Academic literature on the topic 'GaN hetero-junctions'

We report a hybrid hetero p-n junction between Zinc Oxide (ZnO) microspheres and p-GaN thin film or poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS). ZnO microspheres, which have high crystalline quality, were synthesized by ablating a ZnO sintered target with focused pulsed laser at high fluence. Recent investigation has demonstrated that ZnO microspheres had high optical property and laser actions in the WGM (whispering gallery mode) from the microshperes under pulsed laser irradiation. In this study, we fabricated a hybrid hetero p-n junction between ZnO microspheres and p-GaN or PEDOT:PSS, and both p-n junctions with p-GaN or PEDOT:PSS had a good rectifying characteristic. In the case of p-GaN, electroluminescence was observed under forward bias.

ABSTRACTThe valence-band offset at the zincblende AIN/GaN. AIN/InN and InN/GaN (110) interfaces are calculated self-consistently by means of the linear muffin-tin orbital method using up to 5+5 layer supercells. All interfaces have a type I-offset. Assuming interface orientation and polytype effects on the valence-band maximum to be reasonably small, a type I offset can also be expected for wurtzite interfaces. Our results are in very good agreement with experimental values for AIN/GaN, the only nitride interface for which they are available.Subject index terms: band offsets, interfaces, hetero junctions, superlattices. III-nitrides.

Hall sensor has proved to be an attractive solution for sensing requirements in electric machines for direct measurement of fields or indirect estimation of physical quantities such as current, speed and torque. Current probes, which measure terminal currents and switching currents in power converters for protection, monitoring and closed-loop current control, typically use Hall-effect sensors. Recently there has been a demand for electric machines with high operational speeds and high power densities for use in electric vehicles, power generation and precision machining applications. High speed machines operate with greater reliability using active magnetic bearings as they eliminate friction and guarantee longer machine life. Research in high-power and high-speed machines and active magnetic bearings can be aided by direct measurement of the internal magnetic field distribution. These electromechanical devices can operate in harsh environments and require stable Hall sensing operation at extreme temperatures. Most commercially available Hall sensors are based on silicon and have a limited operating temperature range. For field sensing at extreme temperatures, wide band gap-based materials offer a viable alternative. This work evaluates Hall-effect sensing using AlGaN/GaN hetero-junctions grown on Si substrates for extreme temperatures. Hall-effect sensors are fabricated using AlGaN/GaN heterojunctions grown on Si substrates. The square-shaped Hall-sensing element is realised by means of a simple fabrication methodology employing shadow masking. An array of greek-cross shaped Hall effect sensors is batch fabricated on a single GaN-on-Si wafer. A process flow for batch fabrication is proposed. In particular, an insulating layer of SixNy, deposited initially in the process, is shown to result in a lower sheet resistance of the Hallsensing elements. The fabricated samples are extensively characterised at temperatures ranging from 75 K to 500 K and at magnetic field strengths up to 2 Tesla. Notwithstanding wide fluctuations in sheet resistance and carrier mobility with the operating temperature, the plot of sensitivity against temperature is reasonably flat. The operating temperature range from 75 K to 500 K spans those of the military grade, industrial grade and commercial grade Hall sensors. Additionally, the fabricated sensors can also be used for field sensing in a cryogenic environment. Small variations in sensitivity, however exist. It is suggested that these variations can be compensated using the terminal measurements such as the transresistances. The geometrical correction factors of the fabricated sensors are also studied over the complete temperature range of interest. It is shown to be very close to unity and exhibit a variation as small as 2%. The offset voltage in the Hall sensor output and its dependence on the biasing currents and operating temperatures are of particular interest in this study. The offset voltage of each of the characterised samples shows a linear dependence on the bias current and a non-linear dependence on the sample temperature. For a given sample, the offset voltage is shown to vary with a change in its biasing configuration. The method of current spinning is shown to nullify the offset at any operating temperature, field or bias current. A micro-controller based electronic subsystem is developed to implement the current spinning scheme to cancel the offsets in the sensed Hall signal. The subsystem achieves the necessary signal amplification and filtering of the Hall voltage. In addition, the subsystem estimates the field and provides a visual read-out of the same. The square-shaped sensing element along with the electronic subsystem has been integrated into a suitable package for use as a magnetic field probe in the air-gap of an electromagnet or a magnetic circuit. A Helmholtz coil based magnetic field producing setup is used for testing and calibration of the electronic subsystem.

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.