Semiconductor material band gap know how much

What is a semiconductor material band gap?

In an atom or molecule, electrons usually occupy different energy levels. In solids, however, these energy levels coalesce to form bands. The energy band is divided into valence band and conduction band. The electrons used to hold the atoms together are in the valence band, while the remaining unpaired electrons are in the conduction band [1].

The gap between the conduction and valence bands is called the band gap. It is  the minimum energy required to excite an electron to jump from the valence band  (bound state) to the conduction band (free state).It is the size of the band gap  that  has a great effect on some properties of the material. For example, if enough energy is given to an electron used to form a bond, it can be excited to jump to the conduction band and thus conduct electricity freely.

How to classify materials according to band gap?

For conductors, the gap between the conduction band and the valence band is zero, so the conduction band is full of electrons and the material is highly conductive. On the contrary, there is a large band gap between the valence band and the conduction band of an insulator, which makes it almost impossible for electrons in the valence band to jump into the conduction band, and the material is manifested as non-conductive. The band gap of semiconductor is between the two, which is generally reflected as non- conductive, but through the form of energy injection (photoexcitation, thermal excitation, etc.), the electrons in its valence band can be made to transition, thus possessing conductive ability.

Figure 1: Schematic diagram of the energy band of conductors, semiconductors and insulators

According to the energy and momentum conditions required for the electron transition, semiconductors can be further classified into direct and indirect bandgap semiconductors. In a direct bandgap semiconductor, only energy needs to be absorbed for electrons to jump to the conduction band to produce conducting electrons and holes (forming a half-full band). The representatives are GaN (project number: G119228), GaAs (project number: G119227), InAs (project number: I119233), GaSb (project number: G119230) and other  III-V  semiconductors. For indirect bandgap semiconductors, the transition of electrons to the conduction band to produce conducting electrons and holes (forming a half-full band) requires not only the absorption of energy, but also the change of momentum. Represented by Si (project number: S108980), Ge (project number: C105165) and other elemental semiconductors.

Figure 2: Schematic diagram of photoluminescence of different types of Semiconductors[2]

At different temperatures, the band gap values of semiconductor materials will vary. The following table shows the band gap values of common semiconductor materials [3]:

Material	Symbol	Eg （eV）
		T = 0 K	T = 300 K
Silicon	Si	1.17	1.11
Germanium	Ge	0.74	0.66
Indium antimonide	InSb	0.23	0.17
Indium arsenide	InAs	0.43	0.36
Indium phosphide	InP	1.42	1.27
Gallium nitride	GaP	2.32	2.25
Gallium arsenide	GaAs	1.52	1.43
Gallium antimonide	GaSb	0.81	0.68
Cadmium selenide	CdSe	1.84	1.74
Cadmium telluride	CdTe	1.61	1.44
Zinc oxide	ZnO	3.44	3.2
Zinc sulfide	ZnS	3.91	3.6

How to calculate the band gap of semiconductor materials?

Generally speaking, there are two ways to judge the band gap size of semiconductor materials:

Method one: transversal method. Trans-section method is a simple method to calculate the band gap width of semiconductor, based on the principle of semiconductor absorption threshold λg and its band gap Eg Inversely proportional, the relations hip between the two is as follows:

Eg (eV) =1240/λg (nm)

The absorption of materials at different wavelengths can be obtained from UV-vis DRS (UV-visible diffuse reflect ion) spectra. The wavelength-absorption curve is  differentiated once, and then a transversal is made at the extreme point (slope is the value of the vertical coordinate of the extreme point), and the  intersection  point  between the transversal and the horizontal coordinate is λg. The band gap width E of the material can be obtained by substituting the above equation.

Method two:  Tauc plot method. This method was developed by Tauc, Davis and Mott [4] The specific expression is as follows:

(αhv)1/n=A (hv-Eg) Hv = hc/lambda

Where, α is the absorption index, h is Planck's constant, c is the speed of light, and λ is the wavelength of light.

