Tweet

Posted on 09 August 2019

Semiconductor Doping

 

Doping is the process of introducing impurity atoms, called dopants, into semiconductor materials during their production. The presence of dopants in semiconductor materials increases the number of available charge carriers, thus altering the material's electrical properties. There are two types of doping processes, n-doping and p-doping, which depend on the dopants that are introduced into the semiconductor material.

 

n-Doping

If a silicon atom is replaced by a phosphorous aton in a silicon crystal, four of the electrons in the outermost shell of the phosphorous atom develop bonds with the neighbouring silicon atoms, freeing the fifth electron which increases the conduction ability of the silicon crystal. The phosphorous atom remains as a static positive charge. The installation of atoms with 5 valence electrons (donors e.g. phosphorus, arsenic, antimony) is refered to as n-doping since negatve free radicals are released due to this installation. The installation of phosphorus (n-doping) increases the number of available charge carriers, thus increasing the conductivity of the silicon crystal.

n-doping with phosphorous in silicon crystal

Figure 1. n- Doping

p-Doping

Whereas a silicon atom contains 4 valence electrons, a Boron atom contains only three. So if a silicon atom is replaced by a boron atom in a silicon crystal, there is a shortage of one electron to form a bond with neighbouring silicon atoms. Due to this shortage, a hole is created (a so-called defect electron) which is equivalent to the presence of positive charge. As with n-doping, the result is an increase in available charge carriers and thus an increase in the electrical conductivity of the crystal. When the hole moves, the boron atom remains behind as a static negative charge. The installation of atoms with 3 valence electrons (acceptors, which include atoms such as Boron, Aluminium, Gallium) is refered to as p-doping since positive free radicals are produced as a result.

p-Doping with Boron in silicon crystal

Figure 2. p-Doping

Majority and Minority Charge Carriers

The amount of available charge carriers per volume of material is refered to as the intrinsic carrier concentration:

 n \cdot p = n_i^2

where n is the concentration of conducting electrons, p is the concentration of holes, and ni is the intrinsic carrier concentration. The intrinsic carrier concentration is dependent on temperature and type of material.

The charge carriers (electrons or holes) present in greater number in the semiconductor are refereed to as the majority charge carriers and the charge carriers in lesser number are refered to as minority charge carriers. Table 1 describes this situation for n-doping and p-doping.

Doping Majority Charge Carriers Minority Charge Carriers
n-Doping Electrons Holes
p-Doping Holes Electrons

Table 1. Majority and Minority Charge Carriers

The Band Structure

A common model for the silicon crystal describes a band structure, which may seem rather abstract but makes use of quantum mechanics phenomena to forecast the behaviour of silicon.

Band Structure in semiconductor material

Figure 3. Band Structure

The electrons move within energy bands within the band structure. Each band contains a given number of electrons. The bands are separated by an energy area refered to as a band gap (figure 3).

The bottom band is refered to as the valence band and is filled to capacity at low temperaturse. This means that no electric conduction takes place, which makes silicon an insulator at very low temperatures.

The top band is refered to as the conduction band and is essentially empty at very low temperatures. At low temperatures, the band gap between the valence and conduction band does not contain any moving charge carriers.

Electric Conduction in the Band Structure

The size of the band gap determines the electrical conductivity of a given material. Insulators have the largest band gap and conductors have no band gap since the conduction and valence band overlap and there is free movement of electrons.

Figure 4 compares the band structures of insulators, semiconductors, and conductors.

Band structures of insulators, semiconductors, and conductors

Figure 4. Band structures of insulators, semiconductors, and conductors

Individual electrons can move from the valence band on to the conductor band when energy (in the form of heat or radiation) is applied to insulators and semiconductors. This electron can move freely within the conduction band. A hole (defect electron) develops in the valence band within the space left by the electron within the valence band, so the hole also moves freely wihin the valence band as the electrons move. This process is referered to as the generation of a charge carrier pair.

The wider the band gap, the more energy is required to move an electron into the conduction band.

Band structure in insulator or semiconductor

Figure 5. Insulator or Semiconductor

Another phenomenon is recombination. Recombination occurs when an electron that has been elevated into the conduction band returns to the valence band due to the exertion of energy in form of a photon (light particle) or phonon (lattice vibration). The recombined particles can no longer be used for conduction. Generation and recombination of charge carriers occur in equal amounts when the system is in thermodynamic equillibrium.

Doping in the Band Structure

Band structure in doped semiconductor

Figure 6. Doping in band structure

New energy levels can be developed within the the band gap in two ways:

  • by doping using donors, an energy level is developed close to the conduction band
  • by doping using acceptors, an energy level is developed close to the valence band

An electron from the donor level can be transfered very easily into the conduction band by applying very little energy. This implies that even at low temperatures an electron can be transfered from the donor energy level into the conductor band leaving behind a positively charged donor atom in the donor energy level.

In the same way, an electron can move from the valence band into a acceptor energy level. A defect electron develops within the valence band as a result of this and the acceptor atom receives a negative charge.

In this manner, the electrical properties of semiconductor materials is modified by controlling the concentration of dopants introduced during semiconductor production.

 

For more information, please read:

Basic Principles of Electricity and Physics of Semiconductors

What is a Semiconductor?

Neutron Transmutation Doping of Silicon Rods

 

VN:F [1.9.17_1161]
Rating: 0.0/6 (0 votes cast)

This post was written by:

- who has written 197 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

One Response

  1. avatar Jahangir says:

    nice got much info..

    VA:F [1.9.17_1161]
    Rating: 5.0/5 (1 vote cast)

Leave a Response

You must be logged in to post a comment.