Self-heating in high-power AlGaN-GaN HFETs

We compare self-heating effects in AlGaN-GaN heterostructure field effect transistors (HFETs) grown on sapphire and SiC substrates. Heat dissipation strongly affects the device characteristics soon after the application of the source-drain voltage (in less than 10^-7 s). Our results show that in HFET's with the total epilayer thickness less than 1.5 μm, the thermal impedance, Θ is primarily determined by the substrate material and not by the material of the active layer. For our devices grown on 6H-SiC substrates, we measured Θ of approximately 2°C·mm/W, which was more than an order of magnitude smaller than Θ=25°C mm/W measured for similar AlGaN/GaN HFET's grown on sapphire. Our results demonstrate that AlGaN-GaN HFET's grown on SiC substrates combine advantages of superior electron transport properties in AlGaN/GaN heterostructures with excellent thermal properties of SiC, which should make these devices suitable for high-power electronic applications     

Thermal management of AlGaN-GaN HFETs on sapphire using flip-chip bonding with epoxy underfill

Self-heating imposes the major limitation on the output power of GaN-based HFETs on sapphire or SiC. SiC substrates allow for a simple device thermal management scheme; however, they are about a factor 20-100 higher in cost than sapphire. Sapphire substrates of diameters exceeding 4 in are easily available but the heat removal through the substrate is inefficient due to its low thermal conductivity. The authors demonstrate that the thermal impedance of GaN based HFETs over sapphire substrates can be significantly reduced by implementing flip-chip bonding with thermal conductive epoxy underfill. They also show that in sapphire-based flip-chip mounted devices the heat spread from the active region under the gate along the GaN buffer and the substrate is the key contributor to the overall thermal impedance.     

Thermal modeling and measurement of AlGaN-GaN HFETs built on sapphire and SiC substrates

We present thermal modeling and measurement results of AlGaN-GaN heterojunction field effect transistors fabricated on sapphire and SiC substrates, respectively. The device structures are identical except for the substrate material used to grow the AlGaN-GaN heterostructure. One objective is to study the effect of substrate material on the thermal and electrical performance of the resulting devices. To compute the temperature profiles, in-house PAMICE code developed for a three-dimensional structure was used. To measure the temperatures on the chip surface, nematic liquid crystal thermography was used. This technique is nondestructive and can be performed in realtime during device operation. It has submicrometer spatial resolution and /spl plusmn/1/spl deg/C temperature accuracy. The measured temperatures agree well with the calculated ones. The relationship between the measured temperature and power is almost linear for both types of devices. The junction-to-case thermal resistance of the device fabricated on sapphire substrate is 4.4 times that of the device built on SiC substrate.     

12 W/mm AlGaN-GaN HFETs on silicon substrates

Al/sub 0.26/Ga/sub 0.74/N-GaN heterojunction field-effect transistors were grown by metal-organic chemical vapor deposition on high-resistivity 100-mm Si (111) substrates. Van der Pauw sheet resistance of the two-dimensional electron gas was 300 /spl Omega//square with a standard deviation of 10 /spl Omega//square. Maximum drain current density of /spl sim/1 A/mm was achieved with a three-terminal breakdown voltage of /spl sim/200 V. The cutoff frequency and maximum frequency of oscillation were 18 and 31 GHz, respectively, for 0.7-/spl mu/m gate-length devices. When biased at 50 V, a 2.14-GHz continuous wave power density of 12 W/mm was achieved with associated large-signal gain of 15.3 dB and a power-added efficiency of 52.7%. This is the highest power density ever reported from a GaN-based device grown on a silicon substrate, and is competitive with the best results obtained from conventional device designs on any substrate.     

