This paper deals with the application of ZnO varistors—an area which has not been treated in a systematic way in the literature. The paper starts with a brief description of the fundamental properties comprising the electrical behavior as well as the physics, chemistry, and microstructure of the varistor. These properties then form the basis for defining the application parameters that are directly related to the nonlinear current-voltage characteristics of the varistor. This paper provides a detailed description of these parameters and their relation to microstructure and the processing of the varistor. Finally, a discussion is presented on the reliability of the varistor by considering a grain-boundary defect model which explains both the instability and the stability under use conditions.

Application of Zinc Oxide Varistors

The physics of the growth mechanisms, characterization of epitaxial structures and device properties of GaAs and other compound semiconductors on Si are reviewed in this paper. The nontrivial problems associated with the heteroepitaxial growth schemes and methods that are generally applied in the growth of lattice mismatched and polar on nonpolar material systems are described in detail. The properties of devices fabricated in GaAs and other compound semiconductors grown on Si substrates are discussed in comparison with those grown on GaAs substrates. The advantages of GaAs and other compound semiconductors on Si, namely, the low cost, superior mechanical strength, and thermal conductivity, increased wafer area, and the possibility of monolithic integration of electronic and optical devices are also discussed.

Gallium arsenide and other compound semiconductors on silicon

Polar-on-nonpolar epitaxy

Advanced epitaxial growth techniques permit the use of pseudomorphic Si/sub 1-x/Ge/sub x/ alloys in silicon technology. The smaller bandgap of these alloys allows for a variety of novel band-engineered structures that promise to enhance silicon-based technology significantly. The authors discuss the growth and properties of pseudomorphic Si/sub 1-x/Ge/sub x/ structures and then focus on their applications, especially the Si/sub 1-x/Ge/sub x/-base heterojunction bipolar transistor (HBT). They show that HBTs in the Si/sub 1-x/Ge/sub x/ system allow for the decoupling of current gain and intrinsic base resistance. Such devices can be made by using a variety of techniques, including molecular-beam epitaxy and chemical vapor deposition. The authors describe the evolution of fabrication schemes for such HBTs and describe the DC and AC results obtained. They show that optimally designed HBTs coupled with advanced bipolar structures can provide performance leverage. >

Heterojunction bipolar transistors using Si-Ge alloys

A new sno2-based varistor system

For the formation of nonohmic ZnO ceramics based on the system ZnO-Bi2O3-Sb2O3, the reactions on heating (→) and on slow cooling (←), 2PY+17ZnO\xleftarrow\xrightarrow950°~1050°C3Xzso+3Bi2O3 [L] \ding"C0, and Xzso\xleftarrow\xrightarrow\gtrsim1300°CSP \ding"C1, play important roles, where PY is Zn2Bi3Sb3O14 pyrochlore, Xzso a polymorph of Zn7Sb2O12 [SP] and L is a liquid. Oxide additives markedly influence these reactions and consequently the phases and microstructure of the ceramics, dissolving into the above phases and relating to composition and sintering conditions. In the case of Co, Mn and Cr oxides, the degree of influence is as follows : Co<Mn\lesssimCo+Mn\lesssimCr<Co+Mn+Cr \ding"C2. On heating, those reactions occur at lower temperatures in this order, and finally for Cr oxide the Xzso is not formed. On slow cooling after the SP is formed, the reaction \ding"C1 and then \ding"C0 partially occur only for Co oxide; for the others, the reaction, 3SP+3Bi2O3 [L]→2PY+17ZnO, occurs only partially and the degree of this reaction becomes smaller in that order \ding"C2; the nonreacting liquid crystallizes into Bi2O3-rich phases. On quenching, only the crystallization of the liquid occurs. The formation of various Bi2O3-rich phases is related to composition change of the liquid and cooling manner.

https://iopscience.iop.org/article/10.1143/JJAP.19.409/pdf

Formation Mechanism of Nonohmic Zinc Oxide Ceramics

The molecular‐beam epitaxial growth of GaP on Si was investigated, with the aim of at least approaching device‐quality interfaces. Gallium‐primed growth on (211)‐oriented substrates yielded layers which were free of antiphase domains, and which were of much higher quality than growths on other orientations. A tentative energy‐band lineup is proposed, which is consistent with the electrical data. Heterojunction bipolar transistors were fabricated with emitter injection efficiencies up to 90%, in spite of indications that the epitaxial emitter layer was far less heavily doped than the base.

