Naohiro Tsurumi

AlN Passivation Over AlGaN/GaN HFETs for Surface Heat Spreading

Reduction of thermal resistance in AlGaN/GaN heterojunction field-effect transistors (HFETs) is critical for further increase in their output power to be handled in these promising material systems. In this paper, we present a new technique to reduce it using polycrystalline AlN passivation deposited by dc sputtering as a surface heat spreader over AlGaN/GaN HFETs. The AlN is deposited by dc sputtering, and it is well oriented to the c-axis. The measured thermal resistances of AlGaN/GaN HFETs with AlN passivation are lower than those with SiN passivation, which well agree with the results of the thermal simulation. The most notable change of the characteristics by AlN passivation is that the drain current increases by 30%, and the on-state resistance is reduced by 66% by the passivation; moreover, the current collapse suppresses successfully. As a result, the radio frequency output power is higher than that with conventional SiN passivation and is increased by increasing the thickness of AlN passivation. Thus, reduction of the thermal resistance and higher output power by AlN passivation is experimentally achieved.

200 W Output Power at S-Band in AlGaN/GaN Heterojunction Field Effect Transistors with Field Plates on Si Substrates

Use of Si substrates for the fabrication of microwave AlGaN/GaN heterojunction field effect transistors (HFETs) has been strongly desired for the low cost fabrication. The performance so far has never been satisfactory in view of the output power and the gain as compared with those on SiC substrates. In this paper, AlGaN/GaN HFETs on Si with high output power of 203 W and high linear gain of 16.9 dB at 2.5 GHz are demonstrated. The HFETs have field plates to reduce the feedback capacitance leading to higher gain, of which a new design of the field plates enables high power as well. The structural design is based on the equivalent circuit model using the device parameters extracted from the small signal RF performances. Here, it is found that shortening the field plate length down to 0.6 µm results in the high output power owing to the stable output impedance for various drain voltage. Note that the conditions of the epitaxial growth are optimized to achieve high current density of 850 mA/mm with both the high mobility and high sheet carrier concentration. The device processing is established so as to achieve the high power operation free from the current collapse. The device can be operated at the drain voltage as high as 50 V, which enables the 200 W output power. The presented AlGaN/GaN HFETs are very promising for various microwave applications including cellular base stations, which would lower the system cost taking advantage of cost-effective Si substrates.

High-Power and High-Gain S-band AlGaN/GaN HFETs with Source Field Plates on Si Substrate

1. Introduction AlGaN/GaN heterojunction field-effect transistors (HFETs) have been widely investigated for high-frequency and high-power applications such as base stations of cellular phones, taking advantages of the superior material properties. Higher gain is also required in such applications, which reduces the number of amplifiers leading to smaller system in size. Introduction of field plate structures in the HFETs increases the gain by reducing gate-drain feedback capacitances (C gd) [1]. However, there has been a limitation of increasing the maximum output power keeping the high gain in the reported devices. In this paper, we present 203W output power with high gain of 16.9dB at 2.5GHz in AlGaN/GaN HFETs on Si substrates with source field plates. The detailed simulation using the device parameters at various biasing conditions reveals that shortening the field plate length achieves high output power together with the high gain, which well agrees with the experimental esults. r

GaN Transistors for Power Switching and High Frequency Applications

We review our state-of-the-art GaN-based device technologies for power switching at low frequencies and high frequency operation aiming at future millimeter-wave communication systems. These two applications are emerging in addition to the widely investigated power amplifiers at microwave frequencies for cellular base stations. As for the power switching GaN devices, we present a novel device structure called Gate Injection Transistors (GIT), which enables normally-off operation with high drain current. Here we also present the world highest breakdown voltage of 10400 V in AlGaN/GaN HFETs. In the last part of this paper, we present GaN-based MIS-HFETs which exhibits as high fmax as 203 GHz. The successful integration of low- loss microstrip lines with via-holes onto sapphire enables compact 3-stage K-band amplifier MMIC of which a small-signal gain is as high as 22d B at 26 GHz with a 3dB bandwidth of 25-29 GHz. The presented devices are promising for the two emerging future applications demonstrating high enough potential of GaN-based transistors.

Improved hysteresis in a normally-off AlGaN/GaN MOS heterojunction field-effect transistor with a recessed gate structure formed by selective regrowth

A normally-off AlGaN/GaN MOS heterojunction field-effect transistor (MOS-HFET) with a recessed gate structure formed by selective area regrowth is demonstrated. The fabricated MOS-HFET exhibits a threshold voltage of 1.7 V with an improved hysteresis of 0.5 V as compared with a device fabricated by a conventional dry etching process. An analysis of capacitance–voltage (C–V) characteristics reveals that the dry etching process increases interface state density and introduces an additional discrete trap. The use of the selective area regrowth technique effectively suppresses such degradation, avoiding the MOS interface from being exposed to dry etching. The results presented in this paper indicate that the selective area regrowth technique is promising for the fabrication of normally-off AlGaN/GaN MOS-HFETs.