Showing papers by "Oliver Ambacher published in 2016"

The role of surface electron accumulation and bulk doping for gas-sensing explored with single-crystalline In2O3 thin films

Abstract: Single crystalline and textured In2O3 thin films with (1 1 1) surface orientation, grown by plasma-assisted molecular beam epitaxy, were used as a model system to study the role of bulk and surface electron accumulation layer conductance for ozone sensing. Both conductance contributions, which add to the total film conductance, were systematically varied. The resulting ozone sensitivity was determined by total conductance measurements in synthetic air with defined ozone concentration using UV irradiation instead of heating to regenerate the In2O3 surface. Depletion of the surface electron accumulation by an oxygen plasma treatment, confirmed by X-ray photoelectron spectroscopy, rendered the films ozone insensitive. The ozone response of films with an accumulation layer was increased by thickness reduction or by designing the bulk of the film semi-insulating using deep acceptor doping by Mg. Our results of using electron accumulation layers for gas sensing and bulk doping by deep acceptors to increase sensitivity can be generalized to other gas sensing materials. The use of single crystalline films allows selecting the most sensitive crystallographic surface orientation and may have further advantages over polycrystalline films, such as increased stability and sensing speed.

High-Current Submicrometer Tri-Gate GaN High-Electron Mobility Transistors With Binary and Quaternary Barriers

Abstract: Through implementation of the 3-D tri-gate topology, GaN-based high-electron mobility transistors (HEMTs) have been fabricated and high-frequency performances as well as the short-channel effects are investigated. The designed tri-gate transistors are highly-scaled having 100 nm of gate length, which introduces the condition of a short channel. It is demonstrated that higher sub-threshold slopes, reduced drain-induced barrier lowering and better overall off-state performances have been achieved by the nano-channel tri-gate HEMTs with an AlGaN barrier. A lattice-matched InAlGaN barrier with the help of the fin-shaped nano-channels provide improved gate control, increasing current densities, and transconductance $g_{\mathrm{ m}} $ . In a direct comparison, very high drain current densities ( $\sim 3.8$ A/mm) and $g_{\mathrm{ m}} $ ( $\sim 550$ mS/mm) have further been obtained by employing a pure AlN barrier.

A 40 dBm AlGaN/GaN HEMT power amplifier MMIC for SatCom applications at K-band

Abstract: The design, realization, and characterization of a K-band high power amplifier with a saturated output power of 40dBm is described in this paper. The amplifier is realized using a 250nm gate length AlGaN/GaN HEMT MMIC technology on semi-insulating SiC substrates. The two-stage amplifier is designed with two 6×90 µm HEMT cells in the driver and four 8×100 µm HEMT cells in the final stage and thus exhibits a relatively aggressive staging ratio of 1∶3. When measured with a supply voltage of 32V, the amplifier delivers a saturated output power of 40dBm at 18 GHz. The peak PAE at this frequency is 30%, and the linear gain exceeds 20 dB. These results are state-of-the-art performance with regard to power/efficiency at K-band.

A 100 GHz FMCW MIMO radar system for 3D image reconstruction

Abstract: We present a frequency modulated continuous wave (FMCW) multiple input multiple output (MIMO) radar demonstrator system operating in the W-band at frequencies around 100 GHz. It consists of a two dimensional sparse array together with hardware for signal generation and image reconstruction that we will describe in more detail. The geometry of the sparse array was designed with the help of simulations to the aim of imaging at distances of just a few up to more than 150 meters. The FMCW principle is used to extract range information. To obtain information in both cross-range directions a back-propagation algorithm is used and further explained in this paper. Finally, we will present first measurements and explain the calibration process.

Soft-switching 3 MHz converter based on monolithically integrated half-bridge GaN-chip

Abstract: This paper presents switching operation of a monolithically integrated half-bridge stage in a 600 V-class GaN-on-Si technology. The integrated power chip includes two high performance GaN-HEMTs with integrated freewheeling Schottky diodes, on a total chip area of 4×4 mm2. Operation of the half-bridge chip is demonstrated in a soft-switching buck converter at switching frequencies up to 3 MHz, input voltages up to 400 V, and output power up to 250 W.

