Student: Rohith Soman
Research Advisor: Prof. Navakanta Bhat and Prof. Srinivasan Raghavan
DAY AND DATE : Wednesday, 18th July, 2018 - 11:00AM
VENUE : Multi Media Class Room (MMCR), First Floor, CeNSE
Abstract:
AlxGa(1-x)N/GaN HEMTs find application in power electronics systems as high-frequency power switches. Formation of 2 dimensional electron gas (2DEG) at the heterointerface and wide band gap nature make AlxGa(1-x)N/GaN HEMTs a promising candidate. High on current and high breakdown voltage are the key features of AlxGa(1-x)N/GaN devices. However, achieving these qualities with a normally-off operation is a challenge. Normally-off operation is desired in power electronics applications for safe operation and simple gate drive circuits. Scattering of electrons at the damages created by the gate engineering techniques, which is required for realisation of normally off device, will result in low field effect mobility and thereby low on current for the normally off devices. Another
challenge in AlxGa(1-x)N/GaN systems is buffer leakage current, which limits the breakdown voltage of the device.
This work aims at development of a new device architecture for normally-off operation in AlxGa(1-x)N/GaN HEMTs which has promise to deliver high on current and high breakdown voltage without compromising on threshold voltage.
First, we design a buried channel normally off AlxGa(1-x)N/GaN MOS-HEMT device with a p-n junction in the GaN buffer on Si substrate. The conduction channel in this architecture forms away from the gate oxide - GaN interface hence low scattering of electrons at the interface and high on current for the device. The depletion region associated with the p-n junction in the buffer also aids in achieving high breakdown voltage. P-type doping in GaN is very challenging, therefore an in depth study of Magnesium (Mg) doping, the common p-type dopant, in GaN is carried out. The interdependence between Mg doping, polarity inversion, dislocation evolution and stress generation during growth of GaN is analyzed. With the understanding gathered, a buried channel stack with p-n junction in the
buffer is grown on Si substrate and normally off device is fabricated. The device featured a threshold volta ge of +1.3 V with a drain saturation current of 287 mA/mm. The field effect mobility of the buried channel device increased more than five times compared to a reference device owing to lesser interface scattering effects. To increase the mobility even further an in-situ etching process is executed during growth. A Mg doping profile with 24 nm/dec decay rate is archived and the 2DEG mobility increased from 641 cm2/Vs to 1178 cm2/Vs. Next, we designed a novel thick buffer stack for achieving high breakdown voltage, which has multiple p-n junciton disposed over a transition layer. Lastly, we aslo demostrate fabrication of a normally off AlxGa(1-x)N/GaN FinFET, which is reported to
deliver the highest on current density. The FinFet fabricated featured a thrshold voltage of +2 V and drain current of 270 mA/mm.