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

1. According to the law of superposition, if the terrain was undisturbed, the layer E would be the youngest and the layer A would be the oldest.

2. The law of horizontality states that sediment deposited into water will settle at the bottom in flat, horizontal layers. This would make us expect that we could find layer D in sector 3. To explain the absence, if none of the stratum D species lived there, there would be no stratum D fossils there.

3. The law of faunal and floral succession suggests that the graphic did not go through any geologic processes after deposition.

4. According to the law of cross-cutting relationships, Layer A is the oldest, and Magma F is the youngest.