RF Performance of Trigate GaN HEMTs

Abstract: The impact of the trigate GaN high electron mobility transistor (HEMT) body geometry on the device RF performance is investigated by 3-D numerical simulations. The trigate concept is a viable approach to achieve normally off operation and to suppress short-channel effects. The effect of gate length scaling on the RF behavior is studied and guidelines for design improvements are provided. Furthermore, it is shown that trigate HEMTs with improved body design and/or InAlN barriers causing a higher polarization charge than AlGaN can exhibit better RF figures of merit than planar GaN HEMTs.

Stability Investigation of Large Gate-Width Metamorphic High Electron-Mobility Transistors at Cryogenic Temperature

Abstract: An investigation of metamorphic high electron mobility transistor stability at cryogenic temperature is presented in this paper. Unlike in the case of two-finger transistors, the measurements of cooled four-finger devices with large gate widths exhibit unstable behavior in the form of steps in the current–voltage characteristics, discontinuities in the transconductance, and reduced gain. This unstable behavior has hampered the reliable realization of low-noise amplifiers for cryogenic applications. We study different gate-width devices with a multiport transistor model, allowing the separation of gate and drain feeder structures from the active part of the transistor. The simulation reveals the presence of resonances in the frequency region of several hundreds of gigahertz. We demonstrate that the resonances disappear when an air bridge is placed across the fingers of the drain feeder structure, and confirm the stabilizing effect of the air bridge both on device and circuit level by cryogenic measurements.

Analysis and optimization of sputter deposited AlN-layers for flexural plate wave devices

Abstract: Aluminum nitride (AlN) thin films deposited by reactive radio frequency magnetron sputtering in an Ar/N2 discharge on Si(001) substrates were studied with respect to structure, stress, and piezoelectric properties. In order to optimize the AlN layers for flexural plate wave (FPW) devices, the influence of process pressure and N2 concentration has been evaluated by means of spectroscopic ellipsometry, residual stress measurements, x-ray diffraction, atomic and piezoresponse force microscopy, along with analysis of the piezoelectric charge coefficient d33,f. FPW devices with low compressively stressed (−200 to −300 MPa) AlN layers were prepared and characterized by white light interferometry and Raman measurements. With increasing pressure from 3×10−3 to 8×10−3 mbar, a transition from −840 MPa compressive stress to +300 MPa tensile stress was measured. Increasing the nitrogen concentration from 3.3% to 50% resulted in a change in stress from +150 to −1170 MPa. All films exhibited a high degree of c-axis ori...

Prospects and Limitations of Stacked-FET Approaches for Enhanced Output Power in Voltage-Controlled Oscillators

Abstract: This paper reports on the emerging potential of a stacked field-effect transistor (FET) approach with respect to maximum achievable RF output power with special remarks on high electron-mobility transistor (HEMT) based series feedback oscillator monolithic microwave integrated circuits (MMICs). A stacked-FET oscillator can provide benefit in several ways—in the form of an improved RF output power, a high drain efficiency, or a stabilized oscillation behavior, even for changing load impedances. The limitation of the maximum number of transistors is analyzed with reference to the maximum achievable output power of stacked devices. This is done by describing the compromise between an increased voltage swing and a decreased RF output current. The output current is decreased since the current gain of a common gate device is smaller than unity. Based on the proposed theory a W-band stacked-FET voltage-controlled oscillator MMIC with an output power of 15 dBm and a drain efficiency of 23.3% was realized. The MMIC is based on the Fraunhofer IAF 50-nm gate-length metamorphic HEMT process. For an advanced evaluation of the power capability and the optimum target load of the utilized technology, an active W-band on-wafer load–pull system was implemented.

