Showing papers by "Oliver Ambacher published in 2015"

Appropriate salt concentration of nanodiamond colloids for electrostatic self-assembly seeding of monosized individual diamond nanoparticles on silicon dioxide surfaces.

Abstract: Monosized (∼4 nm) diamond nanoparticles arranged on substrate surfaces are exciting candidates for single-photon sources and nucleation sites for ultrathin nanocrystalline diamond film growth. The most commonly used technique to obtain substrate-supported diamond nanoparticles is electrostatic self-assembly seeding using nanodiamond colloidal suspensions. Currently, monodisperse nanodiamond colloids, which have a narrow distribution of particle sizes centering on the core particle size (∼4 nm), are available for the seeding technique on different substrate materials such as Si, SiO2, Cu, and AlN. However, the self-assembled nanoparticles tend to form small (typically a few tens of nanometers or even larger) aggregates on all of those substrate materials. In this study, this major weakness of self-assembled diamond nanoparticles was solved by modifying the salt concentration of nanodiamond colloidal suspensions. Several salt concentrations of colloidal suspensions were prepared using potassium chloride as an inserted electrolyte and were examined with respect to seeding on SiO2 surfaces. The colloidal suspensions and the seeded surfaces were characterized by dynamic light scattering and atomic force microscopy, respectively. Also, the interaction energies between diamond nanoparticles in each of the examined colloidal suspensions were compared on the basis of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. From these investigations, it became clear that the appropriate salt concentration suppresses the formation of small aggregates during the seeding process owing to the modified electrostatic repulsive interaction between nanoparticles. Finally, monosized (<10 nm) individual diamond nanoparticles arranged on SiO2 surfaces have been successfully obtained.

Integrated reverse-diodes for GaN-HEMT structures

Abstract: This work introduces an enhanced GaN-HEMT structure that uses separated Schottky contacts as integrated free-wheeling diodes for the reverse operation. The principle is investigated and compared to other integrated reverse-diode concepts. Different diode structures are fabricated and evaluated. The new concept is demonstrated on a large gate width 600 V-device with on-state currents up to 30 A and an on-state resistance of 215 mΩ. Furthermore, the device achieves a very low gate-charge of below 3 nC and a reverse recovery charge of 8 nC.

A 183 GHz Metamorphic HEMT Low-Noise Amplifier With 3.5 dB Noise Figure

Abstract: This letter presents a 183 GHz low-noise amplifier (LNA), designed primarily for water vapor detection in atmosphere. The LNA requirements were defined by MetOp Second Generation (MetOp-SG) Microwave Sounder, Microwave Imager and Ice Cloud Imager instruments. MetOp-SG is the European contribution to operational meteorological observations from polar orbit. This LNA advances the current state-of-the-art for the InGaAs metamorphic high electron mobility transistor (mHEMT) technology. The five-stage common-source MMIC amplifier utilizes transistors with a gate length of 50 nm. On-wafer measurements show a noise figure of 3.5 dB at the operative frequency, about 1 dB lower than previously reported mHEMT LNAs, and a gain of $24\pm 2~{\rm dB}$ over the bandwidth 160-200 GHz. The input and output matching are $-11~{\rm dB}$ and $-10~{\rm dB}$ , respectively. Moreover, the dc power dissipation at the optimal bias for noise is as low as 24 mW.

On the Accurate Measurement and Calibration of S-Parameters for Millimeter Wavelengths and Beyond

Abstract: It is well known that, in the millimeter (mm-wave) and sub-mm-wave range, on-wafer S-parameter measurements are often inaccurate and suffer from serious systematic artifacts. In this paper, we confirm that these artifacts are related to spurious wave modes that are excited and propagate in the substrate. These parasitic wave components may be scattered at neighboring structures on the wafer and cause detrimental crosstalk. While these parasitic components deteriorate the measurement itself, an even more serious complication arises from the fact that these modes are already present in the calibration measurement and are unintentionally imported and superposed to the measurement data. In this paper, we present a new type of RF pad with novel screening features and show that these parasitic modes can be efficiently suppressed by the use of proper on-wafer couple structures. Moreover, a novel calibration substrate and method is presented and demonstrated to be capable to remove spurious artifacts from S-parameter measurements up to 450 GHz.

