Kabula Mutamba

First Observation of Bias Oscillations in GaN Gunn Diodes on GaN Substrate

In this paper, we report on the bias oscillation of GaN-based Gunn diodes realized on a n+-GaN substrate. Different contact materials, ambient gases, and pulsewidths were used and compared with regard to device stability. A wide negative- differential-resistance (NDR) region was measured for electrical- field values E larger than a threshold field Eth of 150 kV/cm. Electrical fields much higher than the threshold value did not lead to any electromigration effects or discharging problems from the contacts. The drift velocity derived from the current-voltage characteristics, diode geometry, and doping concentration in the active layer was estimated to be 1.9 times 107 cm/s. Bias oscillations were obtained for the GaN Gunn diodes in the presence of a series inductance.

Quantum-size resonance tunneling in the field emission phenomenon

Theoretical analyses have been performed of the quantum-size (QS) resonance tunneling in the field-emission (FE) phenomenon for different models of the emitting structures. Such experimentally observed peculiarities have been considered as the enhancement of the FE current, the deviation from the Fowler-Nordheim law, the appearance of sharp current peaks, and a negative resistance. Different types of FE cathodes with QS structures (quantized layers, wires, or dots) have been studied experimentally. Resonance current peaks have been observed, from which the values of the energy-level splitting can be estimated.

A GaAs pressure sensor with frequency output based on resonant tunneling diodes

We present a novel bulk semiconductor pressure sensor based on an Al/sub x/Ga/sub 1-x/As/GaAs resonant tunneling diode (RTD) with a frequency output. A relaxation oscillator is obtained with the RTD biased in the negative differential resistance (NDR) region. Pressure applied to the RTD changes the frequency of oscillation due to the shift in current-voltage characteristics. Frequency measurements have been performed on [001]-oriented RTDs with [110]-compressive uniaxial pressure. Sensitivities of up to 0.8 kHz/MPa at 113 kHz have been measured. The main feature of this sensor type is the direct frequency output obtained by using only a maximum of three components including the RTD. Using a simplified differential equation of the oscillator circuit, the pressure-dependent effects of the RTD current-voltage characteristics on the sensor output have been investigated.

Strain sensitivity of AlGaN/GaN HEMT structures for sensing applications

Sensing elements based on AlGaN/GaN HEMT and Schottky diode structures have been investigated in relation with the strain sensitivity of their characteristics. Piezoresistance of the Al 0.3 Ga 0.7 N/GaN HEMT-channel as well as changes in the current-voltage characteristics of the Schottky diodes have been observed with gauge factor (GF) values ranging between 19 and 350 for the selected biasing conditions. While a stable response to strain was measured, the observed temperature dependence of the channel resistance demonstrates the need for a systematic characterisation of the sensor properties to allow compensation of the observed temperature effects.

Emission characteristics of a GaAs wedge emitter monolithically fabricated with an air bridge and a cantilever anode

GaAs wedge emitters with an air bridge anode and a cantilever anode were monolithically fabricated using a micromachining technique. The threshold voltage of electron emission is about 8 V for the diode between the emitter and the air bridge anode, while only 1.5 V for the diode between the emitter and the cantilever anode. The current–voltage characteristic of the air bridge diode is due to a field emission, on the other hand, that of the cantilever diode is recognized as a direct tunneling of electrons through the reduced air gap between the emitter and the cantilever by electrostatic force.

Micromachined GaN-based FBAR structures for microwave applications

This paper reports on microwave characteristics of micromachined GaN-based thin-film bulk acoustic resonator devices. The 2.2 micron active piezoelectric layer was epitaxially grown on (111)-oriented high-resistivity silicon substrate. Bulk micromachining techniques were used for the release of the resonating GaN membrane structure. S-parameter measurements have shown a fundamental mode resonance around 1.2 GHz. Extracted material parameters such as acoustic velocity and effective coupling coefficient are in good agreement with those reported in the literature using other methods. These are, to our knowledge, the first FBAR results with GaN-based active layers.

Interface tuning of the InAs/AlSb heterostructure-based quantum wells

This work shows the importance of interface consideration at the barrier sides for quantum well (QW)-based semiconductor structures, on the example of the InAs/AlSb heterostructure. Various interface combinations of AlAs and InSb type have been included in an InAs/AlSb double-barrier resonant-tunneling-diode structure, and the resulting transmission functions have been calculated. A systematic comparison of the resulting structures with each other and also with the traditional case without interface consideration have been made. Clear tendencies and relationships observed in the transmission characteristics of the different structures let the interface tuning emerge as a quality tool for QW-device tayloring.

Reliability studies on integrated GaAs power-sensor structures using pulsed electrical stress

The transmission line pulse method (TLP) is used to characterise the reliability of bolometric GaAs microwave power-sensors. Two degradation mechanisms are identified during the degradation process of the absorbing NiCr termination, in which the input power is converted into heat. Simulations and a material analysis have been performed in order to characterise the observed degradation mechanisms.

Integrated microwave sensor for cavity-length measurement with sub-millimeter accuracy

A novel measurement procedure using microwaves is presented. The implemented sensor determines the length of a cylindrical cavity (e.g. hydraulic system) with sub-millimeter accuracy in real time. The principle of operation is based on the detection of the resonance-frequency distribution in a cavity resonator.

Micrometer-size GaN Schottky-diodes for mm-wave frequency multipliers

Small-size Pt/n-GaN Schottky diodes are fabricated using electrochemical technique for anode metallisation. Effects of surface passivation and thermal annealing on the interface quality are studied using PL-measurements and electrical characterisation. DC-characteristics of 5 /spl mu/m-diameter anodes result in a cut-off frequency of 390 GHz. The perspectives of GaN-diodes for THz-frequency multipliers are discussed.