Hole The ‘gap’ that is left in the atom when the electron breaks free from its bond is called a hole.
These holes in the structure are often referred to as ‘positive holes’ because the atom has become positively charged since it has lost an electron.
Intrinsic Conduction
INTRINSIC CONDUCTION Intrinsic conduction is the movement of charge through a pure semiconductor.
During intrinsic conduction there are equal amounts of negative electrons moving from negative to positive as there are positive holes moving in the opposite direction. The intrinsic current is very small and depends on the temperature of the semiconductor.
In semiconductors the charge carriers are the ‘positive holes’ and ‘negative electrons’.
V / V
Fig 19.2: I–V graph for conduction in a thermistor
Extrinsic Conduction DOPING
N-TYPE SEMICONDUCTOR
An N-type semiconductor is one in which the impurity added produces more free electrons available for conduction.
A group V element is added. Negative electrons are the majority charge carrier, holes the minority carrier, e.g. phosphorous in silicon (Fig 19.3).
Note: An n-type semiconductor is not negatively charged; it is electrically neutral.
P-TYPE SEMICONDUCTOR A P-type semiconductor is one in which the impurity added produces extra holes which are available for conduction.
334 INVESTIGATING PHYSICS
Fig 19.2 is a current-voltage graph for a thermistor. It shows an increase in current with increasing voltage. The slope of the graph increases, indicating that as the voltage increases, the resistance decreases.This is because as the voltage increases, so does the current. This causes the thermistor to get hotter, which frees more electrons for conduction, thus decreasing the resistance.
EXTRINSIC CONDUCTION Extrinsic conduction is the movement of charge through a doped semiconductor.
Doping is the addition of a small amount of another element to pure semiconductor to increase its conductivity.
