13.3 Fundamental Concepts and Properties of Wide Bandgap

Semiconductors

Each family of WBG semiconductors has a notable chemical, electrical, and optical char­

acteristics, and therefore the materials should be chosen according to the properties that are

required for each application in implantable and wearable devices. Table 13.1 exhibits

the typical properties of common WBG materials. These properties could be changed and

improved with the formation of tertiary and quaternary alloys, or with the addition of

dopants, allowing to form of p-type or n-type materials. Knowing the basics of the main

properties of the semiconductors is fundamental for later understanding how these prop­

erties will make them efficient for bioelectronics applications. Thus, in this section, basic

concepts of the main properties of WBG materials will be present.

13.3.1 Piezoelectric Effect, Piezoelectric Polarization, and Piezoresistive Effect

Discovered by Jacques and Pierre Curie in 1880, the piezoelectric effect is the ability of a

material to generate an electric field (or electric displacement) as a response to mechanical

stress (or strain). G. Lippmann in 1881 predicted that the piezoelectric effect is a reversible

process, and later in 1882, the Curies confirmed it experimentally. Therefore, a reverse

piezoelectric effect is the generation of a mechanical strain in response to an electrical

charge application.

The linear electromechanical interaction can only be observed in a crystalline com­

pound with no inversion symmetry, thus it is an anisotropic material such as the wide

bandgap compounds with WZ and ZB crystal structures. When the pressure is applied to

these crystals, the molecules will re-align, generating a voltage across the crystal along the

c-axis. Thus, the WBG materials with piezoelectric properties are pressure-sensitive for

application in mechanical sensing in the context of bioelectronics. Additionally, the low

dielectric losses and high breakdown field of AlN and ZnO (Table 13.1) make these

piezoelectric semiconductors of high interest to researchers for applications in high

temperatures [7].

The III-nitrides WBG semiconductors show excellent piezoelectric properties, especially

AlN [18]. Besides it, the III-nitride compounds also show spontaneous polarization.

The spontaneous polarization occurs in two types of materials, the ferroelectric and

FIGURE 13.3

Bilayers stacking sequence of 6H-Sic, 4H-SiC and 3C-SiC.

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Bioelectronics