Dopant distribution, doped semiconductor

Fig. 19 The effect of doping on the density of states distribution in a disordered organic semiconductor at variable concentration of charged dopants. The energy scale is normalized to the width of the DOS, expressed through a, of the undoped sample. The parameters are the intrinsic site concentration V and the dopant concentration N. From [125] with permission. Copyright (2005) by the American Institute of Physics...

Dopant atoms chemical impurities that are deliberately introduced into the semiconductor lattice to provide control over the conductivity and Fermi level of the solid Doping the introduction of specific chemical impurities into a semiconductor lattice to control the conductivity and the Fermi level of the semiconductor Effective density of states the number of electronic states within ikT of the edge of an energy band, where k is the Boltzmann constant and T is the temperature Energy bands a cluster of orbitals in which the individual molecular orbitals are packed closely together to form an almost continuous distribution of energy levels... 

This always holds when the semiconductor is clean, without any added impurities. Such semiconductors are called intrinsic. The balance (4.126) can be changed by adding impurities that can selectively ionize to release electrons into the conduction band or holes into the valence band. Consider, for example, an arsenic impurity (with five valence electrons) in gennanium (four valence electrons). The arsenic impurity acts as an electron donor and tends to release an electron into the system conduction band. Similarly, a gallium impurity (three valence electrons) acts as an acceptor, and tends to take an electron out of the valence band. The overall system remains neutral, however now n p and the difference is balanced by the immobile ionized impurity centers that are randomly distributed in the system. We refer to the resulting systems as doped or extrinsic semiconductors and to the added impurities as dopants. Extrinsic semiconductors with excess electrons are called n-type. In these systems the negatively charged electrons constitute the majority carrier. Semiconductors in which holes are the majority carriers are calledp-type. 

The famous Mott-Schottky relationship [25,26] in Eq. 5-21 represents a different potential-dependent surface capacitive case. This relationship was derived to express the electronic properties of passive capacitive films of constant thickness formed on metals. The methods based on the Mott-Schottky equation have been widely used as a valid tool to determine semiconductive character and dopant density of the surface films in the semiconductor industry and in corrosion studies. The change of the space-charge layer capacitance of the passive film (or space charge distribution) depends on the difference between the applied DC potential V and flat band potential V g characteristic of the surface film, where Np = concentration of donors (or acceptors) or "doping density" ( 10 - lO cm" ), and Cg = 1.6 KT C electron charge ... 

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