The substances which have resistivity (102 to 0.5 ohm-m) in between conductors and insulators are known as Semiconductors. For example – Germanium, Selenium, Carbon, Sulphur, etc. The following properties which distinguish semiconductors from conductors and insulators are described below.
|1||Copper||Conductor||1.72 x 10-8 Ωm|
|3||Glass||Insulator||9 x 1011 Ωm|
|4||Nichrome||Resistance Material||10-4 Ωm|
Resistivity of a Semiconductor
The resistivity of a semiconductor is less than an insulator but more than a conductor. The following table below shows the resistivity of the various materials.
From the above table, it is clear that the resistivity of semiconductor Germanium is quite high as compared to the conductor (copper). But it is quite low if compared to a glass which is an insulator. It can also be seen that the resistivity of germanium is much higher than the resistivity of Nichrome, which is one of the resistive material having highest resistivity.
Hence, Germanium cannot be regarded as a conductor, insulator or even a resistance material. It is considered as a semiconductor.
Negative Temperature Co-efficient
Semiconductors have Negative Temperature co-efficient of Resistance. It means that the resistance decreases with the rise in temperature and vice-versa. According to this property, the semiconductors behave like an insulator at very low temperature but act as a conductor at high temperatures.
Current Conducting Property
When a suitable metallic impurity (like arsenic, gallium, etc.) is added to a semiconductor, it changes the current conducting properties of the semiconductor appreciably. It is the property which is exploited to develop various solid state devices like the diode, transistor, thyristor, etc.