In the energy band structure shown in Figure 1, when the forbidden band width Eg is relatively small, as the temperature increases, the number of electrons transitioning from the valence band to the conduction band increases, and the same number of holes are generated in the valence band. The process is called the generation of electron-hole pairs. The semiconductors that can generate such pairs at room temperature and have a certain conductivity are called intrinsic semiconductors, which can only be obtained in extremely pure materials. In general, due to the presence of impurities or defects in the semiconductor, either electrons or holes, which are free carriers, increase, resulting in a doped semiconductor. Those with excess electrons are called n-type semiconductors, and those with excess holes are called p-type semiconductors.
Impurity atoms can be incorporated into the crystal structure in two ways: they can squeeze into positions between the atoms of the host crystal, a condition called interstitial impurities; or they can replace atoms in the host crystal, keeping them in the crystal structure A regular arrangement of atoms, in which case they are called substitutional impurities.
Group IIIA and VA atoms in the periodic table act as substitutional impurities in silicon, and Figure 2 shows a partial lattice in which a group VA impurity (eg, phosphorus) replaces one silicon atom. Four valence electrons form covalent bonds with the surrounding silicon atoms, but the fifth is in a different situation, it is not in the covalent bond and therefore not in the valence band, it is bound to the group VA atoms, so it cannot pass through the crystal. The lattice moves freely, so it is also not in the conduction band.
It can be expected that the release of this excess electron requires less energy than the free electron bound in the covalent bond, much smaller than the 1.leV bandgap energy of silicon. Free electrons are located in the conduction band, so the excess electrons bound to group VA atoms are located at an energy E’ below the bottom of the conduction band, which places an allowable energy level in the “forbidden” interstitial, group IIIA Analysis of impurities is similar. For example, when group VA elements (Sb, As, P) are doped into single-element semiconductor silicon single crystals as impurities, these impurities replace the positions of silicon atoms and enter the lattice points (Figure 3)
In addition to forming a covalent bond with the adjacent silicon atom, its 5 valence electrons have an extra valence electron. Compared with the covalent bond, this remaining valence electron is extremely loosely bound to the impurity atom. Therefore, as long as the impurity atom gets a small amount of energy, it can release electrons to form free electrons, and itself becomes a 1-valent positive ion, but it cannot move due to the constraints of the lattice lattice. In this case, electrons are formed. excess n-type semiconductors. Such impurities that can provide free electrons to the semiconductor are called donor impurities, and their energy band structure is shown in Figure 4. In n-type semiconductors, in addition to electrons generated from these donor levels, there are also electrons excited from the valence band to the conduction band. Since this process is generated in pairs of electrons and holes, there are also the same number of holes. We call the electrons with a large number as majority carriers, and the holes with a small number as minority carriers.
When group IIIA elements (B, Al, Ga, In) are doped as impurities, due to the lack of an electron on the formation of a complete covalent bond, a valence electron is taken from the adjacent silicon atom to form a complete covalent bond. The valence bond, the taken away electron leaves a vacancy, which becomes a hole. As a result, the impurity atom becomes a 1-valent anion and provides a hole that is not tightly bound. This combination can be destroyed with a small amount of energy, and free holes are formed, making the semiconductor a p-type semiconductor with excess holes. The impurity atoms that can accept electrons are called acceptor impurities, and their energy band structure is shown in Figure 5. . In this case, the majority carriers are holes and the minority carriers are electrons.
The above examples are all n-type or p-type semiconductors formed by doping, so they are called doped semiconductors. However, in many compound semiconductors, the conductivity type may change depending on the excess or deficiency of a certain constituent element. In addition, there are cases in which both the n and p conductivity types are not obtained even if impurities are doped due to reasons such as excessively large vapor pressure differences of the constituent elements.