Concept of hole: (or) Effective or negative mass of electron

CONCEPT OF HOLE: (OR) EFFECTIVE OR NEGATIVE MASS OF ELECTRON

The effective mass plays an important role in conduction process of semiconductors and insulators since they have full or almost filled valence bands.

We can find that the effective mass m^* is negative near the zone edges of almost filled valence bands. Physically speaking the electrons in these regions are accelerated in a direction opposite to the direction of the applied field. This is called the negative mass behaviour of the electrons.

The electrons with the negative effective mass is considered as a new entity having the same positive mass of that of an electron but with positive charge. The new entity is given the name "hole".

The holes are not real particles like electrons or positrons, but it is a way of looking at the negative mass behaviour of the electrons near the zone edge.

We look upon the motion of the effective negative mass electrons as the motion of positive holes or positive vacant sites in a nearly full band and allow the electrons in the band to carry the current.

The positive hole conduction and effective negative electron mass conduction are in equilibrium. The calculations made on the hole picture appear to be more convenient and hence the hole concept is advantageously retained.

Several phenomena like Hall effect, Thomson effect, etc find ready explanation on the basis of the hole concept.