Detection of different target-DNA concentrations with highly sensitive AlGaN/GaN high electron mobility transistors

Abstract: This paper presents the successful detection of different target-DNA concentrations using AlGaN/GaN high electron mobility field effect transistors (HEMTs). The gate was bio-functionalized with two different densities of complementary DNA before application of target-DNA. Dosages of 10−16 mol/L up to the gate saturation were tested. The DNA concentration was increased by a factor of 10 each time, and the transistor transfer characteristics were measured. The threshold voltage shift and drain–source current change depend on target concentration and obey the Sips adsorption model. The AlGaN/GaN HEMTs are highly sensitive to very low target-DNA concentrations with a detection limit of 10−14 mol/L. The gates with a low probe-DNA density were saturated with a lower target-DNA concentration and presented a faster hybridization equilibrium constant.

Threshold Voltage Engineering in GaN-Based HFETs: A Systematic Study With the Threshold Voltage Reaching More Than 2 V

Abstract: One of the key challenges for the adoption of gallium nitride (GaN)-based heterostructure field effect transistors (HFETs) in power-switching applications is obtaining enhancement mode behavior. A large variety of methods have been applied to shift the threshold voltage $V_{\mathrm {th}}$ of HFETs. However, most of the time, approaches were discussed individually, neglecting the effects of combinations. Hence, in this paper, a comprehensive study of four different approaches to shift $V_{\mathrm {th}}$ well into the positive range is presented. We show the effects of different gate metallizations, of a backbarrier, of a gate oxide, and of a gate recess. Each approach is discussed individually, and special focus is on the insulator/semiconductor interface, which is apparently different with and without gate recess. The final device exhibits a $V_{\mathrm {th}}$ of +2.3 V, which is shown to be stable when applying OFF-state stress during dynamic characterization.

Charge balancing in GaN-based 2-D electron gas devices employing an additional 2-D hole gas and its influence on dynamic behaviour of GaN-based heterostructure field effect transistors

Abstract: GaN-based heterostructure FETs (HFETs) featuring a 2-D electron gas (2DEG) can offer very attractive device performance for power-switching applications. This performance can be assessed by evaluation of the dynamic on-resistance Ron,dyn vs. the breakdown voltage Vbd. In literature, it has been shown that with a high Vbd, Ron,dyn is deteriorated. The impairment of Ron,dyn is mainly driven by electron injection into surface, barrier, and buffer traps. Electron injection itself depends on the electric field which typically peaks at the gate edge towards the drain. A concept suitable to circumvent this issue is the charge-balancing concept which employs a 2-D hole gas (2DHG) on top of the 2DEG allowing for the electric field peak to be suppressed. Furthermore, the 2DEG concentration in the active channel cannot decrease by a change of the surface potential. Hence, beside an improvement in breakdown voltage, also an improvement in dynamic behaviour can be expected. Whereas the first aspect has already been de...

Monolithic integrated quasi-normally-off gate driver and 600 V GaN-on-Si HEMT

Abstract: This work reports on a 600 V GaN-on-Si power transistor with monolithic integrated gate driver. The circuit is based on Schottky-gate depletion-mode technology and fabricated on a 2×3 mm2 chip. The push-pull gate driver stage implements a quasi-normally-off pull-up transistor, fabricated with monolithic integrated series-connected Schottky diodes for positive voltage-level shifting in the source path of a d-mode HEMT. The measured gate-source threshold voltage of the fabricated quasi-normally-off pull-up transistor is +2.7 V as compared to −2.9 V of the normally-on pull-down transistor. Pulsed-IV measurements determine an effective gate driver resistance of around 2 Ω. On-wafer measurements of the power transistor show low off-state leakage-currents up to 600 V blocking voltage with high wafer yield and 150 mΩ on-resistance. Finally, inductive-load switching measurements up to 450 V, 14.3 A show maximum switch node slew-rates during turn-on and turn-off transitions as high as 250 V/ns.

