K. Mutamba

Formation of conducting nanochannels in diamond-like carbon films

A sharp increase of the emission current at high electric fields and a decrease of the threshold voltage after pre-breakdown conditioning of diamond-like carbon (DLC) films have been measured. This effect was observed for DLC-coated silicon tips and GaAs wedges. During electron field emission (EFE) at high electric fields the energy barriers caused by an sp3 phase between sp2 inclusions can be broken, resulting in the formation of conducting nanochannels between the semiconductor–DLC interface and the surface of the DLC film. At high current densities and the resulting local heating, the diamond-like sp3 phase transforms into a conducting graphite-like sp2 phase. As a result an electrical conducting nanostructured channel is formed in the DLC film. The diameter of the conducting nanochannel was estimated from the reduced threshold voltage after pre-breakdown conditioning to be in the range of 5–25 nm. The presence of this nanochannel in an insulating matrix leads to a local enhancement of the electric field and a reduced threshold voltage for EFE. Based on the observed features an efficient method of conducting nanochannel matrix formation in flat DLC films for improved EFE efficiency is proposed. It mainly uses a silicon tip array as an upper electrode in contact with the DLC film. The formation of nanochannels starts at the interface between the tips and the DLC film. This opens new possibilities of aligned and high-density conducting channel formation.

Microwave FBAR Structures Fabricated using Micromachined GaN Membranes

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 1.3 GHz. Extracted material parameters such as an acoustic velocity of 5700 m/s and an effective coupling coefficient of about 2% are in good agreement with those reported in the literature using other fabrication methods.

Gunn effect in field-emission phenomena

The peculiarities of electron field emission from nanostructured GaN surface have been investigated. The current–voltage characteristics of emission current in Fowler–Nordheim plot show two parts with different slopes. There are emission current oscillations in the changing slope region. As an explanation for the experimental results a model based on the electron-emission analysis from lower (Γ) valley, upper (U) valley, and electron transition between valleys due to heating in electric field has been proposed. The electron affinities for the emission from Γ and U valleys have been determined. The decreased affinities from there valleys have been estimated for quantization in nanostructured GaN.

Stacked crystal resonator: A highly linear BAW device

Besides small size, exceptional insertion loss and outstanding isolation, bulk acoustic wave (BAW) filters and duplexers are often the preferred choice in the transmit path of mobile frontend due to their excellent power handling capabilities. However, they exhibit a nonlinear behavior, causing generation of higher harmonics and a sensitivity of the resonance frequency towards a DC voltage bias. In this paper we present a new BAW device, a stacked crystal resonator (SCR), which intrinsically cancels out the second order nonlinearity and compare it to standard BAW resonators and current solutions for improved linearity. Measurements of first devices demonstrate a 30dB improvement in worst case second order harmonic generation of a SCR over a standard BAW solid mounted resonator. Furthermore, the frequency sensitivity on a DC voltage bias is reduced by a factor of 20 and the coupling coefficient increased by 10%.

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

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

Technology of Wide-Bandgap Diode Structures for Highfrequency Operation

This paper reports on technology development aspects for GaN-based diodes in view of their application at high frequencies. The investigated devices include structures for transferred electron effects for operation at high electric-fields as well as heterostructure varactors (HBV) and Schottky diodes for frequency multiplication function. The impact of factors such as high threshold voltage for transferred electron effects, strong piezoelectric effects and Schottky barrier properties on device technologies and performances are addressed