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Amorphous and crystalline semiconductors have similar basic energy band structures, and also have conduction bands, valence bands, and forbidden bands (see energy bands of solids). The basic energy band structure of materials mainly depends on the conditions near the atoms, which can be explained qualitatively by the chemical bond model. Taking amorphous Ge and Si with tetrahedral bonds as an example, the four valence electrons in Ge and Si are sp hybridized, and covalent bonds are formed between the valence electrons of adjacent atoms, and the bonding state corresponds to the valence band; the antibonding state Corresponds to the conduction band. Regardless of the crystalline or amorphous state of Ge and Si, the basic combination is the same, but the bond angle and bond length are distorted to a certain extent in the amorphous state, so their basic energy band structures are similar. However, the electronic state in the amorphous semiconductor is also fundamentally different from the crystalline state.
The structure of crystalline semiconductors is periodically ordered, or has translational symmetry, the electronic wave function is Bloch function, the wave vector k is a quantum number associated with translational symmetry, and amorphous semiconductors do not have periodicity , k is no longer a good quantum number. The movement of electrons in crystalline semiconductors is relatively free, and the mean free path of electron movement is much longer than the atomic distance; the distortion of structural defects in amorphous semiconductors greatly reduces the mean free path of electrons. When the mean free path is close to the atomic distance When the order of magnitude is smaller, the concept of electron drift motion established in crystalline semiconductors becomes meaningless. The change of the density of states at the band edge of the amorphous semiconductor is not as steep as that of the crystalline state, but has different degrees of band tails.
The electronic states in the energy band of amorphous semiconductors are divided into two categories: one is called extended state, and the other is localized state. Each electron in the extended state is shared by the entire solid and can be found in the entire scale of the solid; it moves in an external field similar to electrons in a crystal; each electron in the localized state is basically confined to a certain area , its state wave function can only be significantly non-zero in a small scale around a certain point, and they need the assistance of phonons to conduct jumping conduction. In an energy band, the central part of the band is an extended state, and the tail part is a localized state. There is a boundary between them, and this boundary is called a mobility edge. Mott first proposed the concept of mobility edge in 1960. If the mobility is regarded as a function of the electronic state energy E, Mott believes that there is a sudden change in mobility at the boundary between Ec and Eg. The electrons in the localized state are jumping conductive, relying on exchanging energy with the lattice vibration, jumping from one localized state to another localized state, so when the temperature T tends to 0K (j to zero degrees), the localized state electrons Mobility tends to zero. The conduction of electrons in the extended state is similar to that of electrons in crystals, and when T tends to 0K, the mobility tends to a finite value. Mott further believed that the mobility edge corresponds to the case where the mean free path of electrons is close to the interatomic distance, and defined the conductivity in this case as Z small metallization conductivity. However, there are still debates surrounding the mobility edge and Z-small metallization conductivity.