Kazuki Ota

A normally-off GaN FET with high threshold voltage uniformity using a novel piezo neutralization technique

In this paper, we successfully demonstrate a recessed gate normally-off GaN FET on a silicon substrate with high threshold voltage (V<inf>th</inf>) uniformity and low on-resistance. In order to realize high V<inf>th</inf> uniformity, a novel V<inf>th</inf> control technique is proposed, which we call “piezo neutralization technique”. This technique includes a piezo neutralization (PNT) layer formed at the bottom of the gate recess. Since the PNT layer neutralizes the polarization charges under the gate, the V<inf>th</inf> comes to be independent of the gate-to-channel span. The fabricated normally-off GaN FET with PNT structure exhibits an excellent V<inf>th</inf> uniformity (σ(V<inf>th</inf>)=18 mV) and a state-of-the-art combination of the specific on-resistance (R<inf>on</inf>A=500 mΩmm²) and the breakdown voltage (V<inf>B</inf>≫1000 V). The normally-off GaN FETs wtih PNT structure show great promise as power devices.

Novel AlGaN/GaN dual-field-plate FET with high gain, increased linearity and stability

We have successfully developed a novel AlGaN/GaN FET with dual field-modulating-plates (FPs). The breakdown voltage is enhanced from 125 to 250 V by adding the second FP to the conventional FP structure. Benefiting from the first FP, no current collapse is observed simultaneously. Since the second FP effectively reduces feedback capacitance, this device provides a 3-dB higher gain along with increased linearity and stability. Under a 2.15-GHz W-CDMA modulation scheme, a dual-FP-FET with a 24-mm gate periphery achieved a state-of-the-art combination of 160-W output power and a 17.5-dB linear gain

A GaAs-based field-modulating plate HFET with improved WCDMA peak-output-power characteristics

A novel GaAs power FET with a field-modulating plate (FP-FET) and operated at a drain bias voltage of 24 V was developed. Regarding the power performance of an FP-FET under wideband code division multiple access (WCDMA) operation, its peak output power significantly degraded in the saturated-output-power region when its FP length was short. However, by introducing a sufficiently large FP length, it was demonstrated that the peak-power degradation can be significantly suppressed. In particular, an FP-FET amplifier fabricated with an FP length of 1.5 /spl mu/m exhibited no WCDMA peak-power degradation and delivered an output power of 120 W with a linear gain of 14.2 dB at 2.14 GHz.

370-W Output Power GaN-FET Amplifier with Low Distortion for W-CDMA Base Stations

High output power and low intermodulation distortion (IMD) GaN-FET amplifiers are highly sought after for W-CDMA base stations. In order to obtain high output power, we have developed a field-modulating plate GaN-FET with a high maximum drain current and a high gate-to-drain breakdown voltage. Also, in order to suppress the memory effects that obstruct digital predistortion (DPD) linearization in W-CDMA power amplifiers, we have newly developed the bias network, in which frequency-dependent impedance at the base band of multi-carrier W-CDMA signals is reduced. As a result, the GaN-FET amplifier developed for W-CDMA base stations achieves a record 370-W W-CDMA peak output power, and a low DPD linearized IMD of less than -50 dBc at the highest W-CDMA average output power of over 60 W, resulting in a significant improvement of DPD linearization not only in IM3 but also in IMS and IM7

A 76 GHz GaN-on-silicon power amplifier for automotive radar systems

This paper describes the first demonstration of a 76 GHz gallium nitride (GaN) power amplifier (PA) on a silicon substrate. The PA microwave monolithic IC (MMIC) was fabricated by using AlGaN/GaN FET with a maximum oscillation frequency of 160GHz and a breakdown voltage of over 50 V. For reducing transmission loss, we used a CPW line on the silicon substrate with low transmission loss of 0.5 dB/mm at 76 GHz. For precise design of the PA, a large signal model of the FET was developed. The developed CPW 3-stage PA exhibited an output of over 12 dBm with over 5 dB gain at 75–81 GHz.

