Improved Thermal Performance of AlGaN/GaN HEMTs by an Optimized Flip-Chip Design

AlGaN/GaN high electron mobility transistors (HEMT) on sapphire substrates have been studied for their potential application in RF power applications; however, the low thermal conductivity of the sapphire substrate is a major drawback. Aiming at RF system-in-a-package, the authors propose a flip-chip-integration approach, where the generated heat is dissipated to an AlN carrier substrate. Different flip-chip-bump designs are compared, using thermal simulations, electrical measurements, micro-Raman spectroscopy, and infrared thermography. The authors show that a novel bump design, where bumps are placed directly onto both source and drain ohmic contacts, improves the thermal performance of the HEMT     

Performance characterization of III-V power devices

The power performance of GaAs/AlGaAs pseudomorphic high electron mobility transistors (PHEMTs) has been modeled by using the statistical Design of Experiment approach. Empirical models for the small signal gain, output power and power added efficiency have been developed. The “walk-out/in” phenomenon has been observed in the devices as a result of power measurements. The evolution of surface photovoltage spectra after RF power stress indicates accumulation of positive electrical charge in the buffer and the surface layer of the devices.     

Determination of the average channel temperature of GaN MOSHFETs under continuous wave and periodic-pulsed RF operational conditions

We present a method to determine the average device channel temperature of AlGaN/GaN metal–oxide–semiconductor heterostructure field effect transistors (MOSHFETs) in the time domain under continuous wave (CW) and periodic-pulsed RF (radiation frequency) operational conditions. The temporal profiles of microwave output power densities of GaN MOSHFETs were measured at 2 GHz under such conditions and used for determination of the average channel temperature. The measurement technique in this work is also being utilized to determine the thermal time constant of the devices. Analytical temporal solutions of temperature profile in MOSHFETs are provided to support the method. The analytical solutions can also apply to generic field effect transistors (FETs) with an arbitrary form of time-dependent heat input at the top surface of the wafer. It is found that the average channel temperature of GaN MOSHFETs on a 300 μm sapphire substrate with the output power of 10 W/mm can be over 400 °C in the CW mode while the average channel temperature of GaN MOSHFETs on a SiC substrate with the same thickness only reaches 50 °C under the same condition. The highest average channel temperature in a pulsed RF mode will vary with respect to the duty cycle of the pulse and type of the substrate.     

Read versus flat doping profile structures for the realization ofreliable high-power, high-efficiency 94 GHz IMPATT sources

An investigation of the potential RF performance of various types of silicon IMPATT homojunction structures was carried out to identify the most efficient type for reliable high-power, high-efficiency CW generation in the 94 GHz window. The study used an IMPATT oscillator model accounting, in a self-consistent manner, for both thermal limitation and diode impedance matching. The main result is that in contrast to lower operating frequencies, the realization of a Read doping profile does not improve the RF performance level compared to structures with a flat doping profile. Operating conditions were optimized for high RF emitted performance conditions. In addition, the fundamental effects on RF performance of both the diode thermal resistance and RF losses have been quantified     

RF MEMS switch integrated on printed circuit board with metallic membrane first sequence and transferring

This letter reports, for the first time, on RF MEMS switches integrated on flexible printed circuit boards (i.e., FR-4) using transfer technology. The devices were first processed on Si-substrate using a modified MEMS sequence and subsequently transferred onto an FR-4 substrate by thermal compressive bonding, mechanical grinding, and wet removal of silicon. The switches were demonstrated with flat metal membrane (top electrode), precisely controlled gap between the membrane and bottom electrode, low insertion loss (/spl les/ 0.15 dB at 20 GHz), and high isolation (/spl sim/ 21 dB at 20 GHz). This technology shows the potential to monolithically integrate RF MEMS components with other RF devices on organic substrate for RF system implementation.     