A compact W-band LFMCW radar module with high accuracy and integrated signal processing

Abstract: In this paper, a compact W-band radar module will be presented, design aspects will be discussed and some measurement results of the finally realized module will be shown. The module features an integrated signal generation, full signal processing and power distribution. The signal generation is based on a phase-locked loop, optimized for linear frequency-modulated continuous-wave operation. A digital signal processor also works as internal controller and provides the digital interfaces. Due to the integrated power distribution, a single 7 to 15V power supply is necessary. In contrast to most state-of-the-art W-band systems using waveguides, a liquid crystal polymer based printed circuit board with integrated antenna and dielectric lens is used to realize a very compact module size of 42mm × 80mm × 27mm and low weight of 160 g. The total power consumption is 5 W, the radio frequency output power is 3dBm and the linear frequency-modulated contimuous-wave radar bandwidth is 14 GHz. The evaluated distance measurement repeatability of the module is 5 ppm (5µm deviation per meter target distance).

High-gain over 30% PAE power amplifier MMICs in 100 nm GaN technology at Ka-band frequencies

Abstract: Two high-efficiency power amplifier MMICs utilizing a 100 nm AlGaN/GaN HEMT MMIC technology at Ka-band frequencies are reported in this paper. They have also been designed in order to ensure high output power and high gain. In continuous-wave (CW) operation, the first three stage power amplifier provides 4.5 W of output power and 33% of power-added-efficiency (PAE) at 30 GHz. The first power amplifier has been mirrored and combined in order to reach higher output power levels. This second MMIC power amplifier provides 8.1 W of output power and 30% of PAE at 30 GHz.

Normally-Off AlGaN/GaN/AlGaN Double Heterostructure FETs With a Thick Undoped GaN Gate Layer

Abstract: In this letter, we report on polarization charge engineering enabling normally-off operation for a double-heterostructure Al0.26Ga0.74N/GaN/Al0.07Ga0.93N-based field effect transistors (DHFETs) using a 35-nm-thick undoped GaN layer underneath the gate metallization. The combined effect of the negative polarization charge induced by the AlGaN back barrier and the undoped GaN gate layer ensures the total depletion of the channel, and provides a positive threshold voltage. The fabricated DHFET exhibits normally-off operation with a threshold voltage of 1.2 V, a maximum drain current density of 370 mA/mm, and a high ON/OFF current ratio of $10^{7}$ , at a gate bias of 7 V. A transistor with gate–drain distance of 6 $\mu \text{m}$ demonstrates 300 V off-state breakdown voltage.

High-voltage stress time-dependent dispersion effects in AlGaN/GaN HEMTs

Abstract: In this work we investigate the dispersion effects of GaN based HEMTs as a function of the off-state stress voltage and the stress time. We characterize the reduction of the drain current in on-state after off-state stress time from 2 μs up to 10 s. In addition, we compare different source- and gate-terminated field plate configurations. High-voltage (HV) pulsed stress tests with several stress rates are carried out to measure the increase of dynamic on-resistance over time. A new analytic model is developed to estimate the relevant trapping and detrapping time constants of the devices. By HV pulsed stress tests at different temperatures it is possible to estimate the activation energies for both processes.

High-Efficiency, High-Temperature Continuous Class-E Sub-Waveform Solution AlGaN/GaN Power Amplifier

Abstract: This letter presents the realization of a high-efficiency, high-temperature Continuous Class-E Sub-Waveform Solution power amplifier using for the first time GaN technology. The recently introduced Continuous Class-E PA mode theory has revealed new and various impedance solutions for which high efficiency switch mode PAs can be realized. The investigation carried out in this work clearly shows that very high efficiency is still delivered despite the smaller voltage waveform peak when compared to standard Class-E, leading to more reliable PAs. The Continuous Class-E Sub-Waveform Solution PA prototype shows efficiency as high as 80% while delivering output power of 4 W and gain ${>} 10$ dB at 2 GHz of frequency and ambient temperature. The PA module has also been tested at high temperature ${\rm T}=150^{\circ}$ C revealing targeted performance with efficiency up to 71% while delivering 3.5 W of output power. Reliability measurements also show satisfactory results in the time frame up to 1500 hours.

