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This paper presents an adaptive biasing method for step recovery diode based comb generators which are controlled by a software routine. The aim is to provide the maximum possible harmonic output power for a given configuration and different input frequencies during input power variation. This increases the usefulness of comb generators as harmonic phase reference sources for non-linear VNA measurements in the millimeter wave range. The effects are studied on a SRD comb generator which is able to generate -30...-20 dBm harmonics in the 50 to 60 GHz range when wideband terminated. It is shown that depending on the combination of input power and frequency, different bias level behaviors for maximum harmonic output power over frequency exist. Data for maximum power level bias is provided for output frequencies up to 60 GHz. Additionally the effect of subharmonic comb frequency generation is demonstrated.

Polarization shift keying (PolSK) is a digital modulation technique using the state of polarization of an electromagnetic wave as the signalling quantity. PolSK comes from fibre communications, where the channel offers two orthogonal states of polarization. This article develops on the idea to adapt this technology to satellite communications, where similar channel conditions exist. For this purpose, a digital PolSK modem was implemented on a programmable logic board. A proposal for constellation design as well as thoughts on synchronization of PolSK over satellite is presented. The modem was used to demonstrate a 16-state Polarization Shift Keying link over a commercial satellite in Ku band. Measurements have been conducted in a back-to-back setup on intermediate frequency and on a Ku band transponder simulator to assess the impact of path-length differences, carrier recovery and non-linearity.

We propose a device concept for a hybrid nanocrystal/III-nitride based nano-LED. Our approach is based on the direct electro-optical pumping of nanocrystals (secondary excitation) by electrically driven InGaN/GaN nano-LEDs as the primary excitation source. To this end, a universal hybrid optoelectronic platform was developed for a large range of optically active nano- and mesoscopic structures. The advantage of the approach is that the emission of the nanocrystals can be electrically induced without the need of contacting them. The proof of principal was demonstrated for the electro-optical pumping of CdSe nanocrystals. The nano-LEDs with a diameter of 100 nm exhibit a very low current of 8 nA at 5V bias which is several orders of magnitude smaller than for those conventionally used. The leakage currents in the device layout were typically in the range of 8 pA to 20 pA/cm2 at 5V bias. The photon-photon down conversion efficiency was determined to be 27%. Microphotoluminescence and microelectroluminescence characterization demonstrate the potential for future optoelectronics and highly secure “green” information technology applications.

Vertically integrated III-nitride based nano-LEDs (light emitting diodes) were designed and fabricated for operation in the telecommunication wavelength range in the (p-GaN/InGaN/n-GaN/sapphire) material system. The band edge luminescence energy of the nano-LEDs could be engineered by tuning the composition and size of the InGaN mesoscopic structures. Narrow band edge photoluminescence and electroluminescence were observed. Our mesoscopic InGaN structures (depending on diameter) feature a very low power consumption in the range between 2 nWand 30 nW. The suitability of the technological process for the long-term operation of LEDs is demonstrated by reliability measurements. The optical and electrical characterization presented show strong potential for future low energy consumption optoelectronics.

Many automatic control applications such as wide scale power grid management and local power conditioning rely on the fast and accurate detection of the voltage signal phase and amplitude. These information are almost all the time given by the Phase –locked Loop (PLL) systems and can be single-phase or three-phase and have many different structures. Many single-phase PLLs core structure are based on the regulation to zero of the voltage direct component in the rotating d-q frame obtained through the Park-transform. Their accuracy and dynamic rely on how quick and accurate the second voltage’s component in the static α-β frame is estimated. This paper presents a new single-phase PLL structure, also based on the same core structure as described before, but implementing a novel voltage β-component generation. A brief review of some single-phase PLLs is made, then the proposed novel PLL structure is detailed and its performance is compared with that of the other PLL’s taking into consideration the following signal disturbances: amplitude variations, phase step variation and frequency fluctuations as well as harmonic distortion.