Small signal modelling approach for submillimeter wave III–V HEMTs with analysation and optimization possibilities

Abstract: In this paper we present a new small signal multiport modelling approach for III–V High Electron Mobility Transistors (HEMT) that is capable for internal transistor analysation and optimization as well as scaleable in gate width and finger-number. The new model decomposes the planar transistor structure into single multiport elements that are separately described by electrical equivalent circuits and connected to each other over discrete ports. With this new modelling topology we only need to extract a couple of multiport elements to predict the correct behavior for a high amount of different planar transistor structures. This point gives the circuit designer a wide range of possibilities to analyze and optimize a given transistor structure according to special needs, like low-noise, input-output matching or cryogenic behavior on a computer based level.

GaN-based E-band power amplifier modules

Abstract: This work discusses the fabrication of two GaN-based power amplifier modules, suitable to increase the available output power levels of E-band multi-Gigabit fixed wireless links dedicated to aeronautics and space applications. The first mounted module contains one GaN-based power amplifier MMIC, packaged in a WR-10 waveguide environment. The module shows a small-signal gain of 13.4 dB with a gain flatness of ± 1 dB, along with a saturated output power of more than 26 dBm (400 mW) for the entire intended frequency range between 71–76 GHz. The second module, parallelizing four MMICs, demonstrates a high saturated output power of more than 31.1 dBm (1290 mW), along with a small-signal gain of typically 11.2 dB and a flatness of ±1 dB within the frequency range of interest.

Trapping Effects at the Drain Edge in 600 V GaN-on-Si HEMTs

Abstract: In this paper, we investigate the influence of the drain electrode on the dynamic switching behavior of AlGaN/GaN high-electron-mobility transistors on Si substrate. By adding a field plate to the drain electrode, a dramatic increase in the dynamic ON-resistance dynR $_{\mathrm{\scriptscriptstyle ON}}$ was identified. The dispersion effect is correlated with the high electric field below the drain field plate (DFP), the onset of which is caused by the full electron depletion from both the channel and the GaN cap layer. We show that the electron distribution is modified by the passivation method, backside bias, or surface charges and, hence, shifts the onset voltage of the trapping effect. Trapped electrons underneath the DFP are thought to be responsible for the measured rise of the dynR $_{\mathrm{\scriptscriptstyle ON}}$ . With the introduction of an extended ohmic drain contact, the influence of a metallization overhang at the drain edge can be suppressed. The detrapping energies associated with the surface defects were determined to 0.2, 0.3, and 0.7 eV, respectively. Simulations and measurements indicate that charges inside the passivation below the DFP worsen the switching behavior.

Broadband E-Band Power Amplifier MMIC Based on an AlGaN/GaN HEMT Technology with 30 dBm Output Power

Abstract: This paper reports on the design of a power amplifier covering the entire E-band satellite communication bands (71-76 GHz & 81-86 GHz) and demonstrating a high saturated output power of more than 1 W across this frequency range of interest. The circuit was fabricated by using an advanced 100 nm GaN high-electron-mobility transistor technology with an AlN-interlayer epitaxy, demonstrating a transit frequency ft of more than 100 GHz and a power density as high as 1.9 W/mm at 94 GHz in continuous-wave load-pull operation. A state-space approach is applied for the device modeling, which enables a successful first-pass circuit design.

Linear temperature sensors in high-voltage GaN-HEMT power devices

Abstract: This work presents a high-voltage GaN-based power HEMT with a highly-linear, monolithically-integrated temperature sensor. The principle is shown and compared to other concepts. The sensor is fabricated by using a interconnect metallization without additional process steps. The performance of the sensor as well as of the power device is characterized. The 600 V power device achieves an on-state resistance of RON = 55 mO at a corresponding drain current ID = 30 A and an advanced dynamic performance with a low gate charge of 20 nC.

