Energy bands and intrinsic semiconductor silicon vs. extrinsic semiconductor silicon

1. Band structure of crystalline silicon

The energy bands of a crystal reflect the interactions between the various atoms in the crystal, especially the outer electrons. One energy level of n isolated atoms is split into n closely spaced near-continuous energy levels, forming an energy band, as shown in Figure 1.

Energy bands and intrinsic semiconductor silicon vs. extrinsic semiconductor silicon
Figure 1 – Atomic energy levels and energy bands

2. Energy Band Model of Semiconductors

According to the energy band theory, at absolute zero (T=0K), the electrons fill the lower energy band, which is called the full band; the empty energy band above the full band is the empty band, as shown in Figure 2. The highest full band in semiconductors and insulators is called the valence band, the empty band closest to it is called the conduction band, and there is a forbidden band between the valence band and the conduction band. When T>0K, because the forbidden band width of semiconductor is generally about 1~2eV, there will usually be a certain number of electrons excited by heat to transition from the valence band to the conduction band and become conductive electrons. At the same time, an equal amount of holes appear in the valence band, and free electrons and holes generate drift motion under the action of an external electric field, resulting in a certain conductivity of the semiconductor. Silicon crystal is a typical semiconductor material with a forbidden band width of 1.11eV.

Energy bands and intrinsic semiconductor silicon vs. extrinsic semiconductor silicon
Figure 2 – Band models of semiconductors, conductors and insulators

Insulators have a wide band gap, and the probability of electron transition from the valence band to the conduction band caused by thermal excitation is small, and the conductivity is very poor. In the conduction band of a metal conductor, the forbidden band is connected with the valence band, and under the action of an external electric field, it has good conductivity. The band gap Eg of silicon varies linearly with temperature over a wide temperature range T, as shown in Figure 3.

Energy bands and intrinsic semiconductor silicon vs. extrinsic semiconductor silicon
Figure 3 – Band gap Eg of silicon as a function of temperature T

3. Intrinsic semiconductor silicon and extrinsic semiconductor silicon

(1) Intrinsic semiconductor silicon Pure and complete ideal single crystal silicon has no other energy levels in the forbidden band and is an intrinsic semiconductor. Carriers in intrinsic semiconductors are generated by intrinsic excitation, and the electron concentration is equal to the hole concentration.

The carrier concentration or conductivity of a semiconductor depends not only on the effective mass of electrons and holes, but also on temperature. This conductivity type is intrinsically conductive. ni is called the intrinsic carrier concentration.

(2) Extrinsic semiconductor The actual semiconductor material of silicon always has a certain amount of impurities. When the conductance formed by the impurities exceeds the intrinsic conductance, it is an extrinsic semiconductor or an impurity semiconductor. The silicon used in crystalline silicon solar cells is an extrinsic semiconductor in which impurities and defects control the performance of solar cells. The energy levels of impurities in silicon are shown in Figure 4.

Energy bands and intrinsic semiconductor silicon vs. extrinsic semiconductor silicon
Figure 4 – Impurity energy levels in silicon

The impurities of group III and V elements in silicon usually produce shallow energy levels in the forbidden band, which are the shallow energy level impurities of silicon, which have a crucial effect on the electrical properties of silicon. Some impurities, defects or complexes of the two, especially heavy metal impurities such as gold, silver and iron, can generate energy levels in the middle of the forbidden band, which are called deep-level impurities. Electrons and holes will reduce minority carrier lifetime through these deep-level recombination, and the reduction of such impurities should be sought during solar cell fabrication.

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