pyroelectric. In the ferroelectric materials, with the application of an electrostatic field, the

polarization can be inverted, while in the pyroelectrics not, where the polarization is par­

allel to the low symmetry axis of the crystal [19]. Thus, III-nitrides are pyroelectric materials

with spontaneous polarization. This property is naturally intrinsic and it corresponds to

the bonding nature, where the geometric center of the negative and positive charges of the

crystal are not coincident. In other words, the bonding between the atoms in these com­

pounds should be asymmetric, and usually, in the hexagonal crystals, the bond along the

c-axis shows to be longer and with different ionicity compared to other ones. On the other

hand, the cubic crystal structure with tetrahedral coordination shows four equivalent bonds

due to the sp³ hybridization, explaining why most of the binary III–V and II-VI semi­

conductors do not have spontaneous polarization, only a piezoelectric effect.

When a mechanical strain induces a change in the electrical resistivity of a material this

phenomenon is denoted by the piezoresistive effect, and the SiC compounds are the most

favorable choice among the WBG materials. The piezoresistance also has a strong de­

pendence on the crystal orientation, and it is quantified by the parameter gauge factor

(GF). This change of resistance in response to applied stress is a function of geometry and

resistivity changes.

In other words, the GF is the ratio of the per unit change in the resistance to the per unit

change in length. The gauge factor for SiC compounds is considerable, where 3C-SiC

showed to be ca. 30 for both p-type and n-type, increasing the importance of this class of

WBG semiconductors for applications in bioelectronics, such as radiofrequency wireless

communication devices [2].

13.3.2 Direct Bandgap and High Optical Transmittance

Semiconductors can have direct or indirect band gaps. In semiconductors with a direct

bandgap, the momentum of the electrons in the highest states of the valence band is

practically the same as the momentum of the holes in the lowest states of the conduction

band. In this case, the optical transitions occur right after the photon energy exceeds the

bandgap energy, and it is observed that the absorption coefficient increase together with the

photon energies. On the other hand, in the materials with indirect bandgap, the momentum

of the electrons in the valence band does not match the momentum of the holes in the

conduction band, requiring an additional emission of a phonon (collective motion of atoms

in a crystal) for change the electron momentum, and allowing the recombination.

The II–VI and III-nitride materials are known for their direct band gaps, allowing their

application in UV photosensors and optogenetics LED [2]. The possibility of synthesizing

different nanostructures of ZnO enables tuning and broadening its emission and ab­

sorption wavelength. Further in ZnO nanostructures, combining the direct bandgap and

the large optical absorption can result in an efficient and sensitive photodetector over the

UV spectral range [2]. Additionally, III-nitrides and their alloys (e.g., InAlN and AlGaN)

also exhibit a tunable bandgap that can vary from the visible spectrum to infrared,

highlighting the GaN-based materials that show to be an efficient UV photodetector [2].

Materials with a bandgap greater than ca. 3.1 eV usually is considered transparent [1].

Optical transmittance can be defined as the ability for light to be conducted through a

material. Therefore, a material with high optical transmittance is essential for applications

in optoelectronics, including in optogenetics. However, it is important to keep in mind

that transmittance is dependent on the thickness, which can result in different accessible

dynamic ranges for measurements of the absorption coefficient [1]. Some WBG materials

have shown good transparency, such as ZnO [20]. From the III-nitride family, bulk GaN

Wide Bandgap Semiconductors

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