Quasi-normally-off GaN gate driver for high slew-rate d-mode GaN-on-Si HEMTs

Abstract: This work presents a quasi-normally-off gallium nitride (GaN) transistor with positive gate threshold voltage based on depletion-mode technology, suitable for gate drivers or logic circuits. Quasi-normally-off behaviour is achieved by the series connection of multiple Schottky diodes in the source path of an initially normally-on transistor. As opposed to conventional approaches, a novel quasi-normally-off gate driver circuit avoids the static shoot-through current path in the driver final stage and ensures a safe blocking state of a d-mode power switch in case of driver failure with only one negative driver supply voltage. For evaluation a hybrid integrated GaN power module is built, comprising a 2.4 A gate driver and 600 V/ 24 A boost converter switching cell. Measurements of pulsed inductive switching up to 274 V/ 12 A show gate voltage rise and fall times of 5.4 ns and 3.8 ns, boost converter switch node transition times as low as 1.6 ns and 1.2 ns, and maximum slew-rates up to 91 V/ns during turn-on transitions, and up to 177 V/ns during turn-off transitions, respectively.

Novel destructive-interference-envelope detector for high data rate ASK demodulation in wireless communication receivers

Abstract: This paper reports on a novel ASK demodulator architecture suitable for receiver circuits in wireless communication applications. The proposed topology targets the theoretical limitation of the maximum achievable data rate or base band bandwidth with respect to the carrier frequency of standard envelope detector approaches. The circuit comprises a balun to split the input signal into two 180° out-of-phase signals and a Schottky diode detector for each branch to convert the modulated RF signal down to base band. Due to the combination of both signal paths the carrier frequency is canceled out, for which reason the novel demodulator topology is named destructiveinterference- envelope detector (DIED). In order to demonstrate the capabilities of the new concept a W-band demodulator MMIC was designed and realized. The MMIC is based on a 35nm InGaAs metamorphic HEMT technology and contains optimized monolithic integrated planar zero bias Schottky diodes. It shows a measured carrier suppression of 65 dB at 102.5 GHz. The measured voltage responsivity for a high load impedance is 6760V/W for low input power levels. The simulated base band bandwidth of 25 GHz leads theoretically to an achievable data rate of up to 50 Gbit/s. First time domain measurements showed a data rate of 26 Gbit/s, which is also the limit of the characterization setup.

Monolithic three-stage 6–18GHz high power amplifier with distributed interstage in GaN technology

Abstract: This paper reports on a three-stage broadband amplifier architecture suitable for wide bandwidth applications such as electronic warfare systems. The proposed topology comprises a distributed active power splitter to function as an interstage driver amplifier, whereas the pre-driver and power amplifier stages are reactively-matched. As a result, purely resistive interstage impedances are obtained and therefore the proposed architecture allows wider bandwidth operation as compared to the conventional reactively-matched multistage topology. A 6GHz to 18GHz three-stage high gain and high power amplifier MMIC is designed and realized in order to demonstrate the capabilities of the concept. The MMIC is based on a 0.25μm gate length AlGaN/GaN HEMT microstrip transmission line technology. It shows a measured small-signal gain of (25 ± 3)over the entire frequency range. The saturated output power is higher than 25W at the center frequency in pulsed operation.

Performance and parasitic analysis of sub-micron scaled tri-gate AlGaN/GaN HEMT design

Abstract: AlGaN/GaN high-electron mobility transistors (HEMTs) with varied Tri-gate topologies have been fabricated and influences of the fin-shaped nano-channels on device parasitics are examined. Through S-parameter measurements and modelling of the designed Fin-FETs, a detailed RF investigation on intrinsic device parameters is performed under different biasing schemes. Corresponding RF performances and transfer characteristics as well as the derived small-signal parameters of the measured devices are extracted by employing 3-D EM FET model analysis at 110 GHz. Comparisons between the designed fin-geometries and intrinsic device parameters have proven flatter gm, gds and fT responses, which are presented through experimental results in detail for the first time.