Monolithic GaN-on-Si Half-Bridge Circuit with Integrated Freewheeling Diodes

Abstract: This work presents the design, realization, and the characterization of a monolithic GaN-on-Si half-bridge circuit with integrated Schottky contacts as freewheeling diodes. The extrinsic- and intrinsic- layouts are realized, analyzed, and compared to other approaches. The high- and low-side switches feature an off-state voltage of 600 V, an on-state resistance of 120 mOmega, and a reverse resistance of below 150 mOmega at corresponding drain currents of 30 A. Furthermore, the switches achieve very low gate-charges of below 5 nC and reverse recovery charges of 12 nC. The on-state- and reverse-state-performances are benchmarked against other state-of-the-art power devices and compared to the theoretical limits.

Interaction of indium oxide nanoparticle film surfaces with ozone, oxygen and water

Abstract: authoren The interaction of defect-rich nanocrystalline indium oxide films, which have previously shown to exhibit excellent ozone sensing properties, with O3, O2, and H2O molecules is investigated using ultra-violet and X-ray photoelectron spectroscopy. The investigated samples are grown by metalorganic chemical vapor deposition at low temperatures resulting in high oxygen deficiency and high defect density. The ozone-induced surface oxidation and UV-induced photoreduction mechanisms of the ozone sensor active material are evaluated with respect to surface stoichiometry and electronic properties including adsorbate features, band bending and surface dipole formation. A strong interaction with ozone and water is found, whereas the interaction with O2 is relatively weak. In all cases the interaction results in the same negatively charged oxygen adsorbate species, which can either be removed by UV light or by annealing resulting in the capability of these films to be used in reversible adsorption induced oxidation and UV/thermal reduction cycles.

A portable W-band radar system for enhancement of infrared vision in fire fighting operations

Abstract: In this paper, we present a millimeter wave radar system which will enhance the performance of infrared cameras used for fire-fighting applications. The radar module is compact and lightweight such that the system can be combined with inertial sensors and integrated in a hand-held infrared camera. This allows for precise distance measurements in harsh environmental conditions, such as tunnel or industrial fires, where optical sensors are unreliable or fail. We discuss the design of the RF front-end, the antenna and a quasi-optical lens for beam shaping as well as signal processing and demonstrate the performance of the system by in situ measurements in a smoke filled environment.

Single-input GaN gate driver based on depletion-mode logic integrated with a 600 V GaN-on-Si power transistor

Abstract: This work presents a monolithically-integrated power circuit with a single control input gate driver based on depletion-mode logic and a 600 V, 150 mΩ power HEMT in GaN-on-Si technology. The gate driver final-stage is a push-pull circuit, in which the pull-up transistor is indirectly driven through a depletion-load logic inverter, whereas the pull-down transistor is directly driven by the single external control input. Measurements of soft- and hard-switching turn-on transitions in an inductive-load half-bridge at 300 V/ 4 A demonstrate controllability of the turn-on speed by adding an external speedup resistor in parallel to the depletion-load. Gate-charge measurements show a 25-fold reduction of external pre-driver drive capability requirement during a 400 V turn-on transition, since the main power transistor gate-charge (8.5 nC)-related losses are provided and dissipated within the GaN power device, and only the pull-down gate driver transistor gate-charge of 0.34 nC has to be provided externally by the pre-driver circuit.

Enhanced actuation of nanocrystalline diamond microelectromechanical disk resonators with AlN layers

Abstract: A great potential of the use of aluminum nitride (AlN) to enhance the actuation of nanocrystalline diamond (NCD) microelectromechanical system disk resonators is revealed. A disk resonator with a unimorph (AlN/NCD) structure is fabricated by depositing a c-axis oriented AlN on a capacitive NCD disk resonator. The unimorph resonator is piezoelectrically actuated with flexural whispering gallery modes with a relatively large electrode gap spacing, i.e., the spacing which is greater than 1 μm, although this is not possible for the capacitive NCD disk resonator. This result is explained by a finite element method simulation where the piezoelectric actuation turns out to be more effective than the capacitive actuation when the electrode gap spacing is >0.8 μm. The simulation also shows that the electrode gap spacing required for the capacitive actuation to be more effective than the piezoelectric actuation exponentially decreases when the resonator dimension is scaled down for higher frequency operations. Our ...

