Fabrication and characterization of AlGaN/GaN high electron mobility transistors

The task of this work was to fabricate and characterize AlGaN/GaN HEMTs on sapphire and silicon substrates. For this reason the HEMT fabrication technology had to be developed and optimized on AlGaN/GaN heterostructures grown on sapphire and silicon substrates. The optimization of AlGaN/GaN heterostructure on sapphire substrate has been done using RoundHEMT technology. Effects of the AlGaN barrier layer thickness and doping density of AlGaN carrier supply layer and its influence on the electrical properties of HEMTs were studied. The doping density of 5x1018cm-3 and 12nm thick barrier layer gave the best DC performance, i.e. IDSS=354mA/mm and gm=132mS/mm. On an optimized layer structure the influence of the transistor layout to its electrical properties has been studied by changing the source to drain distance and by varying the gate width. The maximal frequency of oscillation of 64GHz and cutoff frequency of 37GHz have been measured. The influence of the different gate widths onto the device performance was studied with multi-finger HEMTs. The HEMTs on AlGaN/GaN heterostructure on sapphire substrate have shown desirable performances but due to poor thermal conductivity of sapphire substrate they are not suitable for high temperature and high power application. The suitability of AlGaN/GaN HEMTs on silicon substrates have been proved by RoundHEMT device. After the first difficulties connected with not suitable structure quality and crack structure have been overcame the RoundHEMT with very promised DC performances had been fabricated on the doped AlGaN/GaN heterostructure. The devices with source to drain spacing of 3µm and the gate length of 0.3µm have shown maximal saturation current of 0.82A/mm which was the highest value compared to values published in that time for similar AlGaN/GaN HEMTs. Optimizing the fabrication technology and using better quality of layer structure linear HEMTs with excellent RF performance were fabricated. Devices with 0.5 (respectively 0.7)µm gate length yielded cutoff frequency of 32(20)GHz and maximal frequency of oscillation 27(22)GHz. The obtained fmax to fT ratio was 0.85 and 1.08 for devices with 0.5 and 0.7µm gate respectively. Non-insulating substrates also lower the cutoff frequency. This small value indicates a parasitic conduction due to non-insulating silicon substrate. Evaluated channel temperature for AlGaN/GaN HEMTs on sapphire substrate compared to HEMTs on silicon substrate has shown that (at 6W/mm dissipated power) the temperature in channel of the HEMT on sapphire substrate is 320°C in comparison to 95°C for HEMT on silicon substrate. The effect of doping concentration in the carrier supply layer of AlGaN/GaN HEMTs on silicon substrate on DC and RF performance was studied at four samples with varying doping density. An improvement of the performance of HEMT devices is obtained with use of appropriate passivation. Influence of SiO2 and Si3N4 passivation on undoped and doped AlGaN/GaN HEMTs was studied. Hall effect measurements have shown an increase of the 2DEG concentration after applying both types of passivation. An improvement of DC and RF performance was observed in the case of Si3N4 passivation for all four samples. Nearly two times higher output power density was observed for undoped AlGaN/GaN HEMT after applying Si3N4 passivation. The influence of passivation layers on AlGaN/GaN is still not fully understood and further work requires deeper investigations. Finally, preparation and characterization of AlGaN/GaN HEMTs on silicon substrate have shown that this device can be a good candidate for mass production of devices for high-frequency, high-power and high-temperature applications. However, further studies related to the material structure improvements and to better understanding of DC/RF dispersion (traps, strain effect, etc.) should be performed