Materials can be divided into three types according to their conductivity. Some of these materials that conduct electricity well are called conductors, and metals are good conductors. Materials that do not conduct electricity at all are called insulators, such as wood. Conductivity between the two is called a semiconductor. Semiconductors are neither good conductors nor insulators. Silicon is a semiconductor. Through a process called doping, the conductivity of a semiconductor can be altered to achieve the desired conductivity.
When foreign atoms doped in a semiconductor can release excess electrons, it is called n-type doping, and the doped material is called a donor. Phosphorus is an n-type impurity for silicon. In phosphorus-doped silicon materials, current can be viewed as the flow of electrons. Certain heteroatoms doped in a semiconductor material can trap electrons in the semiconductor. In this case, the doped semiconductor is called a p-type semiconductor, and the impurity material is called an acceptor. After the acceptor captures electrons from the semiconductor matrix atoms, vacancies are generated in the original position called holes. In p-type semiconductors, the current can be regarded as the flow of holes. Boron is a p-type dopant atom for silicon.
A p-n junction is formed when a p-type semiconductor and an n-type semiconductor are in contact. A p-n junction has special properties different from a single p or n type semiconductor. A p-n junction composed of the same material is called a homojunction; a p-n junction composed of two different materials is called a heterojunction. The p-n junction is the basic structure of inorganic solar cells and LEDs.
In solid materials, electrons can belong to two energy bands according to their energy. Electrons belonging to the valence band cannot participate in the conduction of current because they are bound by atoms. Only electrons belonging to the conduction band can participate in the conduction of the current because these electrons are not bound by individual atoms, they can move freely throughout the material.
In metals, the conduction and valence bands are continuous. In an insulator, the conduction band is empty, and the band gap between the conduction band and the valence band is so large that under an applied voltage, the electrons in the valence band cannot transition to the conduction band, so that current cannot be conducted. In semiconductors, the situation is different, because the band gap between the conduction and valence bands is not so large, and electrons can easily jump from the valence band to the conduction band to become conduction electrons. Also, when the semiconductor is n-doped, excess electrons fill the conduction band.
The size of the band gap is described in electron volts (eV). The band gap of metals is 0eV; insulators are generally greater than 5ev, and semiconductors are usually between 1-3eV. The size of the band gap in a semiconductor determines almost all of its electrical and optical properties.
Depending on the band gap, semiconductors can be used in different technical fields, for example, in LEDs and solar cells. The size of the band gap of the semiconductor also determines the color of the LED.
The p-n junction is a polar structure; the current in the p-n junction can only conduct unidirectionally, that is, when the potential applied to the p-type terminal is higher than that of the n-type terminal, the p-n junction is forward biased and the current conducts. In the opposite case, that is, the voltage applied at the n-terminal is positive and the p-terminal is negative, the p-n junction is reverse biased and the current does not conduct.
Figure 1 depicts the band structure of a direct bandgap semiconductor. In fact, semiconductors can be divided into two categories: direct bandgap semiconductors and indirect bandgap semiconductors. Under electrical excitation, the recombination of electrons and holes has different consequences: electroluminescence or absorption of photons due to the photoelectric effect. Direct bandgap semiconductors have a faster response to electroluminescence. For the case of photon absorption, the thickness of the direct bandgap semiconductor material is required to be smaller than that of the indirect bandgap semiconductor material, which is why thin-film solar cells are usually fabricated from direct bandgap semiconductors. Arsenide, (GaAs) is a direct bandgap semiconductor material while silicon is an indirect bandgap semiconductor material.