An ultra-broadband bow-tie with a VSWR < 2 from 5 to 60 GHz is revealed. The bow-tie antenna shows frequency independence properties and resonates periodically at multiples of its resonance frequency of 7.7 GHz with the return loss always less than -10 dB. The antenna is linearly polarized and has a maximum directivity of 6 dBi in both the elevation and azimuth planes in the farfield. The antenna is a symmetrical design with straight ends and a flare angle of 90°. It is a simple metal layer on top of a substrate layer which is fed on the metal layer right at the center with closely spaced coplanar striplines. The flare angle and the straight-end of the bow-tie antenna are varied and the 90° straight-end bow-tie antenna is compared with three other bow-tie antennas with flare angle of 60° and round-ends. All four bow-tie antennas have a low VSWR. The E-fields, H-fields and impedance plots of the ultra-broadband bow-tie antenna are shown. The impedance plot is in fact the impedance of frequency independent antennas of 189Ω or 60π across all frequencies.

This paper reveals a patch antenna with a large bandwidth of 34% and a high gain of 8 dBi in the frequency range from 10.1 to 14.2 GHz. The patch antenna is a four layer design comprising of a superstrate dielectric layer of εr 10.2 on top of a metal layer with six small simply designed parasitic H-shaped elements, which are optimized using CST Microwave Studio. The six H-shaped parasitic elements are centrally located on the metal layer. On the same layer, the antenna is fed through a coupled feedline so as to reduce mutual coupling and the antenna is thus a perfect candidate to preserve its wideband feature in an antenna array. Right underneath the metal layer is the substrate dielectric layer of εr 2.2 with two strategically placed slots underneath the metal layer in order to decrease the back radiation of the H-shaped parasitic elements. The whole design is terminated with a final metal layer to act as a conductor backed antenna to push most radiation out. The final antenna occupies a volume of 25 mm x 25 mm x 2.56 mm. The design has a high gain of 8 dBi in both the φ = 0° plane and the φ = 90° plane with a large beamwidth of 77° in the φ = 0° plane and 83° in the φ = 90° plane in the farfield.

Comparative studies of different single-phase phase-locked loops (PLL) algorithms have been made. They show that the PLL based on sample delay (dPLL), presents the lowest computational load and is as robust as the three-phase synchronous reference frame PLL dqPLL by input signal amplitude and phase variations. Its weakness appears when the input signal frequency differs from its rated frequency: It depicts a steady error on the calculated signal phase-angle. After a brief review of the dqPLL which constitutes debase structure of the dPLL, the following work will present three methods that improve the phase detection accuracy of dPLL. It is shown that the modifications brought in the original structure do not influence the robustness and stability of the algorithm but reduce the phase angle offset error by input signal frequency variation. This is corroborated by tests including not only the fundamental input voltage disturbance like amplitude, phase and frequency variation but also harmonic voltage distortion.

This paper reveals a novel stacked slotted bow-tie antenna element with a large bandwidth resonating at 11.7 GHz for satellite TV broadcasts. The simple, cost effective antenna is built on common dielectrics and is a layered version of the popular patch antenna. It is shaped in a bow-tie and converted into the slotted version in order to increase the antenna radiating surface. The dual polarized antenna element offers a large directivity of 7.2 dBi in the farfield direction. It is terminated with a coplanar waveguide feedline which is ideal to preserve its large bandwidth in an antenna array configuration. In order to appreciate its results and its design simplicity, a comparison of its S-parameters, radiation patterns, E-fields and H-fields are made with five other popular elements at the center frequency of 11.7 GHz.

We report on the impact of thermal annealing to carrier transport and transient, subpicosecond photoresponse of freestanding GaAs mesostructures. Our measurements included micro-photoluminescence and dark current and responsivity studies as well as optical femtosecond characterization. The fabricated GaAs mesostructures consisted of both mesowires and platelets that were integrated into coplanar striplines to form a photoconductive switch. We demonstrate that an optimized annealing process of our mesostructures, performed at 600 ◦C for 20 min, led to restoring bulklike properties of our freestanding devices. They exhibited dark currents below 600 pA at 10 V bias, responsivity of 0.2 A W−1 at 30 V, and mobility as high as 7300 cm2 V s−1. The annealed freestanding GaAs photodetectors were characterized by subpicosecond carrier relaxation dynamics with negligible trapping and a cutoff frequency of 1.3 THz. The latter characteristics make them excellent candidates for THz-bandwidth optoelectronics.

