Thomas Rödle

Comparison of the DC and Microwave Performance of AlGaN/GaN HEMTs Grown on SiC by MOCVD With Fe-Doped or Unintentionally Doped GaN Buffer Layers

In this brief, the authors present a comparative and comprehensive investigation of the effect of the type of resistive GaN buffers on the dc, dynamic, microwave, and power performance of AlGaN/GaN high electron mobility transistors (HEMTs). Two types of buffer layers were investigated: 1) a nonintentionally doped resistive GaN buffer and 2) an Fe-compensated buffer. The Fe modulation-doped buffer is shown to be favorable for better dc isolation. The RF small-signal performance of the HEMTs does not exhibit any significant dependence on the type of resistive GaN buffer. However, the type of GaN buffer influences considerably the dynamic large-signal characteristics of the processed AlGaN/GaN HEMTs. The continuous-wave output power density of the AlGaN/GaN HEMTs at 3 GHz was increased from 3.4 to 9.7 W/mm by using a nonintentionally doped buffer instead of an Fe-doped one. Based on this observation combined with pulsed current-voltage measurements, we ascribe this difference to the deep trapping of electrons by defects in the GaN buffer introduced by the incorporation of Fe

A strong reduction in the density of near-interface traps at the SiO2∕4H‐SiC interface by sodium enhanced oxidation

This paper demonstrates how sodium enhanced oxidation of Si face 4H‐SiC results in removal of near-interface traps at the SiO2∕4H‐SiC interface. These detrimental traps have energy levels close to the SiC conduction band edge and are responsible for low electron inversion channel mobilities (1–10cm2∕Vs) in Si face 4H‐SiC metal-oxide-semiconductor field effect transistors. The presence of sodium during oxidation increases the oxidation rate and suppresses formation of these near-interface traps resulting in high inversion channel mobility of 150cm2∕Vs in such transistors. Sodium is incorporated by using carrier boats made of sintered alumina during oxidation or by deliberate sodium contamination of the oxide during the formation of the SiC∕SiO2 interface.

High field-effect mobility in n-channel Si face 4H-SiC MOSFETs with gate oxide grown on aluminum ion-implanted material

We report investigations of Si face 4H-SiC MOSFETs with aluminum (Al) ion-implanted gate channels. High-quality SiO/sub 2/-SiC interfaces are obtained both when the gate oxide is grown on p-type epitaxial material and when grown on ion-implanted regions. A peak field-effect mobility of 170 cm/sup 2//V/spl middot/s is extracted from transistors with epitaxially grown channel region of doping 5/spl times/10/sup 15/ cm/sup -3/. Transistors with implanted gate channels with an Al concentration of 1/spl times/10/sup 17/ cm/sup -3/ exhibit peak field-effect mobility of 100 cm/sup 2//V/spl middot/s, while the mobility is 51 cm/sup 2//V/spl middot/s for an Al concentration of 5/spl times/10/sup 17/ cm/sup -3/. The mobility reduction with increasing acceptor density follows the same functional relationship as in n-channel Si MOSFETs.

Reliability and degradation mechanism of AlGaN/GaN HEMTs for next generation mobile communication systems

Excellent reliability performance of AlGaN/GaN HEMTs on SiC substrates for next generation mobile communication systems has been demonstrated using DC and RF stress tests on 8x60 μm wide and 0.5 μm long AlGaN/GaN HEMTs at a drain voltage of Vd=50V. Drain current recovery measurements after stress indicate that the degradation is partly caused by slow traps generated in the SiN passivation or in the HEMT epitaxial layers. The traps in the SiN passivation layer were characterized using high and low frequency capacitance voltage (CV) measurements of MIS test structures on thick lightly doped GaN layers.

Physics-Based Modeling of GaN HEMTs

A thorough approach to the investigation of GaN-based high-electron mobility transistors by device simulation is demonstrated. Due to structure and material peculiarities, new comprehensive hydrodynamic models for the electron mobility are developed and calibrated. Relying on this setup, three different independent device technologies are simulated and compared. We further study the pronounced decrease in the transconductance gm at higher gate bias. We show that the electric field distribution and the resulting carrier velocity quasi-saturation are the main source for the transconductance collapse.