Three-dimensional modeling of the heat flow into a GaAs substrate. Influence of thermal phenomena on the RF behavior of power HBTs and technological optimization

Using three-dimensional modeling of the heat flow into the substrate, and taking into account the variation of thermal conductivity with temperature, we study the sensitivity of the thermal resistance R_TH to parameters such as the substrate thickness, its nature (GaAs, Si, AlN or diamond), the topology of the power source (length and width) or even the power density. Thus, we show that the transfer of active GaAs layers onto a host substrate with a higher thermal conductivity such as Si, AlN or diamond is of greater interest than the thinning of the substrate.We developed an accurate electrothermal model for the AlGaAs/GaAs heterojunction bipolar transistor (HBT) allowing prediction of the static and mainly dynamic (RF) behavior at a high signal operating mode. We show that if the HBT layers are transferred onto a diamond substrate, a 50% gain in RF power can be expected at 10 GHz.Finally, we present the technological solutions investigated for the transfer and the heteroassembly of HBT active layers on substrates better suited for thermal dissipation.     

A New Lossy Substrate Model for Accurate RF CMOS Noise Extraction and Simulation With Frequency and Bias Dependence

A lossy substrate model is developed to accurately simulate the measured RF noise of 80-nm super-100-GHz f_T n-MOSFETs. A substrate RLC network built in the model plays a key role responsible for the nonlinear frequency response of noise in 1-18-GHz regime, which did not follow the typical thermal noise theory. Good match with the measured S-parameters, Y-parameters, and noise parameters before deembedding proves the lossy substrate model. The intrinsic RF noise can be extracted easily and precisely by the lossy substrate deembedding using circuit simulation. The accuracy has been justified by good agreement in terms of I_d,g_m, Y-parameters, and f _T under a wide range of bias conditions and operating frequencies. Both channel thermal noise and resistance induced excess noises have been implemented in simulation. A white noise gamma factor extracted to be higher than 2/3 accounts for the velocity saturation and channel length modulation effects. The extracted intrinsic NF_min as low as 0.6-0.7 dB at 10 GHz indicates the advantages of super-100 GHz f_T offered by the sub-100-nm multifinger n-MOSFETs. The frequency dependence of noise resistance R_n suggests the bulk RC coupling induced excess channel thermal noise apparent in 1-10-GHz regime. The study provides useful guideline for low noise and low power design by using sub-100-nm RF CMOS technology     

High-Power CW Klystron for Fusion Experiments

A 3.7-GHz 700-kW klystron in continuous-wave (CW) operation has been developed to upgrade the lower hybrid RF plasma heating power in a tokamak up to 10 MW. The klystron is equipped with a diode gun, a five-cavity RF structure, two BeO-disk RF windows, and a large-size X-ray-shielded hypervapotron collector. The output power is recombined in a four-port junction which we also developed. The tube is designed to deliver 620-kW CW RF power with a mismatched load (VSWR = 1.4) and 700-kW CW with a matched load. Several prototypes have been built with successive design improvements. The major improvement was to change one single RF output into two RF outputs. The most recently built prototype meets all design specifications at 73.1 kV and 20.7 A, with an efficiency of 47% on a matched load and 40% with a 1.4 : 1 VSWR load, worst case phase. The power losses dissipated in the body have been measured as low as 17 kW, which corresponds to the RF heating and implies low beam interception. The measured temperatures of the output cavity noses and collector wall have been kept below 130degC and 200degC, respectively, which results in large thermal margin.     

Frequency-offset Cartesian feedback for MRI power amplifier linearization

High-quality magnetic resonance imaging (MRI) requires precise control of the transmit radio-frequency (RF) field. In parallel excitation applications such as transmit SENSE, high RF power linearity is essential to cancel aliased excitations. In widely-employed class AB power amplifiers, gain compression, cross-over distortion, memory effects, and thermal drift all distort the RF field modulation and can degrade image quality. Cartesian feedback (CF) linearization can mitigate these effects in MRI, if the quadrature mismatch and dc offset imperfections inherent in the architecture can be minimized. In this paper, we present a modified Cartesian feedback technique called "frequency-offset Cartesian feedback" (FOCF) that significantly reduces these problems. In the FOCF architecture, the feedback control is performed at a low intermediate frequency rather than dc, so that quadrature ghosts and dc errors are shifted outside the control bandwidth. FOCF linearization is demonstrated with a variety of typical MRI pulses. Simulation of the magnetization obtained with the Bloch equation demonstrates that high-fidelity RF reproduction can be obtained even with inexpensive class AB amplifiers. Finally, the enhanced RF fidelity of FOCF over CF is demonstrated with actual images obtained in a 1.5 T MRI system.     