An investigation of millimeter wave switches based on shunt transistors including SPDT SWITCH MMICs up to 300 GHz

Abstract: This paper reports on a design study for millimeter wave single-pole multiple-throw switches, which are based on shunt transistors. For the investigation of an optimized design flow this study focuses on the three main steps of a millimeter wave switch design: identifying the optimum transistor gate width, large signal modeling of shunt transistors and the MMIC design. Based on the investigations of the optimum transistor gate width, it will be shown that insertion loss and output signal dynamic of a switch are directly correlated to the on-resistance of the utilized semiconductor technology. To prove the feasibility of this study, two RF SPDT switch MMICs, operating in the frequency range from 53 to 150 GHz and from 200 to 330 GHz, respectively, were designed and fabricated. Both switches show low insertion loss, high output signal dynamic, high yield and good agreement to the S-parameter simulations, based on the proposed shunt FET model. The proposed W-band and H-band SPDT switch MMICs achieve an insertion loss of 2 dB and 1.7 dB, respectively, and an output signal dynamic of up to 47 and 20 dB, respectively.

Electrostatic Self-Assembly of Diamond Nanoparticles onto Al- and N-Polar Sputtered Aluminum Nitride Surfaces

Abstract: Electrostatic self-assembly of diamond nanoparticles (DNPs) onto substrate surfaces (so-called nanodiamond seeding) is a notable technique, enabling chemical vapor deposition (CVD) of nanocrystalline diamond thin films on non-diamond substrates. In this study, we examine this technique onto differently polarized (either Al- or N-polar) c-axis oriented sputtered aluminum nitride (AlN) film surfaces. This investigation shows that Al-polar films, as compared to N-polar ones, obtain DNPs with higher density and more homogeneously on their surfaces. The origin of these differences in density and homogeneity is discussed based on the hydrolysis behavior of AlN surfaces in aqueous suspensions.

Quasi-Bessel beams from asymmetric and astigmatic illumination sources.

Abstract: We study the spatial intensity distribution and the self-reconstruction of quasi-Bessel beams produced from refractive axicon lenses with edge emitting laser diodes as asymmetric and astigmatic illumination sources. Comparing these to a symmetric mono-mode fiber source, we find that the asymmetry results in a transition of a quasi-Bessel beam into a bow-tie shaped pattern and eventually to a line shaped profile at a larger distance along the optical axis. Furthermore, we analytically estimate and discuss the effects of astigmatism, substrate modes and non-perfect axicons. We find a good agreement between experiment, simulation and analytic considerations. Results include the derivation of a maximal axicon angle related to astigmatism of the illuminating beam, impact of laser diode beam profile imperfections like substrate modes and a longitudinal oscillation of the core intensity and radius caused by a rounded axicon tip.

Enhancement-mode AlGaN/GaN FinFETs with high on/off performance in 100 nm gate length

Abstract: This paper reports on the design and fabrication of enhancement-mode high-electron mobility transistors (HEMTs) in AlGaN/GaN FinFET technology with 100 nm of gate length (L g = 100 nm). Provided by the lateral as well as the vertical modulation of the fin-shaped channels, the threshold voltages of the designed transistors are made possible to be shifted toward the positive direction, enabling the enhancement-mode (E-mode) of operation. The fabricated FinFETs also exhibit highly-improved off-state performance with minimised short-channel effects (SCE) as a result of the enhanced gate control. A very high on/off current ratio of 108 and a sub-threshold swing of 75 mV/decade are recorded by the E-mode devices with a threshold voltage of +0.2 V, showing substantial potential for high speed logic, mixed-signal and power electronics applications.

