In order to illustrate the concept of photovoltaic effect, we have to start with semiconductors. According to the electrical conductivity, solid materials can be divided into insulators, conductors and semiconductors. In layman’s terms, those that can conduct electricity are called conductors; those that cannot conduct electricity are called insulators; those that are between conductors and insulators are called semiconductors. The conductivity of different materials is different.
There are many types of semiconductor materials, which can be divided into inorganic semiconductors and organic semiconductors. According to its chemical composition, it can be divided into element semiconductor and compound semiconductor; according to whether it contains impurities, it can be divided into intrinsic semiconductor and impurity semiconductor. Impurity semiconductors are divided into N-type semiconductors and P-type semiconductors according to their conductivity types. In addition, according to the physical properties of semiconductor materials, there are magnetic semiconductors, piezoelectric semiconductors, ferroelectric semiconductors, organic semiconductors, glass semiconductors, gas-sensitive semiconductors, and so on. At present, the widely used semiconductor materials include germanium, silicon, selenium, gallium arsenide, gallium phosphide, cadmium sulfide, indium antimonide, etc. Among them, the semiconductor production technology of germanium and silicon materials is the most mature and most used.
The forms of matter in nature are gas, liquid and solid. A solid material is composed of atoms. Atoms are composed of the nucleus and the surrounding electrons. When some electrons are free from the bondage of the nucleus and can move freely, they are called free electrons. The reason why metal is easy to conduct electricity is because there are a large number of freely moving electrons in the metal body. Under the action of an electric field, these electrons regularly flow in the opposite direction of the electric field to form an electric current. The greater the number of free electrons, or the higher the average velocity of their regular flow under the action of an electric field, the greater the current. We call this kind of particles that carry electricity as carriers. At room temperature, there are only a small amount of free electrons in the insulator, so it does not exhibit electrical conductivity to the outside. There are a small amount of free electrons in a semiconductor, which can conduct electricity under certain conditions. The conductivity of semiconductors is between conductors and insulators,
Semiconductor materials are a class of electronic materials with semiconductor properties that can be used to make semiconductor devices and integrated circuits, and their electrical conductivity is in the range of 10-3 ~ 10-9S/cm. The electrical properties of semiconductor materials are very sensitive to changes in external factors such as light, heat, electricity, and magnetism. Doping a small amount of impurities in semiconductor materials can control the conductivity of such materials. First of all, the doping of trace impurities can greatly enhance the conductivity of semiconductors. Secondly, the properties of semiconductors can be controlled by controlling the temperature. When the ambient temperature rises, the conductivity of the semiconductor will increase significantly: when the ambient temperature drops, the conductivity of the semiconductor will decrease significantly. This characteristic is called the “heat sensitivity” of semiconductors. Thermistor is made using this characteristic of semiconductors. In addition, many semiconductors are very sensitive to light. When light is irradiated on these semiconductors, these semiconductors are like conductors and have strong conductivity.
Although there are many types of semiconductor materials, they all have the same basic characteristics as follows.
①Resistivity characteristics. Resistivity can produce large-scale fluctuations under the action of impurities, light, electricity, magnetism and other factors, so that its electrical performance can be adjusted.
② Conductive properties. There are two types of conductive carriers: one is an electron, which is a negatively charged carrier; the other is a hole, which is a positively charged carrier. In ordinary metal conductors, the conductive carriers are only electrons.
③Negative temperature coefficient of resistivity. As the temperature increases, the resistivity decreases; while the metal is on the contrary, as the temperature increases, the resistivity also increases.
④ Rectification characteristics. Semiconductors have unidirectional conductivity.
⑤Photoelectric characteristics. It can produce photo-generated charge carrier effect under the light of the sun.
It is the use of these properties of semiconductor materials to manufacture semiconductor devices with diverse functions. According to chemical composition and internal structure, semiconductor materials can be roughly divided into the following categories.
① Elemental semiconductors include germanium and silicon. In the 1950s, germanium was dominant in semiconductors, but germanium semiconductor devices had poor high temperature resistance and radiation resistance, and were gradually replaced by silicon materials in the late 1960s. Semiconductor devices made of silicon have better high temperature resistance and radiation resistance, and are particularly suitable for making high-power devices. Therefore, silicon has become the most widely used semiconductor material.
② Compound semiconductors. A semiconductor material composed of two or more elements. There are many types of it, commonly used are gallium arsenide, indium phosphide, antimony steel, silicon carbide, cadmium sulfide and gallium arsenide silicon. Among them, gallium arsenide is an important material for manufacturing microwave devices and integrated circuits. Silicon carbide is widely used in the field of aerospace technology due to its strong radiation resistance, high temperature resistance and good chemical stability.
③Amorphous semiconductor material. It is a kind of non-crystalline amorphous semiconductor material, divided into two kinds of oxide glass and non-oxide glass. This type of material has good memory characteristics and strong radiation resistance, and is mainly used to manufacture threshold switches, memory switches and solid-state display devices.
④ Organic semiconductor materials. There are dozens of known organic semiconductor materials, some of which have not yet been applied.