Explain concept of donor levels and accepter levels in extrinsic semiconductors and How can Fermi level be defined for conductors , insulators and semiconductors when temperature increases above zero Kelvin.

1 Dopant materials

Typical doping concentrations in semiconductors are in ppm (10�6) and ppb (10�9). This small addition of `impurities' can cause orders of magnitude increase in conductivity. The impurity has to be of the right kind. For Si,

n-type impurities are P, As, and Sb while p-type impurities are B, Al, Ga, and In. These form energy states close to the conduction and valence band and the ionization energies are a few tens of meV . Ge lies the same group IV as Si so that these elements are also used as impurities for Ge. The ionization energy data n-type impurities for Si and Ge are summarized in table 1. The ionization energy data for p-type impurities for Si and Ge is summarized in table 2. The dopant ionization energies for Ge are lower than Si. Ge has a lower band gap (0.67 eV ) compared to Si (1.10 eV ). Also, the Table 1: Ionization energies in meV for n-type impurities for Si and Ge. Typical values are close to room temperature thermal energy.

Material P As Sb

Si 45 54 39

Ge 12 12.7 9.6

1