Charge trapping in gate-drain access region of AlGaN/GaN MIS-HEMTs after drain stress

Abstract: In this paper we investigate the drain stress behavior and charge trapping phenomena of GaN-based high electron mobility transistors (HEMTs). We fabricated GaN-on-Si MIS-HEMTs with different dielectric stacks in the gate and gate drain access region and performed interface characterization and stress measurements for slow traps analysis. Our results show a high dependency of the on-resistance increase on interfaces in the gate-drain access region. The dielectric interfaces near the channel play a significant role for long term high voltage stress and regeneration of the device.

Large area InN terahertz emitters based on the lateral photo-Dember effect

Abstract: Large area terahertz emitters based on the lateral photo-Dember effect in InN (indium nitride) are presented. The formation of lateral photo-Dember currents is induced by laser-illumination through a microstructured metal cover processed onto the InN substrate, causing an asymmetry in the lateral photogenerated charge carrier distribution. Our design uses simple metal structures, which are produced by conventional two-dimensional micro-structuring techniques. Having favoring properties as a photo-Dember material InN is particularly well-suited as a substrate for our emitters. We demonstrate that the emission intensity of the emitters can be significantly influenced by the structure of the metal cover leaving room for improvement by optimizing the masking structures.

Monolithically-Integrated Mulitlevel Inverter on Lateral GaN-on-Si Technology for High-Voltage Applications

Abstract: This work reports on the development of a fully integrated monolithic single-phase diode-clamped multilevel (DCM) converter in a high-voltage AlGaN/GaN-on-Si technology for high-power conversion applications. The power chip operates as an inverter (DC/AC) to produce a three level output voltage from a DC link as well as a rectifier (AC/DC) to feed a DC link with a desired voltage. The application range, the topology, and the design of the inverter are presented in detail. The integrated inverter circuit is dimensioned for Umax = +/- 400 V and Imax = 5 A. Furthermore measurements of the performance of the single devices on the integrated chip are demonstrated.

Broadband low-noise GaN HEMT TWAs using an active distributed drain bias circuit

Abstract: Modern communication and radar systems show an increasing demand for robust ultra-broadband amplifiers for low-noise applications. A set of three different 0.5 GHz to 20 GHz MMIC LNAs using a GaN HEMT technology with a gate length of 0.25 μm was designed and fabricated, each with a noise figure between 3 dB and 7 dB over frequency. Two designs with four and five FET cells feature approx. 10 dB and 11 dB broadband gain, while a third MMIC with a chain connection of both figures more than 20 dB of gain. A distributed active drain bias circuit substitutes large area or off-chip inductor structures and enables a full-MMIC chain connection of both TWA stages.

Switching frequency modulation for GaN-based power converters

Abstract: The effects on the EMI spectrum for various switching frequency modulation (SFM) scenarios in a high frequency boost converter are investigated in this paper. A GaN-device and a Si-device are compared with respect to their EMI behavior, which results from different gate charges and therefore different voltage gradients dv/dt on the power lines. First, the dynamic characteristics of the GaN-HEMT are demonstrated in detail. Then the behavior in the time domain and the frequency domain for switching operations at 300 kHz with various frequency modulation settings and an output power of 250 W are presented.

A dual-band UMTS/LTE highly power-efficient class-ABJ Doherty GaN PA

Abstract: This paper reports on the realization of a dual-band high power Class-ABJ GaN Doherty power amplifier (PA) for base-station transmitter system (BTS) applications. The designed Doherty PA module consists of two individual single-ended high power-efficient AlGaN/GaN PAs with multi-harmonic Class-ABJ termination operating in both UMTS 2.1–2.2 GHz and LTE 2.6–2.7 GHz communication bands. Measurements have been performed on both the single-ended and Doherty PA prototypes driving both modules with a 5 MHz WCDMA signal having different PAR. The measured single-ended PA block yields around 39.3 dBm average POUT as well as 40% and 37% average DE at the two center frequency bands FC=2.15GHz and FC=2.65 GHz when driving it with a WCDMA signal having 5.6 dB PAR. The Doherty PA prototype was tested by using a WCDMA input signal with higher PAR=7.7 dB. These measurement results demonstrate around 40% average drain efficiency and 44 dBm average POUT in the UMTS band at FC=2.15 GHz and around 36% average DE and 42.9 dBm average POUT in the LTE band at FC=2.65 GHz.

