To see how current is generated in a semiconductor, we need to undertand how electrons in the valance and conduction bands respond to an external electric field. Let’s start with a simplistic but incorrect description and then consider a more accurate picture.
An electron is a negatively charged particle. Click on either side of the electron to apply an electric field.
At low temperature, all electrons in Si are in the valance band. If you apply an electric field, no current is generated. One may justify this by arguing that electrons don’t see any empty place to move to.
With this picture, if there is a vacancy in the valance band, then nearby electrons can move to fill the vacancy and a current is generated. What is the direction of the current?
While some use this description for describing conduction via holes, there are several problems with this picture. Can you identify them.
However, remember that electrons at the top of the valance band have a negative mass.
With this picture, if we apply an electric field, electrons with negative mass will move in the same direction as the electric field. What about holes?
If all electrons in the valance band start to move, then why there is no conduction if there are no vacancies?
Remember that only the electrons in the top of valance band have a negative mass. The ones at the bottom of the valance band have a positive mass. If the band is full, then for every electron with a negative mass, there is an electron with a positive mass. What is the generated current if the band is full?
Vacancies only happen at the top of the valance band. Therefore, they would move in the direction of the electric field. Hence, we define a hole as positively charged quasiparticle with positive mass.
The electrons in the conduction band have a positive effective mass, and they move frely and get accelerated in the opposite direction of the electric field.
Hence, when we study conduction in semiconductors, we can focus on two kinds of charge carriers, electrons in the conduction band with negative charge and holes in the valance band with positive charge. In short we call them electrons and holes. Both have positive effective masses which makes life easier for us.