A doping agent , in the field of the Semiconductor S, is an impurity added in minor amounts to a pure substance in order to modify its properties of conductivity.

The properties of the semiconductors are mainly controls by the quantity of charge carrier which they contain. These carriers are the electron S or the holes. The doping of a Matériau consists in introducing into its matrix of the Atome S of another material. These atoms will replace certain initial atoms and thus will introduce more electron S or holes.

The Atome S of doping material are also called impurities, and are in diluted phase: their concentration remains negligible in front of that of the atoms of initial material.

Conduction in the semiconductors

A Semi-conducteur is a solid Cristal flax whose properties of electric conduction are determined by two particular energy bands: on the one hand, the valence band , which corresponds to the electron S implied in the covalent bonds; in addition, the band of conduction , including/understanding the electrons in an excited state, which can move in the crystal.

These two bands are separated by a gap , a forbidden band that the electrons can cross only thanks to one external excitation (for example, the absorption of a Photon). The forbidden band corresponds to a barrier of energy, whose order of magnitude is the electronvolt.

The electrons present in the band of conduction allow the conduction of the current. The conduction of the current can be considered in a completely equivalent way in term of holes of electron moving in the valence band. The density of electrons (concentration per unit of volume) is noted N , that of the holes p .

In a Semiconductor intrinsic, or pure, it has no doping atom there. All the electrons present in the band of conduction thus come from the valence band. There are thus as many electrons as of holes: $n\; =\; p\; =\; n\_i$; $n\_i$ is the intrinsic concentration. Any doping is used to modify this balance between the electrons and the holes, to support electric conduction by one of the two types of carriers.

There is always the law of action of mass : $\backslash \; bar\; \{p\}\; \backslash ,\; \backslash \; bar\; \{N\}\; =\; n\_i^2$

Doping of the type NR and type P

There exist two types of doping:

• the doping of standard NR , which consists in producing an excess of electron S, which is égativement N charged;
• the doping of standard P , which consists in producing a deficiency in electron S, therefore an excess of holes, considered as p ositivement charged.

The following diagrams respectively present examples of doping of the Silicium by Phosphore (doping NR ) and Bore (doping P ). In the case of Phospore (on the left), an additional electron is brought. In the case of Boron (on the right), it misses an electron; it is thus a hole of electron which is brought.

Atoms donors and acceptors

The atom of impurity causes effects which depend on the column that it occupies in the periodic classification of Mendeleïev, compared to the column of the atom that it replaces.

• If the doping atom belongs to the same column as the atom that it replaces, they are isovalents (or isoelectric ). The electrons of valence of the atom of impurity replace the electrons of the initial atom exactly. The properties of electric conduction of material are not modified.

• If the doping atom belongs to the preceding column, it misses an outer-shell electron then to restore the whole of the covalent bonds initial. It appears a then deprives of electron, in other words a hole. The inserted atom is known as acceptor (of electron), because it is able to receive an additional electron, coming from the Valence band. It is a doping P .

• If the doping atom belongs to the following column, it has an additional electron compared to the initial atom. The initial covalent bonds are restored, but one of the electrons is not used in these connections. It is thus on a free state of the system. The inserted atom is known as donor (of electron). It is a doping NR .

The same doping atom can be at the same time donor and acceptor: it is then known as amphoteric . It is for example the case of the Silicium (If, column IV), which is doping Gallium arsenide (AsGa): if puts itself in substitution of an atom of Gallium (column III), it is donor of electron. If it is in substitution of an atom of Arsenic (column V), it is acceptor.

If the energy of Ionization $\backslash \; Delta\; E$ is lower than ambient thermal energy $k\; T$ (where $k$ is the Boltzmann constant and $T$ the temperature), then the atoms of impurities are ionized with ambient Température.

Modification of the structure in energy bands

Doping causes the appearance of new levels acceptors and donors of electrons in the structure of band of doped material. These levels appear in the gap , between the Bande of conduction and the Valence band.

At the time of a doping NR (diagram of left), the introduction of atoms donors of electrons involves the appearance of a pseudo energy level located right under the band of conduction. Thus, energy necessary so that the electrons pass in the band of conduction is much more easily reached than in an intrinsic semiconductor.

At the time of a doping P (diagram of right-hand side), the introduction of atoms acceptors of electrons involves, in a similar way, the appearance of a pseudo level located above the valence band. Energy required to the electrons of valence to pass on this level acceptor is weak, and the departure of the electrons involves the appearance of holes in the valence band.

Technologies of doping in the Micro-electronic

There exist several methods to carry out the doping of a material.

• doping by diffusion;
• the ionic Establishment.
• doping by nuclear transmutation

Doping by diffusion

Doping by diffusion can be made leave:

• of a solid source: the sample to be doped is placed in the enclosure. The impurity is transported to material by a inert Gaz vector, starting from a solid compound which sublimates. Example: P ₂O₅ (doping NR of the Silicon).

• of a liquid source: the carrier gas splashes in the liquid or comes very close to its surface at a selected temperature. The Pression partial of composed in gas is equal to the Tension of vapor of the liquid. Example: P OCl₃ (doping NR of the Silicon).

• of a gas source: the gas containing the doping species is introduced into the atmosphere. Examples: P H₃ (doping NR of the Silicon), B ₂H₆ (doping P of the Silicon), Ace H₃ (doping NR of the Silicon).

Doping takes place in a diffusion furnace , a temperature ranging between 850 °C and 1.150 °C, in order to allow the diffusion doping species in material.

Doping by ionic Establishment

Doping by ionic Establishment consists in accelerating impurities ionized with an electric field, in order to confer energy necessary to them to return in material to dope. This method makes it possible to use a large variety of doping elements. The monoenergetic beam and the vacuum chamber make possible a great reproducibility and localized dopings.

The more a Ion is accelerated, the larger its kinetic energy is, and thus more it will be inserted deeply in the network Cristal flax of the substrate which one dopes. Thus, by controlling the amount and energy, one determines the profile of doping.

One of the disadvantages of doping by ionic establishment is the strong crystalline disorder generated by the shocks between the incidental ions and the atoms of material. That generates defects which increase the probabilities of collision, and decrease the mobility of the charge carriers.

Applications of doping

Modulation of the concentrations of carriers

The doping of semiconductor substrates makes it possible to modulate to them electric Conductivité on a broad range. Thus, of the strongly doped semiconductors (called N++ and P++ ) have a conductivity close to that of metals. These strongly doped zones are in particular met when one wishes to carry out ohmic contacts.

Devices and components

The doping of the semiconductors intervenes in the realization of many electronic devices: Junction P-N, Transistor, LED, Laser diode, Photodiode, Heterojunction with modulation of doping

See too

including Fields

• Semiconductor

• Electronic Physics the solid state
• Manufactoring process of the devices to semiconductors

Related Fields

• Circuits semiconductors

• Transistor S
• Diode S
• Microprocessor S

• Materials semiconductors

• Boron nitride
• Diamond
• Gallium arsenide
• Arsenide of gallium-aluminum
• Germanium
• Phosphide of indium
• Silicon
• Semiconductor Silicon-germanium
• with broad band
• Spintronique

Concepts

• Valence band

• Band of conduction
• Energy band
• Hole

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