Field-modulating plate (FP) InGaP MESFET with high breakdown voltage and low distortion

This paper describes a successfully developed field-modulating plate (FP) InGaP MESFET with an extremely high breakdown voltage of 100 V. The FP-FET, consisting of a 2.62 mm gate width, delivered an output power of 4.3 W and an output power density of 1.6 W/mm at 1.95 GHz operated at a drain bias (Vd) of 55 V. A low 3rd-order intermodulation distortion (IM3) of -31 dBc was also achieved at 8 dB back-off from saturation power. These results show the developed FET is suited for applications in the next generation cellular base station.

Observation of cross-sectional electric field for GaN-based field effect transistor with field-modulating plate

Cross-sectional potential distribution of a GaN-based field effect transistor with a field-modulating plate has been measured by Kelvin prove force microscopy to investigate the effect of the field-modulating plate on an electric field under high-voltage biasing conditions. The observed potential distribution clearly revealed that the introduction of the field-modulating plate is effective in relaxing the peak electric field intensity at the drain side of the gate edge. Furthermore, the authors experimentally observed that the peak of electric field splits into two positions: drain-side edges of the gate and field-modulating plate electrodes.

A low-distortion 230 W GaAs power FP-HFET operated at 22 V for cellular base station

This paper describes a successfully developed L-band depletion-mode GaAs-based heterostructure FET (HFET) with a field-modulating plate (FP) for cellular base station applications. The FP-HFET power amplifier, consisting of four 86.4 mm gate-width chips, delivered a 53.6 dBm (230 W) output power at 2.1 GHz with 11 dB linear gain and 42% PAE operated at a drain bias of 22 V. A low adjacent channel leakage power ratio (ACPR) of -35 dBc with 25% PAE was obtained at an output power of 46 dBm, indicating superior linearity characteristics under elevated drain bias conditions. The device also exhibited a record saturated power density of 0.67 W/mm.

CW 20-W AlGaN/GaN FET Power Amplifier for Quasi-Millimeter Wave Applications

This paper describes an AlGaN/GaN FET power amplifier module delivering a continuous wave (CW) output power of more than 20 W at 26 GHz. To achieve high breakdown characteristics with reduced current collapse and high gain, we have developed a 0.2 µm-long recessed-gate AlGaN/GaN FET with a field-modulating plate (FP), achieving high operation voltage of 25 V even at quasi-millimeter wave frequencies. A single-ended AlGaN/GaN FP-FET amplifier module for quasi- millimeter wave frequency has been fabricated for the first time. The amplifier module developed using a 6.3-mm-wide single chip recessed-gate AlGaN/GaN FP-FET exhibited an output power of 20.7 W, a linear gain of 5.4 dB and a power-aided efficiency of 21.3% at 26 GHz. This is the highest output power in solid state power amplifiers at over 20 GHz.

Reduction of Thermal Resistance of High-Power Amplifiers by Carbon Fiber-Reinforced Carbon Composite-Based Package

This paper deals with the thermal design of an electronics package and a demonstration of reduced thermal resistance for high-power amplifiers (HPAs). The focus is package internal thermal management. A carbon fiber-reinforced carbon composite- (C/C composite) based heat sink is proposed as a means of enhancement over the more conventional CuMo material. The C/C composite has anisotropic thermal properties. Thermal performance of the material with anisotropic thermal properties depends strongly on taking advantage of superior properties in the desired directions. Finite-element analysis is performed to determine the correct orientation of the C/C composite material with anisotropic thermal conductivities to minimize thermal resistance. A 32% reduction in thermal resistance of the HPA has been predicted in the initial simulation. A package incorporating the C/C composite material is built with the optimal orientation of thermal anisotropy obtained by numerical simulations. A 20% reduction in thermal resistance has been successfully obtained by surface temperature measurements for the HPA with the C/C composite material. The difference between numerical (32%) and experimental results (20%) is well explained by the difference in boundary conditions at the package base. Also, nonlinearity in thermal resistance is explained by taking account of temperature dependence of semiconductor materials, such as SiC and GaN.