We developed a novel double-level-T-gate technology based on wet etching of a metal gate interlayer. With the help of this technological process we prepared T-gate feet with widths as small as 200 nm. The major advantage of our process is its use of only standard optical lithography. It allows the fabrication of 100 nanometer size T-gates for transistors. High electron mobility transistors (HEMTs) were fabricated on an AlGaN/GaN/sapphire material structure with an original gate length Lg of 2 μm. Their cutoff frequency of 6 GHz was improved to 60 GHz by etching the gate to a 200 nm length double T-gate contact.

Vertically integrated III-nitride nano-LEDs designed for operation in the telecommunication-wavelength range were fabricated and tested in the (p- GaN/InGaN/n-GaN/sapphire) material system. We found that the band edge luminescence energy of the nano-LEDs could be engineered by their size and by the strain interaction with the masked SiO2/GaN substrates; it depends linearly on the structure size. The results of reliability measurements prove that our technological process is perfectly suited for long-term operation of the LEDs without any indication of degradation effects. The presented technology shows strong potential for future low energy consumption optoelectronics.

We fabricated and tested III-nitride (p-GaN/MQW/n-GaN/sapphire) based nano-LEDs designed for operation in the single photon lithography technique. The band edge luminescence energy of the III-nitride nano-LEDs depends linearly on the structure size. Our studies provide clear evidence that our technological process for the vertically integrated nano-LED emitters is perfectly suited for long term operation without any indication of degradation effects. This novel technology shows strong potential for a future flexible single photon lithography [1] which is applicable for molecular photonic and electronic circuits.

It is now well known that the quality of electric power is a key factor for the electricity service. It defines how good the characteristics (amplitude and frequency) of the supplied power meet the rated ones. Voltage dip (or sag) and swell, short and long interruptions, voltage spike, under and over voltage, harmonic distortion, voltage unbalance are the common power quality problems. Among these last, the harmonics issue is getting more and more important over the last decades and this trend will surely continue its race. This is essentially due to the widespread use of electronic components in electric equipment: it changes the nature of the electric loads from linear to non-linear and makes them, on the one hand, responsible for the harmonics generation in the power grid but very sensitive to the power quality problems on the other hand.

We developed a T-gate technology based on selective wet etching yielding 200 nm wide T-gate structures used for fabrication of High Electron Mobility Transistors (HEMT). Major advantages of our process are the use of only standard photolithographic process and the ability to generate T-gate stacks. A HEMT fabricated on AlGaN/GaN/sapphire with gate length Lg=200 nm and double-stacked T-gates exhibits 60 GHz cutoff frequency showing ten-fold improvement compared to 6 GHz for the same device with 2 um gate length. HEMTs with a double-level-T-gate (DLTG) structure exhibit up to 35% improvement of fmax value compared to a single T-gate device. This indicates a significant reduction of skin effect losses in DLTG structure compared to its standard T-gate counterpart. These results agree with the theoretical predictions.

Polarization Shift Keying is a digital modulation method using the state of polarization of an electromagnetic wave as the signalling quantity. The technique comes from fibre communications, where the channel offers two orthogonal states of polarization. This paper develops on the idea to adapt this technology to satellite communications, where comparable channel conditions exist. For this purpose, a digital Polarization Shift Keying transceiver was implemented on programmable logic. Detection performance of the modem was measured in different set-ups with thermal noise and non-linear distortion and the results compared to simulations. Finally, the implemented system was used to demonstrate a 16-state Polarization Shift Keying link over commercial satellite in Ku-band.

