Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Extrinsic Material
By doping, a crystal can be altered so that it has a predominance of either electrons or holes. Thus there are two types of doped semiconductors, n-type (mostly electrons) and p-type (mostly holes). When a crystal is doped such that the equilibrium carrier concentrations n0 and p0 are different from the intrinsic carrier concentration ni the material is said to be extrinsic.
Energy band model and chemical bond model:
- When impurities or lattice defects are introduced into an otherwise perfect crystal, additional levels are created in the energy band structure, usually within the band gap.
- For example, an impurity from column V of the periodic table (P, As, and Sb) introduces an energy level very near the conduction band in Ge or Si.
- This level is filled with electrons at 0 K, and very little thermal energy is required to excite these electrons to the conduction band (Fig.a).
- Thus at about 50-100 K virtually all of the electrons in the impurity level are "donated" to the conduction band.
- Such an impurity level is called a donor level, and the column V impurities in Ge or Si are called donor impurities.
- From Fig. a we note that the material doped with donor impurities can have a considerable concentration of electrons in the conduction band, even when the temperature is too low for the intrinsic EHP concentration to be appreciable.
- Thus semiconductors doped with a significant number of donor atoms will have n0 > (ni, p0) at room temperature. This is n-type material.
- Atoms from column III (B, Al, Ga, and In) introduce impurity levels in Ge or Si near the valence band.
- These levels are empty of electrons at 0 K (Fig. b).
- At low temperatures, enough thermal energy is available to excite electrons from the valence band into the impurity level, leaving behind holes in the valence band.
- Since this type of impurity level "accepts" electrons from the valence band, it is called an acceptor level, and the column III impurities are acceptor impurities in Ge and Si.
- As Fig. b indicates, doping with acceptor impurities can create a semiconductor with a hole concentration p0 much greater than the conduction band electron concentration n0 (this type is p-type material).
- In the covalent bonding model, donor and acceptor atoms can be visualized as shown in Fig. c.
- An As atom (column V) in the Si lattice has the four necessary valence electrons to complete the covalent bonds with the neighboring Si atoms, plus one extra electron. This fifth electron does not fit into the bonding structure of the lattice and is therefore loosely bound to the As atom.
- A small amount of thermal energy enables this extra electron to overcome its coulombic binding to the impurity atom and be donated to the lattice as a whole. Thus it is free to participate in current conduction. This process is a qualitative model of the excitation of electrons out of a donor level and into the conduction band (Fig. a).
- Similarly, the column III impurity B has only three valence electrons to contribute to the covalent bonding (Fig. c), thereby leaving one bond incomplete.
- With a small amount of thermal energy, this incomplete bond can be transferred to other atoms as the bonding electrons exchange positions.