Pulsed characterisation of GaN HEMTs on Si (1 1 1) substrate

The DC and RF power characteristics of GaN HEMTs under continuous wave (CW) and pulsed load-pull measurement are examined, in this article. The results give a comprehensive understanding of self-heating effects and allow improved heat dissipation, by pulsed measurement. The measured output power increases under the pulse load-pull measurement, due to the isothermal environment. The RF power performance for pulsed mode was measured at 3.5?GHz, with 18.4?dB power gain and a large 3.5?W/mm power output, under pulse load-pull, which is a 3.25?dB improvement, compared to CW operation. The relationship of output power and impedance is determined by load-pull measurement.     

Thermal behavior of tensile-strain InGaAsP-InP lasers with varying ridgewidth

The thermal behavior of tensile-strain InGaAsP-InP laser diodes with varying ridgewidth is investigated experimentally. The temperature rise during continuous-wave (CW) operation was determined by comparing CW light-current (L-I) curves with short-pulsed (L-I) curves at different substrate temperatures. It is shown that the thermal impedance R depends strongly on the width w of the ridge and that this effect is different before and after threshold. In the first case, R decreases with increase of ln(w) and in the second case, R increases with increase of 1/w. However, the wider the ridge, the lower the CW maximum operating temperature because of the higher temperature dependence of dl/sub th/(T)/dT.     

Very long pulse high power klystrons for FEL and their generating conditions

High power RF sources for major scientific machines (accelerators, plasma heating) are often either short pulse devices with very high peak power levels (e.g. 30-40 MW peak, 5μs pulses in S-band) or CW devices (e.g. 500 kW in S-band). Free electron lasers, on the other hand, require klystrons operating in a long-pulse mode (100 μs and 10 ms). In this case, the technology used in the two other cases (short pulse, CW) is not necessarily appropriate. When the peak power levels are very high in nearly-CW operation, new problems are encountered, for example thermal cycling and HV breakdown modes. The most critical parts are the output cavity, output window and the electron gun. We will discuss a family of 1·3 GHz klystrons as an example: 35-40 MW/10μs; 10-15 MW/200 μs and a few MW/1-10ms     

Fabrication and properties of multiple vertically stacked Josephsontunnel junctions

Vertically stacked Josephson tunnel junctions with a high number of junctions (up to n=20) were realized using a fabrication process based on the Nb/Al technology. An experimental study aimed toward their potential applications was made. First, we report on a series of DC measurements, with the main purpose of studying the quality of the I-V curve as a function of the number of barriers in the stack; we found that the junction quality slowly becomes worse as we increase this number. In fact, some thermal effects appear, which result in an increase of the subgap current and a decrease of the width of the current jump at the total gap voltage. Further, we found a low frequency voltage noise on devices with n>10 that depends on the junction area and on the substrate nature. Later, we discuss some RF properties of stacked junctions irradiated with a microwave signal. These measurements show that they seem to behave independently. Once again, the thermal effects drastically influence the response to an external RF drive and seem to limit the use for applications of these devices     

X-Band high burnout resistance Schottky-barrier diodes

An investigation of high burnout resistant Schottky-barrier diodes forX-band frequencies is presented. These diodes exhibit pulse RF burnout capability greater than 100 W (τ = 3 ns, 10³pulses/s) and CW burnout capability of 5 W, which is a two to five times better performance than most Schottky-barrier diodes manufactured today. The diodes also exhibit nearly ideal electrical characteristics and low noise figures. The failure analysis of these devices under CW power and pulsed RF power (τ ≃ 3 ns) is also discussed.     

