Kazuo Eda

Conduction mechanism of non‐Ohmic zinc oxide ceramics

The conduction mechanism of non‐Ohmic ZnO ceramics is investigated. In order to explain the non‐Ohmic property, a new energy‐band model composed of a thin intergranular layer with traps sandwiched between Schottky barriers formed opposite each other is proposed. According to the newly proposed energy‐band model, the non‐Ohmic property of ZnO ceramics is mainly governed by field emission for the reverse‐biased Schottky barrier in the voltage region above the threshold voltage in the V‐I curve and by thermionic emission in the voltage region below the threshold voltage. The energy‐band model and the conduction mechanism discussed in this paper are appropriate for the explanation of experimental results, not only those presented in previous papers, such as the effect of additives, the V‐I curve and its temperature dependence, but, additionally, the dielectric properties, the asymmetrical degradation of the V‐I curve, and the thermally stimulated current.

Degradation mechanism of non‐Ohmic zinc oxide ceramics

The degradation phenomena caused by dc and ac biasing in non‐Ohmic ZnO ceramics are studied from the viewpoints of voltage (V)‐current (I) characteristics, dielectric properties, and thermally stimulated current (TSC). As a result, it is concluded that the degradation caused by dc biasing is attributed to the asymmetrical deformation of Schottky barriers, due to ion migrations in Bi2O3‐rich intergranular layers and in the depletion layers of the Schottky barriers; and that the degradation caused by ac biasing is attributed to the symmetrical deformation of the Schottky barriers, due to ion migration in the depletion layers of the Schottky barriers. Also, the relationship between the thermal runaway life of non‐Ohmic ZnO ceramics and biasing conditions, such as biasing temperature and bias voltage, is obtained.

Destruction mechanism of ZnO varistors due to high currents

The destruction process of ZnO varistors caused by high currents is described. When high currents were applied to ZnO varistors, two kinds of destruction modes were found: a cracking mode and a puncture mode. The puncture mode is caused by melting of the region where current concentrates. The puncture process was simulated by a computer, solving a set of heat transfer equations. The relationships among the destruction energy, uniformity of the sintered body and nonohmic I‐V characteristics were obtained. The simulated results are consistent with the experimental results and show that the destruction energy depends strongly on the uniformity.

Grain growth control in ZnO varistors using seed grains

A method for making low‐voltage ZnO varistors having extraordinarily large grain size without necessity of sintering at much higher temperatures or for much longer times is reported. The device is made by sintering a mixture of ZnO fine powder, additives, and ZnO seed grains having grain sizes of 63–105 μm obtained by washing a ZnO sintered body containing BaO in boiling water. The device has a grain size around 500 μm and low threshold voltage of 6 V/mm. Such anomalous grain growth is caused by the difference between the radii of curvature of ZnO fine powder and ZnO seed grains.

Transient conduction phenomena in non‐Ohmic zinc oxide ceramics

Transient conduction phenomena in non‐Ohmic ZnO ceramics are studied. Several characteristic phenomena are observed by applying a rectangular‐wave‐pulse voltage, such as the gradual increase of current, the voltage overshoot, the current overshoot, and the current oscillation. In order to clarify these phenomena, the experiments of the effect of additives, the temperature dependence, and the effect of preinjection of carriers have been carried out. These phenomena of gradual increase of current and voltage overshoot, current overshoot, and current oscillation can be reasonably explained by the electron trapping in the intergranular layer, by the delay in the formation of conduction electrons, and by the generation of the group of excess electrons in the intergranular layer, respectively.

A novel temperature compensation method for SAW devices using direct bonding techniques

We have developed a novel temperature compensation method for SAW devices using a direct bonding technique. The SAW device applied this method was composed of a conventional piezoelectric substrate such as LiTaO/sub 3/ and LiNbO/sub 3/, directly bonded without any bonding agents to a glass substrate having a relatively low thermal expansion coefficient (TEC). The piezoelectric substrate and the glass substrate were bonded in an atomic scale and the interface was very uniform. Therefore, the thermal strain at the surface of the bonded substrate caused by the difference between the TECs of the substrates was quite uniform and stable. With this structure, the thermal expansion of the piezoelectric substrate was restrained and the elastic constant of the piezoelectric substrate was changed by the thermal strain. Using this technique, we have succeeded to improve the temperature coefficient of frequency (TCF) of the SAW devices without causing any deterioration in the frequency response. This novel temperature compensation method is very promising for RF-SAW device applications.

DIRECT BONDING OF LINBO3 SINGLE CRYSTALS FOR OPTICAL WAVEGUIDES

A new fabrication method of optical waveguides using direct bonding of lithium niobate single crystals without using any bonding agents has been developed. The bonded interface was found very uniform and the bonding was performed in an atomic scale in spite of a relatively low heat‐treatment temperature of 400 °C. It was confirmed to be possible to fabricate optical waveguides using the direct bonding. This method is very attractive to obtain high‐performance optical guided‐wave devices because of its versatility.

Direct bonding of piezoelectric crystal onto silicon

A method for bonding a piezoelectric crystal directly onto silicon, without any bonding agents, is reported. The interface microstructure, procedures of fabricating a lithium tantalate (LiTaO3■‐ on‐silicon resonator, and its resonant characteristics are described. This technique is very promising for miniaturizing electroacoustic integrated devices.

AlGaAs/GaAs heterojunction bipolar transistors with small size fabricated by a multiple self-alignment process using one mask

A multiple self-alignment process for HBTs using one mask is developed to form emitters, emitter contacts, emitter contact leads, buried small collectors, base contacts, and base contact leads. This process makes it possible to produce HBTs of very small size and to reduce parasitic elements. An AlGaAs/GaAs HBT fabricated by the process, with an emitter 1 × 20/µm2in size and a buried collector by O+implantation gives a good performance of ft= 54 GHz and fmax= 42 GHz. The performance may be explained by the reduction of parasitic elements, base transit time, and collector depletion layer transit time.

Photoluminescence in AlGaAs/GaAs heterojunction bipolar transistors

Photoluminescence in AlGaAs/GaAs heterojunction bipolar transistors (HBTs) with heavily doped GaAs layers and a p+‐GaAs/n‐AlGaAs heterojunction grown by molecular beam epitaxy at a low temperature of 600 °C is discussed. Each photoluminescence signal in the HBT is identified, and the relationship between performance as a transistor and photoluminescence discussed. A novel photoluminescence signal related to the p+‐GaAs/n‐AlGaAs heterojunction in the HBT was identified. This signal is thought to originate in the transition of the two‐dimensional electrons in the notch formed at the heterojunction to the acceptor and acceptor‐related defect levels.