19. Lasers and Semiconductors

LASER

• This flow chart shows the requirements expected of students when describing how light is produced using LASER (Light Amplification by Stimulated Emission of Radiation).

BAND THEORY

• Energy bands are formed in solids where there are multiple atoms, each closely packed with their neighbouring atoms. The outer electrons of each atom interacts with those of neighbouring atoms. Each energy level will then split into a band consisting of many sub-levels. These sub-levels are so close to each other that they coalesce to form an energy band.
• The valence band is the highest band that is completely filled.
• The conduction band is the empty or partially filled band that is just above the valence band.
• The forbidden band is the region where no electron state is allowed.

EXTRINSIC SEMICONDUCTORS

• In n-type semiconductors, the impurity energy level (or donor level) lies just below the conduction band. Electrons in this energy level can easily reach the conduction band at room temperature. The majority charge carriers are electrons.
• In p-type semiconductors, the impurity energy level (or acceptor level) lies just above the valence band. Electrons from the valence band can easily jump into it leaving behind positive holes. The majority charge carriers are holes.
• The increase in number of charge carriers increases conductivity (and reduces resistivity).