E+T 75 6 FIGURE 22.8 Experimentally determined density of states for a-Si. A and B are acceptor- like and donor- like states, spectively. The arrow marks the position of the Fermi level Efs in undoped hydrogenated a-Si. The energy spectrum is divided into extended states E, band-tail states T, and gap states G. (Source: M.H. Brodsky, Ed, Amorphous Semiconductors 2nd ed, Berlin: Springer-Verlag, 1985. With per si can also be affected by light and strong field effects. The a-Si is used in applications that require deposition of thin-film semiconductors over large areas [xerography, solar cells, thin-film transistors(TFT)for liquid crystal displays]. The a-Si device performance degrades with time under electric stress (TFTs)or under illu mination(Staebler-Wronski effect) because of the creation of new localized states An impurity band in crystalline semiconductors is another example of a disordered system. Indeed, the impurity atoms are randomly distributed within the host lattice. For lightly doped semiconductors at room temperature, the random potential associated with charged impurities can usually be ignored. As the doping level increases, however, a single energy level of a donor or an acceptor is transformed into an energy band with a width determined by impurity concentrations. Unless the degree of compensation is unusually high, this reduces the activation energy compared to lightly doped semiconductors. The activation energy is further reduced by the overlap of the wave functions associated with the individual donor or acceptor states For sufficiently heavy doping, i.e., for N,> N=(0.2/ag)3, the ionization energy is reduced to zero, and the transition to metal-type conductivity(the Anderson-Mott transition)takes place. In this expression the effective lectron Bohr radius ag=/2m E, where E, is the ionization energy of the donor state. For silicon, Na =3.8 108cm-. This effect explains the absence of freeze-out in heavily doped semiconductors. Defining Terms Conduction/valence band: The upper/lower of the two partially filled bands in a semiconductor Donors/acceptors: Impurities that can be used to increase the concentration of electrons/holes in a semicon- Energy band: Continuous interval of energy levels that are allowed in the periodic potential field of the crystalline lattic Energy gap: The width of the energy interval between the top of the valence band and the bottom of the conduction band e 2000 by CRC Press LLC© 2000 by CRC Press LLC a-Si can also be affected by light and strong field effects. The a-Si is used in applications that require deposition of thin-film semiconductors over large areas [xerography, solar cells, thin-film transistors (TFT) for liquid￾crystal displays]. The a-Si device performance degrades with time under electric stress (TFTs) or under illu￾mination (Staebler–Wronski effect) because of the creation of new localized states. An impurity band in crystalline semiconductors is another example of a disordered system. Indeed, the impurity atoms are randomly distributed within the host lattice. For lightly doped semiconductors at room temperature, the random potential associated with charged impurities can usually be ignored. As the doping level increases, however, a single energy level of a donor or an acceptor is transformed into an energy band with a width determined by impurity concentrations. Unless the degree of compensation is unusually high, this reduces the activation energy compared to lightly doped semiconductors. The activation energy is further reduced by the overlap of the wave functions associated with the individual donor or acceptor states. For sufficiently heavy doping, i.e., for Nd > Ndc = (0.2/aB)3 , the ionization energy is reduced to zero, and the transition to metal-type conductivity (the Anderson–Mott transition) takes place. In this expression the effective electron Bohr radius aB = \/ , where Ei is the ionization energy of the donor state. For silicon, Ndc ª 3.8 · 1018 cm–3. This effect explains the absence of freeze-out in heavily doped semiconductors. Defining Terms Conduction/valence band: The upper/lower of the two partially filled bands in a semiconductor. Donors/acceptors: Impurities that can be used to increase the concentration of electrons/holes in a semicon￾ductor. Energy band: Continuous interval of energy levels that are allowed in the periodic potential field of the crystalline lattice. Energy gap: The width of the energy interval between the top of the valence band and the bottom of the conduction band. FIGURE 22.8 Experimentally determined density of states for a-Si. A and B are acceptor-like and donor-like states, respectively. The arrow marks the position of the Fermi level efo in undoped hydrogenated a-Si. The energy spectrum is divided into extended states E, band-tail states T, and gap states G. (Source: M.H. Brodsky, Ed., Amorphous Semiconductors, 2nd ed., Berlin: Springer-Verlag, 1985. With permission.) 2m En i *