Normally off AlxGa(1-x)N/GaN devices: Materials, process and device architecture innovations
AlxGa(1-x)N/GaN HEMTs find applications in power electronics systems as high-frequency power switches. The formation of a highly mobile 2 dimensional electron gas (2DEG) at a heterointerface in an otherwise insulating wide band gap semiconductor make AlxGa(1-x)N/GaN HEMTs a promising candidate. These attributes enable high on current and high breakdown voltages in normally-on devices. However, Normally off operation is desired in power electronics applications for safety purposes and to allow simple gate drive circuits. Translating the normally-on qualities to normally off devices is a challenge. Scattering of electrons at the damage created by the gate engineering techniques, which is required for realisation of normally off devices, results in low field effect mobility and thereby on-current reduction. Device designs that address this on-state challenge, also need to ensure that buffer leakage current, which limits the breakdown voltage of the device in the off-state should not be compromised. Leakage in AlxGa(1-x)N/GaN HEMTs is a current technological challenge and an architecture that improves on this aspect in addition to reducing on-state losses in enhancement mode HEMTs, would be a welcome development. This work aims at the development of a new device architecture for normally off operation in AlxGa(1-x)N/GaN HEMTs on (111) Si which has the promise to deliver on currents and breakdown voltage comparable to current normally on devices but without compromising on the threshold voltage. First, a buried channel normally off AlxGa(1-x)N/GaN MOS-HEMT device with a p-n junction in the GaN buffer is designed. The conduction channel in this architecture forms away from the gate oxide-GaN interface which would result in reduced interface roughness scattering of electrons due to interface roughness and hence enable high on current. The depletion region associated with the p-n junction in the buffer was also expected to aid in the increase of breakdown voltage. P-type doping in GaN with Mg is very challenging. Hence, prior to realizing a device based on this design, an indepth study of Magnesium (Mg) doping, the common p-type dopant, in GaN was carried out. The interdependence between Mg doping, polarity inversion, dislocation evolution and stress generation during growth of GaN was analyzed. With the understanding gathered, a buried channel stack with p-n junction in the buffer was grown on Si substrates and devices were fabricated. The device featured a threshold voltage of +1.3 V with a drain saturation current of 287 mA/mm. In comparison to a reference device that did not have a buried channel, the field effect mobility of in this device was calculated to be 5 times larger owing to lowered interface effects. The off state performance of the device is also shown to improve with buried channel architecture. To increase the mobility even further, an in-situ etching process was executed during growth, to get a sharper Mg doping profile. A Mg doping profile with 24 nm/dec decay rate, one the best reported till date in the literature, was achieved. This enhanced the 2DEG mobility from 641 cm2/Vs to 1178 cm2/Vs. Following this achievement of on-state performnce in enhancement mode devices that are comparable to depletion mode devices routinely fabricated in our group, to address the off-state performance, a novel thick buffer stack for achieving high breakdown voltage, which has multiple p-n junciton disposed over a transition layer was designed and simulated. Lastly, a normally off AlxGa(1-x)N/GaN FinFET, an architechture that does not require gate engineering and which is reported to deliver the highest on current density was designed and fabricated. The FinFet fabricated featured a thrshold voltage of +2 V and drain current of 270 mA/mm.