Source resistance reduction of AlGaN-GaN HFETs with novel superlattice cap layer

We have developed a novel AlGaN-GaN heterojunction field effect transistor (HFET) with an ultralow source resistance by employing the novel superlattice (SL) cap structure. The particular advantage of the SL cap, i.e., the existence of multiple layers of the polarization-induced two-dimensional electron gas (2DEG) with high mobility and high concentration at each AlGaN-GaN interface, is fully exploited for lowering the lateral resistance and the potential barrier at the interface of the SL cap and the HFET barrier layer. By designing the AlGaN-GaN thickness ratio, we have established a method to obtain the optimized SL structure and have achieved an extremely low source resistance of 0.4 /spl Omega//spl middot/mm which is lower not only than HFETs with the conventional structure but also than those with the n-GaN cap structure. The SL cap HFET fabricated on a sapphire substrate exhibited excellent dc and RF performance, i.e., maximum transconductance of over 400 mS/mm, maximum drain current of 1.2 A/mm, a cutoff frequency of 60 GHz, a maximum frequency of oscillation of 140 GHz, and a very low noise figure minimum of 0.7 dB at 12 GHz.     

Micro-Raman temperature measurements for electric field assessment in active AlGaN-GaN HFETs

Temperature profiles in the source/drain (S/D) opening of a single finger AlGaN-GaN heterostructure field-effect transistor were studied at increasing S/D voltages by micro-Raman spectroscopy with <1 /spl mu/m spatial resolution. These profiles imply high field regions near the gate edge of length /spl sim/0.4 /spl mu/m for S/D voltages between 45 and 75 V. Electric field strengths of /spl sim/1.2 and /spl sim/1.9 MV/cm are estimated for 45 and 75 V S/D voltage. The experimental results are in excellent agreement with 2-D Monte Carlo simulations.     

Low-frequency 1/f noise and persistent transients in AlGaN-GaN HFETs

Persistent photoresponse transients and low-frequency 1/f noise were measured in barrier-controlled devices such as AlGaN-GaN heterostructure field-effect transistors (HFETs). The persistent transients and 1/f noise were observed in drain and gate currents. A model describing trapping in a barrier-controlled device introduced, and the appropriate transient evolution and the noise spectra developed. Excellent agreement was obtained between the experimental measurements and the predicted temporal response and noise spectra. Finally, the correlation between drain and gate 1/f noise was measured, confirming that the same noise source (fluctuation in surface potential) is responsible to both currents.     

The improvement of DC performance in AlGaN-GaN HFETs with isoelectronic Al-doped channels

Enhancement of electrical properties of an AlGaN-GaN heterostructure was achieved through isoelectronic Al-doping of the undoped channel layer during the growth by metal-organic chemical vapor deposition. The two-dimensional electron gas mobility was increased from 3390 to 4870 cm/sup 2//V/spl middot/s at 77 K, and the crystal quality was significantly improved as Al atoms were incorporated in the undoped GaN film. The AlGaN-GaN HFETs were fabricated on this material structure and exhibited a maximum drain current of 909 mA/mm, and a maximum transconductance of 232 mS/mm, corresponding to an increase of 30% and 21%, respectively.     

Polyimide passivated AlGaN-GaN HFETs with 7.65 W/mm at 18 GHz

Current metal-organic chemical vapor deposition-grown AlGaN-GaN heterojunction field-effect transistor devices suffer from threading dislocations and surface states that form traps, degrading RF performance. A passivation scheme utilizing a polyimide film as the passivating layer was developed to reduce the number of surface states and minimize RF dispersion. Continuous-wave power measurements were taken at 18 GHz on two-finger 0.23-/spl mu/m devices with 2/spl times/75 /spl mu/m total gate width before and after passivation yielding an increase from 2.14 W/mm to 4.02 W/mm in power density, and 12.5% to 24.47% in power added efficiency. Additionally, a 2/spl times/25 /spl mu/m device yielded a peak power density of 7.65 W/mm at 18 GHz. This data suggests that polyimide can be an effective passivation film for reducing surface states.     

Thermal resistance calculation of AlGaN-GaN devices

We present an original accurate closed-form expression for the thermal resistance of a multifinger AlGaN-GaN high electron-mobility transistor (HEMT) device on a variety of host substrates including SiC, Si, and sapphire, as well as the case of a single-crystal GaN wafer. The model takes into account the thickness of GaN and host substrate layers, the gate pitch, length, width, and thermal conductivity of GaN, and host substrate. The model's validity is verified by comparing it with experimental observations. In addition, the model compares favorably with the results of numerical simulations for many different devices; very close (1%-2%) agreement is observed. Having an analytical expression for the channel temperature is of great importance for designers of power devices and monolithic microwave integrated circuits. In addition, it facilitates a number of investigations that are not practical or possible using time-consuming numerical simulations. The closed-form expression facilitates the concurrent optimization of electrical and thermal properties using standard computer-aided design tools.     

