Shinji Ujita

A 26GHz Short-Range UWB Vehicular-Radar Using 2.5Gcps Spread Spectrum Modulation

A 26 GHz short-range radar (SRR) system has been developed based on a 2.5 Gcps direct sequence spread spectrum (DSSS) technique combined with simple homodyne detection. Separate frequency-triplers have been provided with transmitter (TX) and receiver (RX) chips to cut off the carrier-leakage path from local signal. By using this configuration, carrier-leakage in TX signal can be significantly suppressed falling into the satisfactory range of the UWB regulation. The present radar system achieved the maximum detection range of 14 m with the resolution of 6 cm. TX, RX, and PN generator chipset has been developed using InGaP/GaAs HBT process, where fT/fmax of 65 GHz/ 83 GHz were attained.

A compact GaN-based DC-DC converter IC with high-speed gate drivers enabling high efficiencies

In this paper, we present a novel compact DC-DC converter IC in which normally-off GaN-GITs (Gate Injection Transistors) and gate drivers are integrated into one chip. The DC-DC converter IC can achieve higher efficiency and smaller chip size by reducing parasitic inductances between switching power devices and gate drivers. The gate driver, having a DCFL (Direct Coupled FET Logic) with a buffer amplifier which is consisted of a GaN-HFET (Hetero-junction FET) and GaN-GITs can operate with higher speed and lower power consumption. The fabricated DC-DC converter IC exhibits a peak efficiency as high as 86.6% at 2MHz for the 12V-1.8V conversion.

GaN Gate Injection Transistor with integrated Si Schottky barrier diode for highly efficient DC-DC converters

In this paper, we present a novel GaN-based normally-off transistor with an integrated Si Schottky barrier diode (SBD) for low voltage DC-DC converters. The integrated SBD is formed by the Si substrate for the epitaxial growth of AlGaN/GaN hetero-structure, which is connected to the normally-off GaN Gate Injection Transistor (GIT) over it with via-holes. The diode can flow the reverse current in the conversion operation with lower forward voltage than that of the lateral GaN transistor enabling lower operating loss. A DC-DC converter from 12V down to 1.3V using the integrated devices with the reduced gate length down to 0.5μm exhibits a high peak efficiency of 89% at 2MHz demonstrating the promising potential of GaN devices for the application.

High-speed switching and current-collapse-free operation by GaN gate injection transistors with thick GaN buffer on bulk GaN substrates

GaN-based normally-off Gate Injection Transistors (GITs) with p-type gate over AlGaN/GaN heterojunction are fabricated on bulk GaN substrates. Thickness of insulating GaN buffer layer is increased up to 16 μm for the presented device from 5 μm for conventional GITs on Si. The thick buffer reduces the parasitic output capacitances, which enables fast turn-off switching. The thick buffer and the use of bulk GaN substrate help to improve the crystal quality of AlGaN/GaN so that the sheet resistance is reduced. Improved crystal quality together with reduced trap density successfully suppresses the current collapse up to 1 kV or higher of the applied drain voltage. The resultant RonQoss (Ron: on-state resistance, Qoss: output charge) as a figure-of-merit for high speed turn-off switching is reduced down to 940 mΩnC that is one third from that of GITs on Si. The resultant turn-off dVds/dt reaches as large as 285 V/ns that is twice higher than reported values by GITs on Si.

GaN-based Gate Injection Transistors for power switching applications

GaN-based Gate Injection Transistors (GITs) with p-type gate over AlGaN/GaN heterojunction serve normally-off operations with low on-state resistances owing to the conductivity modulation by injection of holes. Established basic technologies on the GIT have shown promising features for switching applications. Further improvement of the performances would extend the applications and lead to the widespread use. In this paper, recent technologies on the GITs to improve the performances and extract the full potential are described. These include extension of the wafer diameter of Si up to 8 inch, InAlGaN quaternary alloy to reduce the series resistances, shortening the gate length to improve the device performances, integration of the gate driver and flip-chip assembly for faster switching.

A fully integrated GaN-based power IC including gate drivers for high-efficiency DC-DC Converters

In this paper, we present a state-of-the-art integrated GaN power IC capable of operating in a high frequency (MHz) regime. This realizes system size reduction, 60% maximum, of a power IC. The IC consists of two output power transistors (PT) and two gate drivers (GD). The key devices in the IC are normally-off gate injection transistors (GITs) for PT and GD and a normally-on hetero-junction field effect transistor (HFET) for GD. Novel local control of carrier concentration of an identical 2 dimensional electron gas (2DEG) at an AlGaN/GaN interface which made integration of the transistors with such a large threshold voltage difference possible is described. A specially developed post-passivation interconnection process giving low parasitic components is also described. The IC applied to a 12V-1.8V DC-DC converter shows high frequency switching operation well beyond the limit of Si pointing to future improvement in consumer electronics power supply systems.

Vertical GaN-based power devices on bulk GaN substrates for future power switching systems

We propose a normally-off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0 mΩ·cm2 and high off-state breakdown voltage of 1.7 kV. P-GaN/AlGaN/GaN triple layers are epitaxially regrown over V-shaped grooves formed over the drift layer. The channel utilizes so-called semi-polar face with reduced sheet carrier concentration at the AlGaN/GaN interface, which enables high threshold voltage of 2.5 V and stable switching operations. The employed p-type gate does not give any concern of the gate instability. Note that formation of carbon doped insulating GaN layer formed on p-GaN well layer underneath the channel suppresses the punch-through current at off-state between the source and drain, which enables good off-state characteristics. The fabricated high-current vertical transistor achieves successful fast switching at 400V/15A. We also propose a novel vertical GaN-based junction barrier Schottky (JBS) diode with trenched p-GaN region on a bulk GaN substrate. A specific differential on-resistance of the GaN JBS diode is 0.9 mΩ·cm2 while keeping high breakdown voltage of 1.6 kV. These results indicate that the demonstrated vertical GaN devices are very promising for future high power switching applications.

A 26 GHz Transceiver Chipset for Short Range Radar Using Post-Passivation Interconnection

We demonstrate a 26 GHz SiGe bipolar complementary metal oxide semiconductor (BiCMOS) transceiver chipset for short-range spread-spectrum (SS) radar system. The integrated frequency triplers lower the local oscillation frequency down to 8.8 GHz, and eliminate the carrier leak in the transmitting signal which enables high sensitivity. The on-chip balun connected to the mixer demodulating by the pseudo noise (PN) code in the receiver increases the dynamic range of the receiving signal. Low-loss transmission lines on Si substrate are fabricated by post-passivation interconnection process using thick benzocyclobutene (BCB). We have confirmed the transmitting signal having spread-spectrum with suppressing the carrier leak, the improvement of the dynamic range of the receiver, and the reduction of the transmission loss on Si substrate. We have demonstrated the operation of the 26 GHz ultra wide band (UWB) SS radar, corresponding to detecting a human located at 7 m away from the system.