Conduction Properties of N-Type and P-Type Semiconductors
The presence and properties of electrons and holes are fundamental to understand how semiconductor devices operate
The intrinsic conductivity results from electronic movement in pure materials. However, for a semiconductor to help electronic circuits, its resistivity (the reciprocal of conductivity) must be between 1 or 2 orders of magnitude of 10ˉ² ohm-m at 26.85°C (300°K). The intrinsic resistivity of germanium and silicon has limited utility in electronic circuits (47∙10ˉ² and 2.3 ∙10³ Ω-m, respectively, at 26.85 °C).
The addition of certain electronic impurities increases the conductivity of a material. Impurity is an atom of a different element introduced in the crystal structure. The enhanced conductivity is the extrinsic conductivity.
The fraction of impurity atoms needed in the crystal is about one part in a million. This tiny impurity does not change the crystal’s metallurgical properties noticeably but produces significant changes in the semiconductor’s conductivity.
Doping is the process of introducing impurities to make one particular carrier predominate. The result is a doped semiconductor.
For silicon and germanium, the typical impurities are elements located in the 13th and 15th columns of the modern version of the periodic table.
The number of vacancies, or holes, and free electrons occur in equal numbers in the intrinsic material. Consider adding aluminum to silicon as an impurity. Silicon has the same cubic structure as diamond-shaped carbon. Aluminum has three electrons in its valence shell, so an aluminum atom leaves an electronic vacancy in the cubic structure’s covalent bonds.
Adjacent electrons can move into this vacancy by applying an electric field through the material. When an adjoining electron moves into that hole, the hole moves toward the negative electrode and functions as a positive charge. In this case, we speak of a p-type (or positive) semiconductor. Since the aluminum atom captures an electron from the silicon atoms, it is an acceptor material.
Figure 1 shows a hole (circle) in the covalent bond of silicon and an electron (dot) moving toward the hole under an electric field applied to the material.
Figure 1. Silicon plus aluminum.
Regarding the energy bands, the impurity introduces vacant discrete energy levels close to the valence band, as seen in Figure 2. It is not difficult to excite some of the valence band’s electrons into the impurity levels, producing holes in the valence band.
Figure 2. The energy levels with acceptor or p-type impurities added
Other trivalent atoms used as acceptors are indium, boron, and gallium.
Phosphorus has five valence electrons. Adding phosphorus to silicon rather than aluminum introduces an extra electron per impurity atom. The fifth valence electrons are not in low-energy covalent bonds but remain high-energy, just below the conduction band. They require a small additional amount of energy – a few tenths of an eV – to accelerate in the presence of an electric field and jump into the conduction band. In this case, the predominant carriers are electrons, and we speak of an n-type (or negative) semiconductor.
The phosphorus impurity in the crystal is a donor material because it contributes free electrons for conduction.
Figure 3 shows an extra electron accelerating toward the positive plate.
Figure 3. Silicon plus phosphorus.
Figure 4 shows the discrete energy levels occupied by the additional electrons below the conduction band.
Figure 4. Donors or n-type impurities.
Other pentavalent atoms used as donors are arsenic and antimony.
Applying an external electric field to a crystal lattice will exert a force on the free electrons; in response to the field, and due to their small mass per unit charge, electrons exhibit rapid velocity changes. Under their negative charge, the acceleration will be in a direction opposite to that of the field.
Some frictional forces result from the scattering of electrons by impurities in the crystal lattice, comprising dislocations, impurity atoms, interstitial atoms, vacancies, and the atom’s thermal vibrations. These forces counteract the acceleration driven by the external field. The average distance between collisions is the mean free path.
Electron mobility is an indicator of the frequency of scattering events, and it is limited by the number of deflections or reflections that occur. Greater free paths between direction changes allow larger acceleration.
Electron mobility influences the drift velocity and the electrical conductivity.
The drift is the average electron velocity in the direction of the force imposed by the external field. This motion superimposes random motion due to the thermal energy – diffusion. Such a directed electron flow establishes an electric current.
The electrical conductivity is proportional to the number of free electrons and electron mobility. Factors such as thermal agitation, impurities, and plastic deformation reduce the metals’ conductivity because such imperfections unify, producing irregularities in the crystal’s electric fields. Abnormalities in electric fields reduce electron paths, mean free electron paths, and finally, electrical conductivity.
About Electrons and Holes in Semiconductors
The innate electronic structure in the pure material determines the electrical behavior of the intrinsic semiconductors. In extrinsic semiconductors, the impurity atoms prescribe the electrical characteristics.
Impurities are foreign atoms that enhance the semiconductor’s conductivity. The impurity atom may have fewer or more electrons than the semiconductor atom.
Doping is the process of adding impurities to the semiconductor material to provide free carriers for conduction. The semiconductor is said to have been doped.
The elements in the 13th column in the modern version of the periodic table are acceptor impurities since they accept electrons from the valence structure, creating holes.
The elements of the 15th column in the modern version of the periodic table are donors because they donate free electrons to the crystal. Donors don’t create holes within the valence band.
For silicon and germanium, common acceptor impurities are aluminum, gallium, boron, and indium; typical donor impurities are phosphorus, antimony, and arsenic.
Adding an impurity atom with 3 valence electrons will produce a p-type extrinsic semiconductor; an impurity with 5 valence electrons will make an n-type extrinsic semiconductor.
The electrons in a metal accelerate under the influence of an electric field. The collisions with imperfections in the crystal lattice limit the change in speed. A drift speed superimposes the electrons’ random thermal motions, producing an electric current.