T. Lalinský

AlGaN/GaN metal?oxide?semiconductor heterostructure field-effect transistors with 4 nm thick Al2O3 gate oxide

AlGaN/GaN metal?oxide?semiconductor heterostructure field-effect transistors (MOSHFETs) with 4 nm thick Al2O3 gate oxide were prepared and their performance was compared with that of AlGaN/GaN HFETs. The MOSHFETs yielded ~40% increase of the saturation drain current compared with the HFETs, which is larger than expected due to the gate oxide passivation. Despite a larger gate-channel separation in the MOSHFETs, a higher extrinsic transconductance than that of the HFETs was measured. The drift mobility of the MOSHFETs, evaluated on large-gate FET structures, was significantly higher than that of the HFETs. The zero-bias mobility for MOSHFETs and HFETs was 1950 cm2 V?1 s?1 and 1630 cm2 V?1 s?1, respectively. These features indicate an increase of the drift velocity and/or a decrease of the parasitic series resistance in the MOSHFETs. The current collapse, evaluated from pulsed I?V measurements, was highly suppressed in the MOSHFETs with 4 nm thick Al2O3 gate oxide. This result, together with the suppressed frequency dispersion of the capacitance, indicates that the density of traps in the Al2O3/AlGaN/GaN MOSHFETs was significantly reduced.

The improved performance of GaAs micromachined power sensor microsystem

Thermal effects in a 2-μm thick GaAs cantilever beam of the power sensor microsystem are investigated. The increased thermal sensitivity of the microsystem to the thermal conductance changes of the ambient atmosphere is evaluated by the experiment and simulation. A bimetallic effect in the microsystem cantilever beam is studied using both the microscopic laser optical interferometry and laser optical reflectance measurement. The cantilever beam deflections induced by the differential thermal expansion of the cantilever layers are found to be linear with the power dissipated in the microsystem MESFET-heater. The key microsystem transfer characteristics based on the bimetallic effect are obtained.

Thermal actuation of a GaAs cantilever beam

The design and performance of a thermally-actuated GaAs cantilever beam are presented. To actuate the cantilever beam an integrated GaAs MESFET heater was used. A comprehensive cantilever thermal actuation performance is analysed by experiment and simulation. The thermally-actuated GaAs cantilever shows excellent linearity and a high electro-mechanical conversion efficiency (?E = 0.802??m?mW-1). The method of two-dimensional numerical modelling and simulation is proposed to study its thermal and thermo-mechanical properties. The measured and simulated thermo-mechanical and electro-mechanical conversion characteristics are compared, indicating the viability of the simulation method used for further micromechanical analysis.

AlGaN/GaN high electron mobility transistors with nickel oxide based gates formed by high temperature oxidation

We report on the design of gates of AlGaN/GaN high electron mobility transistors (HEMTs) to be predetermined for high temperature applications. In this design, nickel oxide (NiO) gate interfacial layer is formed by high temperature oxidation (T = 500–800 °C, for 1 min) of 15 nm thick Ni gate contact layer to provide a high temperature stable gate interface. AlGaN/GaN HEMTs with thermic NiO gate contact layer show excellent dc performance with higher peak transconductance, larger gate voltage swing, higher linearity, and thermal stability as compared to the reference device based on Ni gate contact layer.

The fabrication of thin GaAs cantilever beams for power sensor microsystem using RIE

A method of fabricating two independent thin GaAs cantilever beams using a bulk micromachining technology based on RIE has been developed. The principle of the method is outlined and results of a three-dimensional patterning of the cantilever beams successfully applied in a power sensor microsystem development are described.

AlGaN/GaN diaphragm-based pressure sensor with direct high performance piezoelectric transduction mechanism

The piezoelectric response of AlGaN/GaN circular HEMT pressure sensing device integrated on AlGaN/GaN diaphragm was experimentally investigated and supported by the finite element method modeling. The 4.2 μm thick diaphragm with 1500 μm diameter was loaded by the dynamic peak-to-peak pressure up to 36 kPa at various frequencies. The piezoelectric charge induced on two Schottky gate electrodes of different areas was measured. The frequency independent maximal sensitivity 4.4 pC/kPa of the piezoelectric pressure sensor proposed in a concept of micro-electro-mechanical system was obtained on the gate electrode with larger area. The measurement revealed a linear high performance piezoelectric response in the examined dynamic pressure range.

Thermal simulation and characterization of GaAs micromachined power-sensor microsystems

Abstract Thermal characteristics of GaAs power-sensor microsystems based on a three-terminal thermoconversion principle are studied by simulation and experiment. The sensor consists of two MESFET heaters and a Schottky diode temperature detector placed on each of two identical micromachined cantilever beams. The sensor power-temperature dependences, cantilever-to-cantilever heat-transfer characteristics and thermal time constant are obtained from electrical and time-domain optical reflectivity measurements in different ambient atmospheres. A laser mterferometric technique is used to evaluate the temperature in the cantilever active area via Fabry-Perot intensity changes. The spatial temperature distribution in the cantilever, thermal time constant, power-temperature dependences and mutual cantilever-to-cantilevertransfer characteristics are calculated from the time-dependent solution of a two-dimensional heat-flow equation including the effect of different gaseous media. The experimental results are found to be in good agreement with the simulation.

Patterning of cantilevers for power sensor microsystem

Abstract Anisotropic dry etching is widely used in GaAs technology for pattern transfer, mesa formation, grating fabrication and via hole etching and here is presented an RIE technique for patterning of micro-mechanical GaAs cantilevers. The results of the micromachining methods are applied in a power sensor microsystem fabrication. Cantilever were made with a thickening 2 μm and length to width ratio of about three.

Study of thermal effects in GaAs micromachined power sensor microsystems by an optical interferometer technique

Abstract A contactless and non-invasive optical interferometric method is used to study the temperature distribution and thermal time response in a GaAs micromachined power sensor. Temperature variations in the sensor active area-a cantilever beam—are sensed by an infrared laser beam. The temperature increase due to pulsed power dissipation in the cantilever induces an increase both in the GaAs refractive index and in the cantilever thickness. This results in a change in the phase and intensity of the reflected laser beam which is interferometrically detected. The spatial temperature distribution along the cantilever beam is studied using measurements of the optical phase and intensity as a function of the dissipated power. The optical signal is analysed taking into account Fabry-Perot interference. The thermal time constant of the sensor of about 5 msec is obtained from transient optical signal measurements. Results of the optical analysis are consistent with those of electrical characterization of the sensor and with the simulation of the temperature distribution.

Coplanar waveguides supported by InGaP and GaAs/AlGaAs membrane-like bridges

GaAs/AlGaAs and InGaP membrane bridges micromachined on GaAs substrates have been developed for use as supporting micromechanical structures for coplanar waveguides. The internal mechanical stresses potentially induced in these micromechanical devices are evaluated analytically, and also by both the acoustic pressure bulging method and free cantilever deformation measurement. The microwave transmission properties of the micromechanical coplanar waveguides are investigated. The amplitude attenuation at a frequency of 20 GHz is found to be 1.3 dB mm−1 for GaAs/AlGaAs bridge-based devices of a length of 900 μm and a slot of 3 μm. We discuss the potential applications of the fabricated micromechanical devices.