1. Semiconductor Concept

According to the different conductivity (resistivity) of objects, conductors, insulators and semiconductors are classified.

1. Conductor: An object that conducts electricity easily. Such as: iron, copper, etc.

2. Insulator: an object that hardly conducts electricity. Such as: rubber, etc.

3. Semiconductor: A semiconductor is an object whose conductivity is between that of a conductor and a semiconductor. It can conduct electricity under certain conditions. The resistivity of the semiconductor is 10-3 to 109 Ω·cm. Typical semiconductors include silicon Si, germanium Ge, and gallium arsenide GaAs.

Semiconductor characteristics:

1) Under the action of external energy, the electrical conductivity changes significantly. Photosensitive elements and thermal elements belong to this category.

2) Doping impurities into pure semiconductors significantly increases the conductivity. Diodes and triodes belong to this category.

2.1.2 Intrinsic Semiconductor

1. Intrinsic semiconductors-semiconductors with pure chemical composition. The purity of the semiconductor materials used to manufacture semiconductor devices must reach 99.9999999%, often referred to as the "nine nines." It is in the form of a single crystal in physical structure. Silicon and germanium are most used in electronic technology.

Both silicon and germanium are tetravalent elements, and both of them have 4 outer electrons. The simplified atomic structure model is shown below:

The outer layer electrons are the least bound by the nucleus and become valence electrons. The properties of matter are determined by valence electrons.

The outer layer electrons are the least bound by the nucleus and become valence electrons. The properties of matter are determined by valence electrons.

2. Covalent bond structure of intrinsic semiconductor

The atoms in the intrinsic crystal are very close, so that the four valence electrons originally belonging to each atom are attracted by the neighboring atoms at the same time, and they form covalent bonds with the valence electrons of the surrounding four atoms. The valence electrons in the covalent bond are shared by these atoms and bound by them, forming an ordered crystal in space. 

3. Covalent bond

The two electrons on the covalent bond are composed of adjacent atoms each with one electron, and these two electrons are called bound electrons. The bound electron is constrained by two atoms at the same time. If there is not enough energy, it is not easy to leave the orbit. Therefore, at an absolute temperature of T=0°K (-273°C), since the electrons in the covalent bond are bound, there are no free electrons in the intrinsic semiconductor, and it does not conduct electricity. Only when excited, the intrinsic semiconductor can conduct electricity

4. Electrons and holes

When the conductor is at a thermodynamic temperature of 0°K, there are no free electrons in the conductor. When the temperature rises or is irradiated by light, the energy of the valence electrons increases, and some valence electrons can break free from the bondage of the atomic nucleus and participate in conduction and become free electrons. This phenomenon is called intrinsic excitation, or thermal excitation.

At the same time as free electrons are generated, a vacancy appears in the original covalent bond. The neutrality of the atom is destroyed, showing a positive charge, and its positive charge is equal to the negative charge of the electron. People often say that it presents a positive charge. This vacancy of sex is a hole.

Recombination of electrons and holes

It can be seen that the free electrons and holes that appear due to thermal excitation appear in pairs at the same time, which are called electron-hole pairs. The free part of the free electrons may also return to the hole, which is called recombination, as shown in the figure. Intrinsic excitation and recombination will reach dynamic equilibrium at a certain temperature.

The movement of holes

Due to the presence of holes in the covalent bond, under the excitation of the external energy, the neighboring valence electrons may break free from the bond and fill the vacancy, and the original position of this electron will appear again, and other electrons may be transferred to it. That location. In this way, there is a charge migration-current in the covalent bond.

The direction of the current is opposite to the direction in which electrons move, and is the same as the direction in which holes move. In intrinsic semiconductors, the root cause of current generation is the presence of holes in the covalent bond. Due to the limited number of holes, its resistivity is very high

The movement of holes in the crystal (animation)

2.1.3 Impurity semiconductors

Doping certain trace elements as impurities in intrinsic semiconductors can significantly change the conductivity of the semiconductors. The doped impurities are mainly trivalent or pentavalent elements. Intrinsic semiconductors doped with impurities are called impurity semiconductors.

1. N-type semiconductor

Intrinsic semiconductors are doped with pentavalent impurity elements, such as phosphorus, to form N-type semiconductors, also known as electronic semiconductors. Because only four valence electrons in the pentavalent impurity atom can form a covalent bond with the valence electrons in the surrounding four semiconductor atoms, the extra valence electron can easily form a free electron because it is not bound by a covalent bond. In N-type semiconductors, free electrons are majority carriers, which are mainly provided by impurity atoms. In addition, silicon crystals generate a small number of electron-hole pairs due to thermal excitation, so holes are minority carriers.

In N-type semiconductors, free electrons are majority carriers, which are mainly provided by impurity atoms. In addition, silicon crystals generate a small number of electron-hole pairs due to thermal excitation, so holes are minority carriers.

N-type semiconductor structure

The pentavalent impurity atom that provides free electrons becomes a positive ion because it is positively charged, so the pentavalent impurity atom is also called a donor impurity. The structure diagram of the N-type semiconductor is shown in the figure.

Therefore, there are two types of conductive ions in N-type conductors: free electrons-majority carriers (composed of two parts);

Holes-Minority Carriers

2. P-type semiconductor

Intrinsic semiconductors are doped with trivalent impurity elements, such as boron, gallium, indium, etc., to form P-type semiconductors, also known as hole-type semiconductors.

When the trivalent impurity atom forms a covalent bond with the silicon atom, it lacks a valence electron and leaves a hole in the covalent bond. When the electrons on the adjacent covalent bonds are excited to gain energy, they may fill the holes and generate new holes. Holes are their main carriers.

P-type semiconductor structure

In P-type semiconductors, the boron atom easily becomes an anion with a unit negative charge due to trapping an electron. The trivalent impurity is therefore also called acceptor impurity. The covalent bond of the silicon atom forms a hole due to the loss of an electron. Therefore, the schematic diagram of the P-type semiconductor structure is shown in the figure.

Holes in P-type semiconductors are majority carriers and are mainly formed by doping; electrons are minority carriers and are formed by thermal excitation.

3. The influence of impurities on the conductivity of semiconductors

Impurities have a great influence on the conductivity of intrinsic semiconductors. Some typical data are as follows:

1. At room temperature T=300 K, the concentration of electrons and holes in intrinsic silicon: n = p =1.4×1010/cmз

2. Atomic concentration of intrinsic silicon: 4.96×1022/cmз

3. Free electron concentration in N-type semiconductor after doping: n=5×1016/cmз

The above three concentrations basically differ by 1000000/cmз in sequence.