Micro-photoluminescence (μ-PL) studies were performed on AlGaN/GaN heterostructure field effect transistors (HFETs) with different gate-recessing depths. It was found that μ-PL is the method of choice for detecting dry etching damage and simultaneously recording strain and stress in the HFET GaN layer. Lateral sub-μm resolved mapping shows that the strain in the GaN layer after recessing is partially relaxed and non-uniform.

The residual strain in AlGaN/GaN heterostructure field-effect transistors (HFETs) with recessed gates and their non-recessed counterparts were investigated by micro photoluminescence measurements (μ-PL). It is found that strain relaxation accounts for the observed sheet carrier density reduction after gate recessing. The usefulness of the method for device processing optimization is demonstrated. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Performance of AlGaN/GaN heterostructure field-effect transistors (HFETs) with recessed gate was investigated and compared with non-recessed counterparts. Optimal dry etch conditions by plasma assisted Ar sputtering were found for ~6 nm gate recess of a 20 nm thick AlGaN barrier layer. A decrease of the residual strain after the gate recessing (from -0.9 GPa to -0.68 GPa) was evaluated from the photoluminescence measurement. The saturation drain current at the gate voltage VG = 1 V decreased from 1.05 A/mm to 0.85 A/mm after the recessing. The gate voltage for a maximal transconductance (240-250 mS/mm) has shifted from -3 V for non-recessed HFETs to -0.2 V for recessed counterparts. Similarly, the threshold voltage increased after the gate recessing. A decrease of the sheet charge density from 1 x 10 13 cm-2 to4 x 10 12 cm-2 at VG = 0 V has been evaluated from the capacitance measurements. The RF measurements yielded a slight increase of the cut-off frequencies after the gate recessing. All these indicate that the gate recessing is a useful tool to optimize the AlGaN/GaN HFET performance for high-frequency applications as well as for the preparation of normally-off devices.

In this work we investigate two different approaches to generate THz radiation by the use of the unique electrical and thermal properties of GaN. One method is heterodyne photomixing, a compact and inexpensive approach to generate continuous electromagnetic radiation in the terahertz range, with tuneable frequency. It uses two lasers with slightly different wavelengths that illuminate an ultrafast photoconductor. The interference of both laser beams generates a beat frequency of the illumination intensity in the terahertz range. One drawback of the conventionally used LT GaAs as ultrafast photoconductor material is the relatively low THz power in the nW to ?W range. The aim of our work is to increase the output power by replacing the LT GaAs with GaN. This semiconductor is rather known as basic material for blue LEDs and lasers, but it has also remarkable electrical and thermal properties that allow higher laser power and bias voltage. A more conventional, electronic approach to generate THz radiation consists in the fabrication of an oscillator circuit based on ultrafast transistors, e.g. Hetero Field Effect Transistors based on InGaAs. These circuits can be designed up to about 100 GHz oscillation frequency. The THz region is achieved by frequency multipliers, e.g. realized by very small-sized Schottky diodes. However, each multiplier stage considerable reduces the output power. In this field we investigate GaN based transistor devices to profit from the much better power performance of this material, compared to classical semiconductors. Devices in this material system are usually used for high power applications at moderate frequencies, but the very high electron saturation velocity of GaN allows the application above 100 GHz as well.

We have fabricated and characterized ultrafast metal–semiconductor–metal (MSM) photodetectors integrated with metal–semiconductor–field-effect-transistors (MESFETs) integrated in coplanar strip lines in the GaN/AlN/SiC material system. We recorded electrical transients of the single photodetector as short as 0.9 ps wide by optoelectric pump–probe measurements using 360-nm-wavelength and 100-fs-duration laser pulses. Electric photoresponse transients of the photodetector with 6-mV peak amplitude were amplified by the MESFET, resulting in 4-ps-wide and 35-mV peak amplitude signals. This monolithically integrated optoelectronic circuit is presented as a potential candidate for high-speed ultraviolet optoelectronics