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In order to change the electrical properties of a semiconductor, it is necessary to dope it. Impurities are added to the semiconductor so that the number of carriers in the semiconductor are increased.

A semiconductor may be doped p-type. When a group III impurity, such as Boron, is introduced into a group IV element such as Si, each boron atom has one less electron than the surrounding lattice. These type of dopants are called acceptors because they accept electrons. The accepted electron leaves in place a hole, and thus the semiconductor is called P type because holes carry positive charge.

Conversely, a semiconductor may be doped n-type. When a group V impurity, such as arsenic, is introduced into a group IV element such as Si, each arsenic atom has one more electron than the surrounding lattice. These type of dopants are called donors because they donate electrons. The donated electron means that there are many mobile electrons, and the semiconductor is called N type because electrons carry negative charge.

Donor states are typically just a little bit below the bottom of the conduction band edge, and acceptor states are a little bit above the top of the valence band edge.

Interfaces use analogous terms for states. At a semiconductor interface, there is an energy level referred to as the charge neutrality level. This energy level is generally at a different energy than the semiconductor Fermi level. If $\phi_0 > E_f$ then, the surface is donor like and traps holes. These states are positively charged above the electron quasi-Fermi level (the state carries the charge of one hole when occupied) and neutral below. The band bend down near the surface and there is positive charge at the surface. If the substrate is p-type, then the band bending is more significant, but if it is n-type, the band bending is less.

If $\phi_0 < E_f$ then, the surface is acceptor like and traps electrons. These states are negatively charged below the quasi-Fermi level (the state carries the charge of one electron) and neutral above. The band bend down up the surface and there is negative charge at the surface. If the substrate is n-type, then the band bending is more significant, but if it is p-type, the band bending is less.

Mnemonic: acceptor – negative below quasi-Fermi, neutral above; donor – positive above quasi-Fermi, neutral below