Fundamentals of Doping and Electronic Band Structure in Doped semiconductor

Doping in semiconductor

Doping is purposeful introduction of impurities into a semiconductor material to the defined modification of conductivity by controlling the number of electrons in the conduction band. The impurity material can be interstitial impurities or substitutional impurities. Interstitial impurities are squeezed into the interstitial spaces between the atoms of the host crystal, and substitutional impurities are a substitute for an atom of the host crystal while maintaining the regular crystalline atomic structure. There are two types of dopants for silicon semiconductors. Two of the most used materials silicon can be doped with, are boron (valence electrons = 3) and phosphorus (valence electrons = 5). Other trivalent materials are aluminum, indium and pentavalent materials are Arsenic, antimony.

Fig: k -space diagram of indirect bandgap

The dopant is integrated into the lattice structure of the semiconductor crystal, the number of valence electrons defines the type of doping. The elements with 3 valence electrons give output for p-type doping, and 5 valence elements give results for n-doping. The conductivity of a deliberately contaminated silicon crystal can be increased by a factor of 10⁶.

 

n doping

The pentavalent dopant has an outer electron more than the silicon atom. Its four outer cell electrons combine with the silicon atom to complete an octave state, while the fifth valence electron is free to move and serves as the carrier. The free-electron can be lifted to the conduction band from the valence band with significantly less energy than the electron causing intrinsic conductivity of silicon semiconductor. The energy level of this fifth electron corresponds to an isolated energy level lying in the forbidden gap region. This level can be called a donor level, and the impurity atom which emits an electron is called a donor.

Fig : n-doping 

The conductivity of a semiconductor increases with the concentrator of the donors so drastically that conduction due to impurities becomes the dominant conductance mechanism. In an n-type semiconductor, the conductivity is due almost entirely to negative charge (electron) motion. Doped semiconductors whose conductivity is based on the free electrons (majority charge carrier) are n-doped. The diffusion coefficient of Arsenic is low and used as an alternative to phosphorus.

 

p doping

when a trivalent impurity (boron) is introduced, there are only three valence electrons. The material has an affinity to attract electrons from the material, thus leaving the hole in the valence band of silicon atoms. Hole movement collectively creates an energy level in the forbidden gap close to the valence band.

Fig : p doping 

The energy required to lift an electron into the energy level of indium as a dopant is only 1 % of the energy needed to raise a valence electron of silicon into the conduction band. With the inclusion of an electron, the dopant is negatively charged; such dopants are called acceptors. The dopant is fixed into the crystal lattice, and the only hole can move. The level can be called an acceptor level, as shown in fig below and the impurity atom responsible is called an acceptor. The material is called a p-type semiconductor.

Band structure of p and n type doped semiconductor

In n doped semiconductors, there is free electron in crystal which is not bound and moves with relatively low energy into conduction band. So, the donator energy level is near the conduction band edge, the band gap to overcome is very small. While in p doped semiconductor has free moving hole, which may be already occupied at low energy level by and electron from valence band of the silicon. For p-doped semiconductors one finds an acceptor energy level near the valence band.

Fig : Band structure of p and n type doped semiconductor

YOU MAY LIKE THESE POSTS

Recombination in Semiconductor
Working of Generic Photovoltaic Cell
Biogas Technology : Methane Production from Anaerobic Digestion of Organic Matter