Rutherford backscattering, Van der Pauw and TEM measurements were used to characterise the annealing behaviour of antimony implanted LPCVD polysilicon. High electrical activity without dopant redistribution was obtained for 600°C annealing of 1 × 1015 cm−2 and 3×1015 cm−2 antimony implanted samples. Subsequent annealing at temperatures ≥900° C resulted in considerable grain-boundary-assisted redistribution of antimony within the polycrystalline layers and associated changes in sheet resistance. Our results suggest that the sheet resistance of the films is controlled by dopant segregation at grain boundaries and the fraction of antimony distributed on active sites within individual grains.

We demonstrate, for the first time, a technique for the fabrication of polysilicon resistors with excellent control and temperature characteristics: a standard deviation of 2.5 Ω/□ and temperature variation of -5.6% from 25° to 125°C for 1.96kΩ/□, 4.5 × 1014 cm˗2 boron implanted polysilicon resistors. The technique is based on fabricating resistors in laser recrystallized polysilicon on SiO2. Resistor performance is compared to that of similar resistors fabricated in standard LPCVD polysilicon and in single crystal silicon.

The theory of ZnO varistors has evolved along with the increasingly detailed experimental description [1-2]. The extreme nonlinearity in the currentvoltage curves naturally led the early investigators to suggest the well-known nonlinear mechanisms, such as space-charge limited currents or electron tunneling. Later experiments made these first proposals untenable [3-9]. Recently a second generation of theories has been proposed, with many different mechanisms suggested for the extreme nonlinearity [10-15]. Here we wish to review these different theories, and also the experiments which bear upon their validity.

The paper resumes essential grain boundary phenomena which were found experimentally by microcontact measurements on adjacent grains. The results suggest a ZnO homojunction model. Calculations of defect equilibria in the Schottky defect model show that homojunctions can be formed during the cooling period under the influence of an extrinsic donor species by a shift of the bulk defect equilibrium and intersecting diffusion profiles of oxygen- and zinc vacancies near the grain boundary.

Recent experiments indicate that heterogeneous Bi2O3-segregation layers with a dissolved zinc-antimony-oxide compound are transformed to ZnO epitaxial layers plus a fine grained pyrochlore phase of zinc-bismuth-antimony oxide during the cooling period. Thus epitaxial ZnO layers eliminate thin Bi2O3-films and form homogeneous connections of adjacent grains.

A new varistor based on TiO2 ceramic was developed. TiO2 ceramics with the dopant ranges of 0 < Nb < 2% and 0 < Ba < 2% were prepared and their electrical properties were measured. In these dopant ranges, several compositions were discovered to have useful varistor properties with a voltagecurrent nonlinearity index of α > 3. It was found that Ba dopant which segregates at grain boundaries and an oxidizing atmosphere during cooling are necessary to produce varistor properties in (Nb,Ba)-doped TiO2. Nb was introduced to decrease the TiO2 lattice resistivity to a range useful for device applications. Resistivity data were measured as functions of dopant composition and sintering atmospheres. Data were analyzed in terms of the defect structure in TiO2.

The electronic properties of TiO2 surfaces are of interest to further understanding of grain boundary effects in TiO2 and ZnO based varistor devices. Schottky barriers have been fabricated by Au or Al metallization on Nb doped TiO2-rutile [001] single crystal surfaces. Surface conditions included the presence or absence of either mechanical polishing induced surface defects and/or adsorbed oxygen. I­V measurements on Au samples indicate that surface defects yield ohmic behavior, but the defects may be electrically compensated by adsorbed oxygen to give non-ohmic I­V curves. All Al junctions show ohmic behavior which is interpreted in terms of reaction with surface oxygen. These results explain the necessity of firing varistor materials in an oxidizing atmosphere and suggest that the role of additives in these materials is to stabilize the oxygen concentration at the grain boundaries.

Much of the research on ZnO varistors has concentrated on the explanation of their dc current-voltage characteristics. However, varistors also have unusual ac properties which can be technologically important, and must be described by any comprehensive model. In an ideal varistor with identical grain boundaries throughout, there should be no dispersive capacitance at zero bias. In real varistors this capacitance varies considerably with frequency. This dispersion has two causes, charge trapping in the depletion regions and differing grain boundary barriers. Calculations for each process are given. For voltages well below the varistor breakdown value, the low frequency capacitance increases with applied voltage. At even higher voltages the capacitance turns over and becomes negative. All of these effects can be described with a double depletion layer/thermionic emission model. The anomalous capacitance behavior with bias is due to the modulation of the potential barriers by charge trapping at the grain boundaries. In the varistor breakdown regime minority carriers created by impact ionization are important.

Zinc Oxide (ZnO) Ceramics-Bismuth Oxide (Bi2O3) Metal Oxide thin film heterojunction made by sputtering technique showed a highly non-Ohmic property. The voltage-current characteristics and the dielectric properties showed dependence on Bi2O3 metal oxide thin film thickness.

In this paper after reviewing and discussing the electrical properties of ZnO varistors, the role of intergranular layers in the ZnO varistor is discussed based on experimental results with the heterojunction.

A reversible change in barrier voltage is observed in assintered ZnO varistors upon application and removal of the applied voltage. This is attributed to a metastable component in the grain boundary Schottky barriers. Varistors show improved stability when the metastable component is removed or fixed by heat treatment.

The effect of ambient gases on degradation and rejuvenation of ZnO varistors was investigated from the viewpoint of voltage-current characteristics. Ambient oxygen gas suppressed increases in the leakage current during ac biasing, while it rejuvenated degraded samples. The time dependence of rejuvenation indicated that ion migrations, possibly the desorption and adsorption of oxygen ions at the grain boundaries, occur reversibly in degradation and rejuvenation. Moreover, a noticeable difference was observed between V–I curve shapes after ac biasing in nitrogen and in oxygen atmospheres.

The conditions leading to thermal runaway in metal oxide varistors are discussed in terms of its power dissipation. An equivalent circuit is used to rationalize the similarities and differences between the DC and 60 Hz ac behavior.