10.3 : 半導体の基礎

A pure silicon wafer used in integrated circuits is an intrinsic semiconductor that lacks impurities and exhibits low electrical conductivity.

At zero kelvin, its conduction band is empty. The probability of electron occupancy at different energy levels is given by the Fermi level. Here, it lies in the middle of the band gap.

When the temperature increases, several electrons move to the conduction band, resulting in an equal concentration of electrons and holes in their respective bands.

The conductivity of intrinsic silicon can be tailored by introducing impurities through doping, which converts it into an extrinsic semiconductor.

Silicon has four valence electrons. When doped with a pentavalent impurity, it replaces a silicon atom and donates an excess electron, which acts as the majority carrier. Such semiconductors are called N-type semiconductors. Here, the Fermi level is shifted near the conduction band.

Conversely, trivalent impurities create vacant sites called holes, forming the majority charge carriers. These semiconductors are called p-type semiconductors. Here, the Fermi level is shifted towards the valence band.