A millimeter-wave low-noise amplifier MMIC with integrated power detector and gain control functionality

Abstract: A compact E-band amplifier circuit has been developed for use in ultra-high capacity point-to-point communication links. The millimeter-wave monolithic integrated circuit (MMIC) consists of a two-stage low-noise amplifier (LNA), a 10 dB line coupler, an integrated detector diode and a single-stage variable gain amplifier (VGA). The multifunctional MMIC was realized by using a 50 nm InAlAs/InGaAs based metamorphic high electron mobility transistor (mHEMT) technology in combination with grounded coplanar waveguide topology (GCPW) and cascode transistors, thus leading to a very low noise figure in combination with high gain and large operational bandwidth at millimeter-wave frequencies. The fabricated LNA circuit achieved a maximum gain of 37 dB at 78 GHz and more than 34 dB in the frequency range from 69 to 98 GHz. Furthermore, a room temperature (T = 293 K) noise figure of 2.3 dB and a detector responsivity of 39.000 V/W have been obtained at the frequency of operation.

Analysis and modeling of GaN-based multi field plate Schottky power diodes

Abstract: Lateral GaN-based Schottky diodes are promising for high-voltage, high frequency, low-loss power applications due to their low forward- and reverse charges resulting from the physical properties of the AlGaN/GaN-heterojunction. For GaN-Schottky diodes in the 600V/10A power range, reverse charges as low as 6nC can be achieved. The reverse current I RRM does not exceed 150mA at a switch-off slope of 25A/μs and reverse voltage of 400V, for higher switching-speeds of 170A/μs an I RRM of 510rmA is measured. This work focuses on compact modeling of non-linear capacitances of lateral, high-voltage Schottky diodes realized in a multi field plate design. All parameters for the model in forward and reverse direction are extracted from capacitance-voltage (C-V)- and IV-measurements presented in this work.

Performance of tri-gate AlGaN/GaN HEMTs

Abstract: The performance of the GaN-based tri-gate HEMT is investigated by 3D numerical simulations. The tri-gate concept is shown to provide normally-off operation and to effectively suppress short-channel effects (SCEs). Furthermore, it is shown from our simulations that tri-gate AlGaN/GaN HEMTs can exhibit higher breakdown voltages and operate closer to the theoretical limit for GaN devices than their planar counterpart. Moreover, the RF performance of tri-gate HEMTs with optimized body design can be superior to that of conventional planar devices.

Study of a silicon parallel plate capacitor as a dew point sensor

Abstract: In this work we study a silicon parallel plate capacitor as a dew point sensor. Accumulation of water between the parallel plates below dew point temperature leads to an increase in capacitance. The expected 80-fold increase in capacitance (ratio of relative permittivity of water to air) holds true at high frequencies while at low frequencies an increase up to four orders of magnitude has been measured. We report that the condensed water models more as a conductor than as a dielectric at low frequencies, which, along with the native oxide of silicon, explains the unexpected increase in capacitance.

A W-band wireless communication transmitter utilizing a stacked-FET oscillator for high output power performance

Abstract: This paper reports on an efficient transmitter monolithic microwave integrated circuit (TX MMIC) suitable for high-speed wireless communication. In order to achieve high output power, the TX is based on a direct modulation approach, containing a stacked-FET voltage-controlled oscillator (VCO) and an amplitude modulator. Thus, the modulation scheme is based on amplitude-shift keying (ASK). The MMIC utilizes the Fraunhofer IAF 50nm gate-length metamorphic high-electron-mobility transistor (mHEMT) technology. The stacked-FET oscillator generates the carrier signal and achieves an output power of about 14 dBm. The carrier frequency can be tuned from 87.8 to 98.2 GHz. Due to the FET-stacking approach the amplitude modulator can be simplified to a single-pole, single-throw (SPST) switch. Hence, the transmitter MMIC achieves a peak output power of 12.5dBm and a maximum data rate of 18 Gbit/s. The maximum continuous wave (CW) efficiency of the entire TX MMIC yields 17.6 %.