Surface-passivated high-resistivity silicon substrates for RFICs

Surface passivation of high-resistivity silicon (HRS) by amorphous silicon thin-film deposition is demonstrated as a novel technique for establishing HRS as a microwave substrate. Metal-oxide-silicon (MOS) capacitor measurements are used to characterize the silicon surface properties. An increase of the quality factor (Q) of a 10-nH spiral inductor by 40% to Q=15 and a 6.5-dB lower attenuation of a coplanar waveguide (CPW) at 17 GHz indicate the beneficial effect of the surface passivation for radio frequency (RF) and microwave applications. Regarding CPW attenuation, a nonpassivated 3000-/spl Omega//spl middot/cm substrate is equivalent to a 70-/spl Omega//spl middot/cm passivated substrate. Surface-passivated HRS, having minimum losses, a high permittivity, and a high thermal conductivity, qualifies as a close-to-ideal radio frequency and microwave substrate.     

Low-threshold oxide-confined GaInNAs long wavelength verticalcavity lasers

We report, for the first time, room temperature continuous-wave (CW) operation of GaInNAs vertical-cavity surface-emitting laser diodes emitting at a wavelength of 1.2 μm and grown all-epitaxially in a single step on a GaAs substrate. Oxide-apertured devices demonstrated CW threshold currents as low as 1 mA, slope efficiency above 0.045 W/A, and thermal impedance of 1.24 K/mW. Larger sized devices exhibited a pulsed threshold current density of 2-2.5 kA/cm² and slope efficiency above 0.09 W/A     

基于76.2 mm圆片工艺的GaN HEMT可靠性评估

报道了利用76.2 mm圆片工艺实现了SiC衬底GaN HEMT微波功率管的研制,并对其进行了多项试验以评估其可靠性.器件工艺中通过引入难熔金属作器件肖特基势垒,有效提高了GaN HEMT器件肖特基势垒的热稳定性,经过500℃高温处理30 s后器件肖特基特性依然保持稳定.随后的高温工作寿命试验表明,该GaNHEMT能够...     

Power transistors fabricated using isotopically purified silicon (/sup 28/Si)

It is well known that isotopic purification of group IV elements can lead to substantial increases in thermal conductivity due to reduced scattering of the phonons. The magnitude of the increase in thermal conductivity depends on the level of isotopic purification, the chemical purity, as well as the test temperature. For isotopically pure silicon (/sup 28/Si) thermal conductivity improvements as high as sixfold at 20 K and 10%-60% at room temperature have been reported. Device heating during operation results in degradation of performance and reliability (electromigration, gate oxide wearout, thermal runaway). In this letter, we discuss the thermal performance of packaged RF LDMOS power transistors fabricated using /sup 28/Si. A novel technique allows the cost effective deployment of this material in integrated circuit manufacturing. A clear reduction of about 5/spl deg/C-7/spl deg/C in transistor average temperature and a corresponding 5%-10% decrease in overall packaged device thermal resistance is consistently measured by infrared microscopy in devices fabricated using /sup 28/Si over natural silicon.     

GaAs TUNNETT diodes on diamond heat sinks for 100 GHz and above

Single-drift GaAs TUNNETT diodes were mounted on diamond heat sinks for improved thermal resistance and evaluated around 100 GHz in a radial line full height waveguide cavity. The diodes were fabricated from MBE-grown material originally designed for diodes that operate in CW mode around 100 GHz on integral heat sinks. An RF output power of more than 70 mW with a corresponding DC to RF conversion efficiency of 4.9% was obtained at 105.4 GHz. This is the first successful demonstration of GaAs TUNNETT diodes mounted on diamond heat sinks. To the authors' knowledge, these DC to RF conversion efficiencies and RF power levels are the highest reported to date from TUNNETT diodes and exceed those of any single discrete device made of group III-V materials (GaAs, InP, etc.) at this frequency. Free-running TUNNETT diode oscillators exhibit clean spectra with an excellent phase noise of less than -94 dBc/Hz, measured at a frequency off-carrier of 500 kHz and an RF output power of 40 mW     

短波射频数字化接收机的设计与实现

针对传统中频数字化接收机复杂度高、灵活性差等问题,设计并实现了一种新型短波射频数字化接收机.该接收机由A/D、数字下变频器(DDC)、DSP、D/A、CPLD等几部分组成,实现了短波信号的射频数字化处理.详细介绍了接收机的设计方案,并对常见的调幅(AM)、等幅报(CW)、单边带(ISB)等信号进行了接收测试.在输入电平为-103～0 dB动态范围内,解调输出质量都能达到12 dB左右信纳得(SND).