Reliability status of GaN transistors and MMICs in Europe

Recent DC- and RF-reliability results of European GaN HEMTs for high frequency power and MMIC applications between 2 and 18 GHz will be presented. The DC-stress test experiments have been performed at high current and high voltage settings in order to test the devices in the different regimes during large signal operation. GaN HEMTs and one stage MMICs have also been tested under RF-operation conditions and the correlation to DC-stress tests has been investigated.

Comparison between oxidation processes used to obtain the high inversion channel mobility in 4H-SiC MOSFETs

In this work two oxidation methods aimed at improving the silicon face 4H-SiC/SiO2 interface are compared. One is an oxidation in N2O performed in a quartz tube using quartz sample holders and the other is a dry oxidation performed in an alumina tube using alumina sample holders. In n-type metal oxide semiconductor (MOS) capacitors the interface state density near the SiC conduction band edge is estimated using capacitance–voltage (C–V) and thermal dielectric relaxation current (TDRC) measurements. N-channel metal oxide semiconductor field effect transistors (MOSFETs) are characterized by current–voltage (I–V) techniques and the inversion channel mobility is extracted. It is shown that the high inversion channel mobility (154 cm2 V−1 s−1) seen in samples oxidized using alumina correlates with a low interface trap density (3.6 × 1011 cm−2). In the case of N2O oxidation the mobility is lower (24 cm2 V−1 s−1) and the interface trap density is higher (1.6 × 1012 cm−2). Room temperature C–V measurements are of limited use when studying traps near the conduction band edge in MOS structures while the TDRC measurement technique gives a better estimate of their density.

Sodium Enhanced Oxidation of Si-Face 4H-SiC: A Method to Remove Near Interface Traps

We demonstrate how sodium enhanced oxidation of Si face 4H-SiC results in removal of near-interface traps at the SiO2/4H-SiC interface. These detrimental traps have energy levels close to the SiC conduction band edge and are responsible for low electron inversion channel mobilities (1-10 cm2/Vs) in Si face 4H-SiC metal-oxide-semiconductor field effect transistors. The presence of sodium during oxidation increases the oxidation rate and suppresses formation of these nearinterface traps resulting in high inversion channel mobility of 150 cm2/Vs in such transistors. Sodium can be incorporated by using carrier boats made of sintered alumina during oxidation or by deliberate sodium contamination of the oxide during formation of the SiC/SiO2 interface.

High channel mobility 4H-SiC MOSFETs

We report investigations of MOS and MOSFET devices using a gate oxide grown in the presence of sintered alumina. In contrast to conventionally grown dry or wet oxides these oxides contain orders of magnitude lower density of near-interface traps at the SiO2/SiC interface. The reduction of interface traps is correlated with enhanced oxidation rate. The absence of near-interface traps makes possible fabrication of Si face 4H-SiC MOSFETs with peak field effect mobility of about 150 cm2/Vs. A clear correlation is observed between the field effect mobility in n-channel MOSFETs and the density of interface states near the SiC conduction band edge in n-type MOS capacitors. Stable operation of such normally-off 4H-SiC MOSFET transistors is observed from room temperature up to 150°C with positive threshold voltage shift less than 1 V. A small decrease in current with temperature up to 150°C is related to a decrease in the field effect mobility due to phonon scattering. However, the gate oxides contain sodium, which originates from the sintered alumina, resulting in severe device instabilities during negative gate bias stressing.

Efficient AlGaN/GaN HEMT Power Amplifiers

This paper describes efficient GaN/AlGaN HEMTs and MMICs for L/S-band (1-4 GHz) and X-band frequencies (8-12 GHz) on three-inch s.i. SiC substrates. Dual-stage MMICs in microstrip transmission-line technology yield a power-added efficiency of ¿40% at 8.56 GHz for a power level of ¿11 W. A single-stage MMIC yields a PAE of ¿55% with 6 W of output power at VDS= 20 V. The related mobile communication power HEMT process yields an average power density of 10 W/mm at 2 GHz and VDS= 50 V. The average PAE is 61.3% with an average linear gain 24.4 dB and low standard deviation of all parameters. The devices yield more than 25 W/mm of output power at 2 GHz when operated in cw at VDS= 100 V with an associated PAE of ¿60%. The GaN HEMT process with 0.5 ¿m gate-length yields an extrapolated lifetime of 105 h when operated at VDS= 50 V at a channel temperature of 90°C. When operated at 2 GHz devices with 480 ¿m gate-width yield a change of the RF power-gain of less than 0.2 dB under high gain-compression at VDS= 50 V and a channel temperature of 250°C.