Cat-CVD SiN-passivated AlGaN-GaN HFETs with thin and high Al composition barrier Layers

The effect of SiN surface passivation by catalytic chemical vapor deposition (Cat-CVD) on Al/sub 0.4/Ga/sub 0.6/N-GaN heterostructure field-effect transistors (HFETs) was investigated. The channel sheet resistance was reduced by the passivation due to an increase in electron density, and the device characteristics of the thin-barrier HFETs were significantly improved by the reduction of source and drain resistances. The AlGaN(8 nm)-AlN(1.3 nm)-GaN HFET device with a source/drain distance of 3 /spl mu/m and a gate length of 1 /spl mu/m had a maximum drain current density of 0.83 A/mm at a gate bias of +1.5 V and an extrinsic maximum transconductance of 403 mS/mm. These results indicate the substantial potential of Cat-CVD SiN-passivated AlGaN-GaN HFETs with thin and high Al composition barrier layers.     

High performance BCB-bridged AlGaAs/InGaAs power HFETs

A novel low-k benzocyclobutene (BCB) bridged and passivated layer for AlGaAs/InGaAs doped-channel power field effect transistors (FETs) with high reliability and linearity has been developed and characterized. In this study, we applied a low-k BCB-bridged interlayer to replace the conventional air-bridged process and the SiN/sub x/ passivation technology of the 1 mm-wide power device fabrication. This novel and easy technique demonstrates a low power gain degradation under a high input power swing, and exhibits an improved adjacent channel power ratio (ACPR) than those of the air-bridged one, due to its lower gate leakage current. The power gain degradation ratio of BCB-bridged devices under a high input power operation (P/sub in/ = 5 /spl sim/ 10 dBm) is 0.51 dB/dBm, and this value is 0.65 dB/dBm of the conventional air-bridged device. Furthermore, this novel technology has been qualified by using the 85-85 industrial specification (temperature = 85 C, humidity = 85%) for 500 h. These results demonstrate a robust doped-channel HFET power device with a BCB passivation and bridged technology of future power device applications.     

Microwave noise performance of AlGaN-GaN HEMTs with small DC power dissipation

We report low microwave noise performance of discrete AlGaN-GaN HEMTs at DC power dissipation comparable to that of GaAs-based low-noise FETs. At 1-V source-drain (SD) bias and DC power dissipation of 97 mW/mm, minimum noise figures (NF/sub min/) of 0.75 dB at 10 GHz and 1.5 dB at 20 GHz were achieved, respectively. A device breakdown voltage of 40 V was observed. Both the low microwave noise performance at small DC power level and high breakdown voltage was obtained with a shorter SD spacing of 1.5 /spl mu/m in 0.15-/spl mu/m gate length GaN HEMTs. By comparison, NF/sub min/ with 2 /spl mu/m SD spacing was 0.2 dB greater at 10 GHz.     

Improved power performance for a recessed-gate AlGaN-GaN heterojunction FET with a field-modulating plate

A recessed-gate AlGaN-GaN field-modulating plate (FP) field-effect transistor (FET) was successfully fabricated on an SiC substrate. By employing a recessed-gate structure on an FP FET, the transconductance was increased from 150 to 270 mS/mm, leading to an improvement in gain characteristics, and current collapse was minimized. At 2 GHz, a 48-mm-wide recessed FP FET exhibited a record output power of 230 W (4.8 W/mm) with 67% power-added efficiency and 9.5-dB linear gain with a drain bias of 53 V.     

