Germanium impurity energy levels

The spectrum of energy levels for electrons at the germanium surface usually differs from that in the interior of the crystal. This situation arises because of the unsaturated energy levels of the surface atoms, misfits in the structures of germanium and the oxide, and other impurities like chemisorbed gases. All these factors, either independently or combined together, result in the formation of the... 

Solid state detectors consist of three layers, a layer of pure silicon sandwiched between a p-type and an n-type conductor. We recall that an example of an n-type conductor is germanium to which is added P or As, an impurity. The extra electron in the phosphorus or arsenic atoms is thought of as being in an energy level close to the conduction band. These electrons are readily thermally excited into the conduction band increasing the conductivity. A p-type semiconductor may be silicon to which a trivalent element such as boron or aluminum is added as an impurity. This creates holes close to the valence band. Electrons are readily promoted to these holes leaving positive holes in the valence band that provide for a conduction pathway. 

Here silicon or. germanium is impregnated (or doped) with an impurity like P, As (group 15) which has more valence electrons (5) than the parent insulator. Each of the impurity atom contributes one more electron. This extra electron occupies an energy state well above the valence band. It is called the donor state or donor level. At room temperature the energy gap is quite close enough to excite the extra electron from the donor state to the conduction band and thus can conduct electricity. Such semiconductors in which current flow is due to the movement of electrons from donor state to conduction band is called n-type (negative electrons) semiconductors. 

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