Technical Analysis of Semiconductor Wide Bandgap

The continuous innovation of science and technology has led to the continuous development of semiconductors. In semiconductor circuits, a word has appeared, which is our theme. Semiconductor bandgap? What problem does this word appear to solve in circuits? With various questions , let’s learn together~

In crystals, including semiconductors, the electrons in them are different from free electrons in a vacuum and from electrons in isolated atoms. Free electrons in a vacuum have a continuous energy state, that is, they can take any amount of energy; while electrons in atoms are in a so-called separated energy level state. The electrons in the crystal are in the so-called energy band state. The energy band is composed of many energy levels. There is a forbidden band separating the energy bands. The electrons are distributed on the energy levels in the energy band. The forbidden band does not exist. The energy range of the publicized state of motion. The highest energy and most important energy bands of semiconductors are the valence band and conduction band. The energy difference between the bottom of the conduction band and the top of the valence band is called the forbidden band width (or band gap or energy gap).

use

Although there are no energy levels in the forbidden band that belong to all public electrons in the entire crystal, energy levels in non-public states such as impurities and defects - bound energy levels - can appear. For example, donor energy level, acceptor energy level, recombination center energy level, trap center energy level, exciton energy level, etc. By the way, these bound energy levels can not only appear in the forbidden band, but can also appear in the conduction band or valence band, because these energy levels do not originally belong to the energy bands that characterize the public electronic state of the crystal. .

physical meaning

The bandgap width is an important characteristic parameter of semiconductors. Its size is mainly determined by the energy band structure of the semiconductor, which is related to the crystal structure and the bonding properties of atoms.

A large number of electrons in the valence band of semiconductors are electrons on valence bonds (called valence electrons) and cannot conduct electricity, that is, they are not carriers. Only when the valence electrons transition to the conduction band (i.e. intrinsic excitation) and generate free electrons and free holes can conduct electricity. Holes are actually valence bond vacancies left after valence electrons transition to the conduction band (the movement of a hole is equivalent to the movement of a large group of valence electrons). Therefore, the size of the forbidden band width is actually a physical quantity that reflects the degree to which valence electrons are bound, that is, the minimum energy required to generate intrinsic excitation.

Electrons and holes as carriers are in the conduction band and valence band respectively; generally, electrons are mostly distributed near the bottom of the conduction band (the bottom of the conduction band is equivalent to the potential energy of electrons), and holes are mostly distributed near the top of the valence band (The top of the valence band is equivalent to the potential energy of the hole). The energy above the bottom of the conduction band is the kinetic energy of the electron, and the energy below the top of the valence band is the kinetic energy of the hole.

Influencing factors

The semiconductor bandgap width is related to temperature and doping concentration: the semiconductor bandgap width can change with temperature, which is a weakness of semiconductor devices and their circuits (but in some applications this is an advantage). The bandgap of a semiconductor has a negative temperature coefficient. For example, the bandgap width of Si is 1.17eV when extrapolated to 0K, and drops to 1.12eV at room temperature. If many isolated atoms are combined to form a crystal, an atomic energy level simply corresponds to an energy band. Then when the temperature increases, the volume of the crystal expands, the atomic spacing increases, and the energy band width narrows, then the band gap width will increase, so the temperature coefficient of the band gap is positive.

However, for commonly used semiconductors such as Si, Ge, and GaAs, when atoms are combined to form crystals, the valence bonds will undergo so-called hybridization (mixing of s-state and p-state - sp3 hybridization), resulting in an atomic energy level It does not simply correspond to an energy band. Therefore, when the temperature increases, the atomic spacing of the crystal increases, and although the energy band width becomes narrower, the forbidden band width decreases - a negative temperature coefficient.

When the doping concentration is very high, the bandgap width may become narrower due to the appearance of impurity energy bands and band tails.

The impact of the bandgap width on the performance of semiconductor devices is self-evident. It directly determines the withstand voltage and maximum operating temperature of the device; for BJT, when the bandgap width becomes narrower in the emitter region due to high doping, it will causing the current gain to be greatly reduced.

The atomic number of Si is smaller than that of Ge, so the valence electrons of Si are tightly bound, so the bandgap width of Si is larger than that of Ge. The valence bonds of GaAs are also polar, binding the valence electrons more tightly, so the bandgap width of GaAs is larger. The bandgaps of so-called wide-bandgap semiconductors such as GaN and SiC are much larger because their valence bonds are more polar. The bandgap widths of Ge, Si, GaAs, GaN and diamond are 0.66eV, 1.12 eV, 1.42 eV, 3.44 eV and 5.47 eV respectively at room temperature.

Diamond is an insulator under normal circumstances, because the atomic number of carbon (C) is very small, and its binding effect on valence electrons is very strong. Valence electrons cannot escape from the constraints of valence bonds under normal circumstances, so the band gap width is very large, at room temperature. No carriers can be produced, so it does not conduct electricity. However, it also exhibits semiconductor characteristics at high temperatures of hundreds of degrees, so it can be used to make transistors with operating temperatures as high as 500 degrees Celsius or more.

Summary: By learning the use, physical meaning and influencing factors of the wide bandgap, we can further understand the significance of the existence of the wide bandgap. The above is an explanation of some technical information about the wide bandgap of semiconductors.