Donor and Receiver
The conductivity of semiconductors is not strong, basically equivalent to insulating materials, not much use. However, if appropriate impurities are added, it will be found that the conductivity of semiconductors can be greatly adjusted. This kind of semiconductor doped with impurities is called extrinsic semiconductor. The concept of doping impurities to change the conductivity of semiconductor materials can be Learn from Figure 1.1. Take the silicon semiconductor material as an example. When the silicon material is doped with arsenic (As) element of group V [Figure 1.1(a)], a silicon atom in the lattice is replaced by an arsenic atom with 5 valence electrons. Arsenic atoms tend to form covalent bonds with 4 adjacent silicon atoms. Although the fifth valence electron is still bound, the arsenic atom forms a covalent bond with the surrounding silicon atoms, resulting in the binding energy of the fifth valence electron of the arsenic atom. It is greatly weakened and can be ionized into conduction band electrons near room temperature. The arsenic atom seems to play the role of providing conduction band electrons, therefore, the arsenic atom is called the “donor”. At this time, the number of conductive electrons in the semiconductor material is determined by the concentration of doped impurities, and is generally much greater than the intrinsic carrier concentration. At this time, the transport in the semiconductor is dominated by electrons (negative charges), so it is called It is “N-type semiconductor”.
Figure 1.1 Doping (a) V-valent arsenic atoms and (b) Ⅲ-valent boron atoms in silicon semiconductor materials; (c) arsenic atoms will generate additional confined energy in the forbidden band between the conduction band and the intrinsic Fermi level The ED is called the donor level. The electrons on the donor level are easily heated and vibrated to transition to the conduction band and become conductive electrons; (d) the boron atom will be in the valence band and the intrinsic Fermi An additional limited energy level (EA) is generated in the forbidden band in the middle of the energy level, which is called the acceptor level. There is a lack of an electron on the acceptor level, so the electrons in the valence band are easily subjected to thermal vibration and jump to the acceptor level. The main energy level leaves a vacancy in the valence band and becomes a conductive hole.
In the same way, when silicon material is doped with boron (B) element of group III [Figure 2.17(b)], a silicon atom in the lattice is replaced by a boron atom with 3 valence electrons, and the boron atom will tend to Yu forms a covalent bond with four adjacent silicon atoms, but lacks a valence electron, just like there is a vacancy on the covalent bond. At close to room temperature, the valence electrons in the covalent bond formed by the surrounding silicon atoms are extremely It may be ionized to replace the insufficient valence electrons of the boron atom and generate conductive holes in the valence band. Boron atoms seem to play the role of accepting valence electrons, therefore, boron atoms are called “acceptors”. At this time, the number of conductive holes in the semiconductor material is determined by the concentration of doped impurities. At this time, the transport in the semiconductor is dominated by holes (positive charges), so it is called “P-type semiconductor”.
From the perspective of energy and energy level, the valence electrons of arsenic atoms are ionized to make them conductive electrons, or valence band electrons are excited to boron atoms, which are bound by boron atoms. The required energy is called ionization energy, so arsenic The atom will generate an additional limited energy level (ED) in the forbidden band between the conduction band and the intrinsic Fermi level, called the donor level (Figure 1.1(c)), the electrons on the donor level , It is easy to be heated and vibrated and ionized to transition to the conduction band and become conductive electrons. The arsenic atom that loses one valence electron becomes arsenic positive ion (As+). The boron atom will generate an additional limited energy state (EA) in the forbidden band between the valence band and the intrinsic Fermi level, called the acceptor level (Figure 1.1(d)), the acceptor energy There is a lack of an electron in the valence band, so the electrons in the valence band are easily subjected to thermal vibration and ionization transition to the acceptor energy level, leaving a hole in the valence band and becoming a conductive hole. Therefore, the boron atom has one more electron and becomes a boron anion (B–).
Figure 1.2 shows the donor or acceptor energy levels corresponding to semiconductors such as germanium, silicon and gallium arsenide doped with different impurities. It must be noted that a single atom impurity may form several energy levels. Taking carbon into a silicon semiconductor as an example, it will form a donor level and an acceptor level.
Figure 1.2 Donor or acceptor energy levels and ionization energies (in eV) corresponding to different impurities in semiconductors such as germanium, silicon and gallium arsenide. The energy level is higher than the energy gap center, except for the energy level marked A as the acceptor energy level, it is the donor energy level. The energy level is lower than the energy gap center, except for the energy level marked D as the donor energy level, it is the acceptor energy level. The ionization energy of all donor impurities is measured from the bottom of the conduction band. The ionization energy of all acceptor impurities is measured from the top of the valence band.