Molecular beam epitaxial growth of GaP on Si

The crystal phases of nonohmic ZnO ceramics, having Bi2O3, Sb2O3, Co2O3, MnO2 and Cr2O3 as additives, have been investigated by classifying and clarifying them as a function of the amounts of the additives and sintering temperature, and synthesizing their single phases. These ceramics consist of ZnO phase dissolving Co and Mn components, spinel phase of Zn7Sb2O12 dissolving Co, Mn and Cr components, and Bi-rich phases (B, C, D or B'). The B-phase is a δ-Bi2O3 type phase (a=5.59 A) having a fundamental composition of 12Bi2O3Cr2O3. The C-phase is a tetragonal phase (a=7.76 A, c=5.75 A) having a fundamental composition of 14Bi2O3Cr2O3. The D-phase is a β-Bi2O3 type tetragonal phase (a=10.93 A, c=5.62 A) dissolving a large amount of Zn component and a small amount of Sb component. The B'-phase is also a δ-Bi2O3 type phase (a=5.48 A) formed in the system Bi2O3–ZnO–Sb2O3. A solid solution between B and B'-is formed. The nonohmic property of ZnO ceramics is improved with the changes of phases, B→C→D or D+B', by raising the sintering temperature. The extinction of the nonohmic property caused by the change in the amounts of additives and sintering temperature corresponds to that of ZnO or Bi-rich phases in the ceramics.

https://iopscience.iop.org/article/10.1143/JJAP.17.1/pdf

Crystal Phases of Nonohmic Zinc Oxide Ceramics

The existing and distributing states of crystal phases in the nonohmic zinc oxide ceramics with five additives, 0.5Bi2O3, 1.0Sb2O3, 0.5Co2O3, 0.5MnO2 and 0.5Cr2O3 (mol.%) were investigated by using various analytical techniques. The ceramics consist of the ZnO and spinel phases and mixed phases of β-and δ-Bi2O3. The ZnO phase corresponds to the main constituent grains, which are single crystals 10 to 20 microns in size with a twinning structure and randomly orientated. The spinel phase corresponds to single crystal grains a few microns in size, which are discontinuously located along comparatively wide grain boundaries. The Bi2O3 phases correspond to continuously spreading intergranular layers, which separate the ZnO grains from each other. The width of the interanular layers in the narrow grain boundaries between two adjacent ZnO grains is less than 500 A.

https://iopscience.iop.org/article/10.1143/JJAP.17.673/pdf

Microstructure of Nonohmic Zinc Oxide Ceramics

An investigation was carried out of the effects of heat-treatment (h.t.) on the crystal phases, microstructure, and nonohmic properties of ZnO ceramics consisting of ZnO containing dissolved Co and Mn; Zn7Sb2O12 spinel (SP) containing dissolved Co, Mn, and Cr; and β, δ-Bi2O3 containing dissolved Zn and Sb. The properties change with h.t. temperature, and the changes are closely related to each other. The nonohmic exponent α decreases greatly at 700° to 800°C, where the intergranular (i.g.) layer is coarsened by the phase transformation of β, δ-Bi2O3 into γ-Bi2O3. The α-value again changes abruptly and almost recovers at 800° to 900°C, where the dense i.g. layer is reconstructed from the Bi2O3-rich liquid (B.r.l.) formed by the melting of γ-Bi2O3. The α-value gradually recovers at 900° to 1100°C with the decrease of Cr in δ-Bi2O3 solid solution (δ-S.S.) and the decrease of Zn2Bi3Sb3O14 pyrochlore (PY) by reaction with ZnO. The formation of δ-S.S. and the decrease of Cr in it are brought about by the movement of Cr from the SP to the B.r.l. around 900°C and from the B.r.l. to the SP above 900°C, respectively. The PY is formed by partial reaction between the SP and the B.r.l.around 900°C. The phases, microstructure, and α-value recover completely around 1100°C.

Masanori Inada

Papers

Formation Mechanism of Nonohmic Zinc Oxide Ceramics

Molecular beam epitaxial growth of GaP on Si

Crystal Phases of Nonohmic Zinc Oxide Ceramics

Microstructure of Nonohmic Zinc Oxide Ceramics

Effects of Heat-Treatment on Crystal Phases, Microstructure and Electrical Properties of Nonohmic Zinc Oxide Ceramics