9.4-W/mm power density AlGaN-GaN HEMTs on free-standing GaN substrates

High power microwave AlGaN-GaN high electron-mobility transistors (HEMTs) on free-standing GaN substrates are demonstrated for the first time. Measured gate leakage was -2.2 /spl mu/A/mm at -20 V and -10 /spl mu/A/mm at -45 V gate bias. When operated at a drain bias of 50 V, devices showed a record continuous-wave output power density of 9.4 W/mm at 10 GHz with an associated power-added efficiency of 40%. Long-term stability of device RF operation was also examined. Under room conditions, devices driven at 25 V and 3-dB gain compression remained stable in 200 h, degrading only by 0.18 dB in output power. Such results illustrate the potential of GaN substrate technology in supporting reliable, high performance AlGaN-GaN HEMTs for microwave power applications.     

AlGaN-GaN double-channel HEMTs

We present the design, fabrication, and characterization of AlGaN-GaN double-channel HEMTs. Two carrier channels are formed in an AlGaN-GaN-AlGaN-GaN multilayer structure grown on a sapphire substrate. Polarization field in the lower AlGaN layer fosters formation of a second carrier channel at the lower AlGaN-GaN interface, without creating any parasitic conduction path in the AlGaN barrier layer. Unambiguous double-channel behaviors are observed at both dc and RF. Bias dependent RF small-signal characterization and parameter extraction were performed. Gain compression at a high current level was attributed to electron velocity degradation induced by interface scattering. Dynamic IV measurement was carried out to analyze large-signal behaviors of the double-channel high-electron mobility transistors. It was found that current collapse mainly occurs in the channel closer to device surface, while the lower channel suffers minimal current collapse, suggesting that trapping/detrapping of surface states is mainly responsible for current collapse. This argument is supported by RF large-signal measurement results.     

Thermal networks for electro-thermal analysis of power devices

This work presents a derivation of the equivalent thermal network associated to the distributed heat diffusion process which takes place in an electrical circuit. Precise definition of temperature and dissipated power density for the elementary electrical element are presented. It is shown that these definitions lead to an equivalent thermal network that preserves the physical properties of the original distributed phenomenon. Using these results an accurate electro-thermal network model of a power DMOS is derived.     

Transient Thermal Analysis of GaN Heterojunction Transistors (HFETs) for High-Power Applications

Transient thermal analysis of GaN heterojunction field-effect transistors (HFETs) was carried out in this letter, with a hybrid nonlinear finite element method (FEM) employed, i.e., combining the element-by-element FEM with the preconditioned conjugated gradient technique. The maximum temperature of the HFETs, strongly depending on the input power density and the duration time of the pulsed heat source, was captured numerically. The effects of temperature-dependent thermal conductivities of the substrates on the maximum temperature were also examined and compared for different substrate materials, such as sapphire, silicon, and SiC     

基于GaAs基大功率激光器的热分析

The thermal characteristics of 808 nm Al Ga As/Ga As laser diodes(LDs) are analyzed via electrical transient measurements and infrared thermography. The temperature rise and thermal resistance are measured at various input currents and powers. From the electrical transient measurements, it is found that there is a significant reduction in thermal resistance with increasing power because of the device power conversion efficiency. The component thermal resistance that was obtained from the structure function showed that the total thermal resistance is mainly composed of the thermal resistance of the sub-mount rather than that of the LD chip, and the thermal resistance of the sub-mount decreases with increasing current. The temperature rise values are also measured by infrared thermography and are calibrated based on a reference image, with results that are lower than those determined by electrical transient measurements. The difference in the results is caused by the limited spatial resolution of the measurements and by the signal being captured from the facet rather than from the junction of the laser diode.     

Drain current DLTS of AlGaN-GaN MIS-HEMTs

The transient behavior of AlGaN-GaN MIS-HEMTs were studied by drain current deep level transient spectroscopy. Two electron traps were observed, one of which had similar activation energy to that of defect that was commonly observed in epitaxial GaN. We compared the results with those of AlGaN-GaN HEMTs. The hole-trap-like positive peaks in the DLTS, which were observed in the HEMTs, were not observed in the MIS-HEMTs. It has been pointed out that the positive peaks did not originate from change in hole trap population in the channel but reflected the change in the electron population in the surface states of the HEMT access regions. The gate insulator was effective to suppress not only the gate leakage current but also the surface-state-related signals.