon wafer DOPED with Boron. Now, check the Periodic table, how many electrons does Boron have in its outer shell?
One less than Si, therefore and due to the low concentrations of Boron in the Sample it 'fits' in to the sample in the regular rows of Silicon. In fact sometimes the Boron might think its Silicon and hold an extra electron, the space where this electron sometimes sits is called a hole. It is truly nothing but a shortage of an electron and thus a percieved extra positive charge (there is not net charge on this) but the extra percieved positive charge makes this material be known as positive type or P-Type for those who like to shorten things.
mmm, do you wonder what n type might be ? .... could you name an element that could dope Silicon to make a N type wafer?
The p-n junction:
basic principles underlying current flow across a p-n junction.
Demonstration of current flow across a p-n junction in forward and reverse bias, e.g. using a bulb
Rectification of a.c.
LED, computers, integrated circuits.
A Thermistor is an electrical component whose resistance decreases rapidly with increasing temperature. VPL
A Light Dependant Resistor (LDR) is an electrical component whose resistance decreases rapidly when light shines on it. VPL
Very good but advanced explanation of the power of transistors.