Toshihide Kikkawa

Enhancement-Mode GaN MIS-HEMTs With n-GaN/i-AlN/n-GaN Triple Cap Layer and High-

This letter presents details of high-performance enhancement-mode GaN MIS high-electron-mobility transistor (MIS-HEMT) devices. Devices with an n-GaN/i-AlN/n-GaN triple cap layer, a recessed-gate structure, and high- k gate dielectrics show high drain current and complete enhancement-mode operation. The maximum drain current and threshold voltage (V th) are 800 mA/mm and +3 V, respectively. These results indicate that a recessed AlGaN/GaN MIS-HEMT with the triple cap could be a promising new technology for future device applications.

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A state-of-the-art highly reliable 250 W AlGaN/GaN high electron mobility transistor (HEMT) push-pull transmitter amplifier operated at a drain bias voltage of 50 V is described. The amplifier, combined with a digital predistortion (DPD) system, also achieved an adjacent channel leakage power ratio (ACLR) of less than -50 dBc for 4-carrier wideband code division multiple access (W-CDMA) signals with a drain supply voltage of 50 V. I also demonstrate its stable operation under RF stress testing for 1000 h at a drain bias voltage of 60 V. Stable gate-leakage current for high-temperature operation was verified. Device fabrications on 4 inch sapphire and 3 inch semi-insulating (S.I.) SiC substrates were also addressed. These performances clarify that an AlGaN/GaN HEMTs amplifier is suitable for 3G W-CDMA systems.

Gate Dielectrics

AlGaN/GaN high electron mobility transistors (HEMTs) have been developed for current-collapse-free operation at high drain bias voltages. The newly designed single-chip GaN HEMT amplifier for W-CDMA base station applications achieves a record CW output power of 150 W with a high power-added efficiency (PAE) of 54% at 2.1 GHz. The amplifier, combined with a digital pre-distortion (DPD) system, also demonstrates a state of the art efficiency of 40% with an adjacent channel leakage power ratio (ACLR) of less than -50 dBc for 4-carrier W-CDMA signals and reaches the saturated peak power level of 174 W with a drain supply voltage of 63 V. We prove for the first time that the AlGaN/GaN HEMT amplifier can completely fulfills the W-CDMA system requirement.

Highly Reliable 250 W GaN High Electron Mobility Transistor Power Amplifier

We describe a state-of-the-art 250-W output power AlGaN/GaN HEMT push-pull transmitter amplifier operated at a drain bias voltage of 50 V. We also demonstrated stable operation under RF stress testing for 1000 h at a drain bias voltage of 60 V, for the first time. The amplifier, combined with a digital pre-distortion (DPD) system, also achieved an adjacent channel leakage power ratio (ACLR) of less than -50 dBc for 1-carrier W-CDMA signals with a drain supply voltage of 50 V. We show, for the first time, that an AlGaN/GaN HEMTs push-pull amplifier can fulfill the requirements of W-CDMA systems.

A 174 W high-efficiency GaN HEMT power amplifier for W-CDMA base station applications

We demonstrate high voltage HFET operation with suppression of gm dispersion and current collapse using an n-type thin GaN cap layer combined with SiN passivation and a recessed ohmic structure. Polarization-induced surface charge was controlled. Off state and on-state breakdown voltages were 140 V and 70 V. We obtained power HFETs with high CW operation voltage of 35 V without any heat sinking method.

An over 200-W output power GaN HEMT push-pull amplifier with high reliability

Carbon nanotubes (CNTs) have been successfully developed as thermal and source bumps for flip-chip high power amplifiers (HPAs). The newly developed 15 mum long CNT bumps exhibit thermal conductivity of 1400 W/m-K. A flip-chip AlGaN/GaN HEMT HPA with a gate width of 2.4 mm utilizing CNT bumps, operating voltage of 40 V, exhibits an output power of 39 dBm at, a frequency of 2.1 GHz without any degradation due to heat-up. To our knowledge, this is the first report about, a practical application of CNTs using their high thermal conductivity

Surface-charge controlled AlGaN/GaN-power HFET without current collapse and gm dispersion

The prevention of disastrous leakage of AsH3 is a requirement for metalorganic vapor phase epitaxy systems. Less hazardous organoarsine has been investigated as an alternative to AsH3 , and recently, tertiarybutylarsine (tBAs) has been used as an arsenic source. So far the use of tBAs has been restricted to fundamental experiments. The growth of GaAs, AlGaAs, and selectively doped AlGaAs/GaAs heterostructures has been studied using tBAs and AsH3, and the properties of the epilayers grown using both sources have been compared. From the PL spectra at 4.2 K, it was determined that GaAs films using tBAs were of high purity and equivalent to those using AsH3. The properties of AlGaAs grown using tBAs are as good as those using AsH3. A higher V/III ratio results in high‐quality AlGaAs layers. The epitaxial uniformity of growth rate and AlAs mole fraction along a wafer using tBAs was poorer than those using AsH3 due to vapor phase reactions in the trimethylgallium‐tBAs mixture. However, the increase of total gas...

Thermal and source bumps utilizing carbon nanotubes for flip-chip high power amplifiers

In this paper, we report a C-band power amplifier with over 340-W output power using 0.8-µm GaN-HEMTs and an X-band power amplifier with over 100-W output power using 0.25-µm GaN-HEMTs. We used two-chip configurations and the three-stage impedance transformers to extend the bandwidth for both circuits. The input and output lines adjacent to each chip are divided by four to suppress the non-uniform heat distribution in a chip at high frequencies. As a result, we obtained 343-W output power and 53-% power added efficiency (PAE) at 4.8 GHz. This is the highest output power ever reported C-band power amplifiers. We also obtained 101-W output power and 53-% PAE at 9.8 GHz. This is also the highest PAE ever reported X-band power amplifiers with over 50-W output power.

The growth of GaAs, AlGaAs, and selectively doped AlGaAs/GaAs heterostructures by metalorganic vapor phase epitaxy using tertiarybutylarsine

In this paper, we present the current status of GaN-high electron mobility transistor (HEMT) for power-supply-applications. Advantages of GaN HEMT are summarized with discussing required characteristics applying for power supplies. Then, we introduce our Enhancement-mode (E-mode) GaN MIS-HEMT technology. We focused on realizing both normally-off operation and high current density with high breakdown voltage. Detailed results are discussed in this paper. In particular, we developed a unique device structure called triple layer cap structure. High current density with normally-off mode was successfully achieved, which is preferable for power-supply application. We also demonstrated high speed performance with low on-resistance.

C-band 340-W and X-band 100-W GaN power amplifiers with over 50-% PAE

In this paper, we describe highly reliable GaN high electron mobility transistors (HEMTs) for high-power and high-efficiency amplifiers. First, we present the reliability mechanisms and progress on the previously reported GaN HEMTs. Next, we introduce our specific device structure for GaN HEMTs for improving reliability. An n-GaN cap and optimized buffer layer are used to realize high efficiency and high reliability by suppressing current collapse and quiescent current (Idsq)-drift. Finally, we propose a new device process around the gate electrode for further improvement of reliability. Preventing gate edge silicidation leads to reduced gate leakage current and suppression of initial degradation in a DC-stress test under high-temperature and high-voltage conditions. Gate edge engineering plays a key role in reducing the gate leakage current and improving reliability.