High-Gain AlGaN/GaN HEMT Single Chip E-Band Power Amplifier MMIC with 30 dBm Output Power

Abstract: This paper reports on the design, fabrication and measured performance of an E-band power amplifier MMIC delivering >1 W of output power. AlGaN/GaN based high-electron-mobility transistors (HEMTs) with 100 nm of gate length have been processed in a grounded coplanar transmission line technology in order to realize the four-stage high power amplifier chip. For the designed prototype, measurement results yielded small-signal gain of 16-18 dB in the frequency range from 71 to 76 GHz (low E-band). A saturated output power of 30.8 dBm (1.2 W) has been reported under continuous wave (CW) operation with more than 11 dB of power gain, corresponding to a power density of around 1 W/mm at the output of the final stage. This demonstrates superior performance of GaN technology in the millimeter-wave design over other competing technologies.

Admittance–voltage profiling of AlxGa1−xN/GaN heterostructures: Frequency dependence of capacitance and conductance

Abstract: Admittance–voltage profiling of AlxGa1−xN/GaN heterostructures was used to determine the frequency dependent capacitance and conductance of FET devices in the frequency range from 50 Hz to 1 MHz. The nominally undoped low pressure metal-organic vapor-phase epitaxy structures were grown with an Al-content of 30%. An additional 1 nm thick AlN interlayer was placed in one structure before the Al0.3Ga0.7N layer growth. For frequencies below 108 Hz it is convenient to use equivalent circuits to represent electric or dielectric properties of a material, a method widely used, for example, in impedance spectroscopy. We want to emphasize the relation between frequency dependent admittance–voltage profiling and the corresponding equivalent circuits to the complex dielectric function. Debye and Drude models are used for the description of the frequency dependent admittance profiles in a range of depletion onset of the two-dimensional electron gas. Capacitance- and conductance-frequency profiles are fitted in the entire measured range by combining both models. Based on our results, we see contributions to the two-dimensional electron gas for our samples from surface states (80%) as well as from background doping in the Al0.3Ga0.7N barriers (20%). The specific resistance of the layers below the gate is above 105 Ω cm for both samples and increases with increasing negative bias, i.e., the layers below the gate are essentially depleted. We propose that the resistance due to free charge carriers, determined by the Drude model, is located between gate and drain and, because of the AlN interlayer, the resistance is lowered by a factor of about 30 if compared to the sample without an AlN layer.

Aluminium nitride membranes with embedded buried idt electrodes for novel flexural plate wave devices

Abstract: Novel flexural plate wave resonators based on bilayer AlN membranes were fabricated and investigated with respect to their piezoelectric response and residual film stress In the proposed device design, the interdigital transducers (IDTs) are embedded between two AlN films Here, chromium and aluminum were evaluated as suitable materials for IDT Using Cr is beneficial with respect to conventional microfabrication technology, whereas Al enhances overall device performance due to enhanced conductivity Finally, the proof-of-concept Lamb wave resonators were fabricated and characterized by means of laser Doppler vibrometry

A 200 GHz driver amplifier in metamorphic HEMT technology

Abstract: A driver amplifier operating at 200 GHz has been design and manufactured with compact size, high gain, broadband performance and high power added efficiency (PAE). This monolithic microwave integrated circuit (MMIC) is realized in grounded coplanar waveguides (GCPW) technology in conjunction with a 35 nm gate length metamorphic high electron mobility transistor technology (mHEMT). The four-stage driver amplifier provides more than 20 dB linear gain between 180 GHz and 270 GHz (40 % bandwidth) and PAE higher than 3.3% with more than 7.4 dBm saturated output power at 200 GHz.