V is frequency, A is the constant and Eg is the band gap width of the semiconductor. The exponential n is related to the type of semiconductor: where the direct bandgap semiconductor is 1/2 and the indirect bandgap semiconductor is 2.

It should be noted that the ordinate of UV-vis DRS spectrum read should be the absorption value Abs. If the transmittance is T%, it can be converted by the formula Abs=-lg (T%).

Applications of wide-band gap semiconductors

Wide band gap (WBG or "wide band gap") semiconductors are key to a variety of electronic devices, such as transparent contacts, p-n junctions, and thin film transistors. Since the 1950s, oxide wideband  gap  semiconductors  have  been extensively studied, especial y for their contradictory properties of high transparency

and high conductivity. Transparent conductive oxide (TCO) doped with tin In2O3, ITO, has been a key possible alternative to various commercial devices over the past few decades, such as F-doped SnO2(FTO)[5]al doped ZnO (AZO) [6], has been deeply studied and applied.

In the 21st century, multi-transparent amorphous oxide semiconductors (TAOS), such as indium gal ium zinc oxide (IGZO), are also widely studied as channel layers in thin film transistors (TFT) due to their high transparency, high mobility, and good uniformity [7] Leading to its commercial application in liquid crystal display (lcd).

Figure 3: Flexible TTFTs a) TTFT structures prepared on plastic sheets; b) Photos of flexible TTFT sheet bent to R=30 mm; c) Photos of flexible TTFT sheet [7]

Although these N-type TCOs exhibit excel ent properties, P-type doping and high pore mobility in oxides have proved much more difficult in practice. This is mainly

due to (1) the inherent limitations of locating valence band holes in dispersion and (2) the challenges of introducing holes and minimizing compensation defects through P- type doping. Decalcified CuAlO2As a preliminary example, the proposed strategy of "valence band chemical modulation" (CMVB) has made a breakthrough [8]. This method uses the hybridization of the O 2p state with the metallic 3d state at the valence band maximum (VBM), increasing the dispersion. Subsequent use of this strategy and other methods has led to a variety of copper-based P-type TCOs, although the photoelectric properties of P-type TCOs are still not comparable to those of N-type TCOs. Due to these challenges, only a small number of P-type TCO

materials have been incorporated into commercial device applications, such as Cu2O and SnO, which are mainly used in thin film transistors.

References

[1] Kittel C. and McEuen P. （2019）, Introduction to Solid State Physics, 8th Ed. Hoboken, NJ: John Wiley & Sons.

[2] Tsagli K, Dordevic S V. Temperature Dependence of Photoluminescence Spectra in Polystyrene[J]. Materials Performance and Characterization, 2020, 9（1）: 675-681. https://www.astm.org/mpc20200093.html

[3] Semiconductors W B. Fundamental Properties and Modern Photonic and Electronic Devices/Eds[J]. K. Takahashi, A. Yoshikawa, A. Sandhu. New York, 2007.

[4] Tauc J, Menth A. States in the gap[J]. Journal of non-crystalline solids, 1972, 8: 569-585. https://doi.org/10.1016/0022-3093（72）90194-9

[5] Rakhshani A E, Makdisi Y, Ramazaniyan H A. Electronic and optical properties of fluorine-doped tin oxide films[J]. Journal of applied physics, 1998, 83（2）: 1049-1057.

https://doi.org/10.1063/1.366796

[6] Jiang X, Wong F L, Fung M K, et al. Aluminum-doped zinc oxide films as transparent conductive electrode for organic light-emitting devices[J]. Applied Physics Letters, 2003, 83（9）: 1875-1877. https://doi.org/10.1063/1.1605805

[7] Nomura K, Ohta H, Takagi A, et al. Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors[J]. nature, 2004, 432（7016）: 488-492. https://www.nature.com/articles/nature03090?free=2

[8] Kawazoe H, Yasukawa M, Hyodo H, et al. P-type electrical conduction in transparent thin films of CuAlO2[J]. Nature, 1997, 389（6654）: 939-942. https://www.nature.com/articles/40087