AlGaN/GaN Dual Channel HFETs and Realization of GaN Devices on different substrates

GaN-based HFETs demonstrate ubiquitous high power and high frequency performance and attract tremendous research efforts. Even though significant advances have been achieved, there still exist some critical issues needed to be investigated and solved. In particular, high defect densities due to inhomogeneous growth and operation under high power conditions bring many unique problems which are not so critical in the conventional Si and GaAs materials systems. In order to reduce the defect density and heat dissipation of GaN-based HFETs, research work on the realization of GaN-based HFETs on bulk GaN substrate has been carried out and the key problems have been identified and solved. Hot phonon scattering is the bottleneck which limits the enhancement of electron velocity in the GaN 2DEG channel. It is found that the plasmon-phonon coupling is the mechanism for converting of hot phonons into high group velocity acoustic phonons. In order to push more electrons into the GaN 2DEG channel in the plasmon-phonon coupling regime and to further reduce the hot phonon lifetime, a novel AlGaN/GaN dual channel HFET structure has been proposed. The growth, fabrication and characterization of such a AlGaN/GaN dual channel HFET structure has been carried out. Conventionally GaN-based light emitting diodes and laser diodes are grown and fabricated using the c-plane III-nitride expitaxy layers. In c-plane III-nitride epi-layers, the polarization-induced electric field introduces spatial separation of electron and hole wave functions in quantum wells (QW)s used LEDs and laser diodes LDs and degrades quantum efficiency. As well, blueshift in the emission wavelength becomes inevitable with increasing injection current unless very thin QWs are employed. The use of nonpolar orientations, namely, m-plane or a-plane GaN, would solve this problem. So far, m-plane GaN has been obtained on LiAlO2 (100), m-plane SiC substrates, and m-plane bulk GaN, which all have limited availability and/or high cost. Silicon substrates are very attractive for the growth of GaN due to their high quality, good thermal conductivity, low cost, availability in large size, and ease with which they can be selectively removed before packaging for better light extraction and heat transfer when needed To realize the low cost and improve the internal quantum efficiency of GaN based light emitting diodes, the process for m-plane GaN growth on Si (112) substrates has been studied and optimized. The continuous m-plane GaN film is successfully grown on Si (112) substrates

Electron – phonon interaction in multiple channel GaN based HFETs: Heat management optimization

New power applications for managing increasingly higher power levels require that more heat be removed from the power transistor channel. Conventional treatments for heat dissipation do not take into account the conversion of excess electron energy into longitudinal optical (LO) phonons, whose associated heat is stored in the channel unless such LO phonons decay into longitudinal acoustic (LA) phonons via a Ridley path. A two dimensional electron gas (2DEG) density of ~5×1012cm-2 in the channel results in a strong plasmon–LO phonon coupling (resonance) and a minimum LO phonon lifetime is experimentally observed, implying fast heat removal from the channel. Therefore, it is desirable to shift the resonance condition to higher 2DEG densities, and thereby higher power levels. The more convenient way to attain the latter is by widening the 2DEG density profile via heterostructure engineering, i.e. by using multiple channel heterostructures. A single channel heterostructure (GaN/AlN/AlGaN), a basic heterostructure used to obtain a 2DEG, exhibits a resonance condition at low 2DEG densities (~0.65×1012 cm-2). Successful widening of the 2DEG density xv profile was predicted by simulation results for two types of multiple (Al)GaN channel heterostructures, i.e. coupled channel GaN/AlN/GaN/AlN/AlGaN and dual channel GaN/AlGaN/AlN/AlGaN. Because of a reduction of carrier confinement, it is experimentally observed that control of the channel is moderate in the case of dual channel heterostructures. On the other hand, carrier confinement provides a better control of the channel in coupled channel heterostructures. Furthermore, unlike in a dual channel heterostructure, alloy scattering does not affect carrier transport properties, which results in a higher cut-off frequency. It was found experimentally that the coupled channel heterostructure successfully reaches resonance condition at a 2DEG density that is 23% higher than in a single channel heterostructure. Multiple channel heterostructures therefore provide a convenient way to shift the plasmon-LO phonon resonance to higher 2DEG densities. However, in our grown heterostructures, high power levels under optimal channel working conditions and minimum heat accumulation, all desirable benefits for the development of high power transistors, were only observed in coupled channel heterostructures

Design, Microfabrication, and Characterization of Polar III-Nitride HFETs

ABSTRACT Design, Microfabrication, and Characterization of Polar III-Nitride HFETs Alireza Loghmany, Ph.D. Concordia University, 2016 With excellent performance in high-frequency power amplifiers, AlGaN/GaN heterojunction field-effect transistors (HFETs) as next generation power amplifiers have drawn a great deal of attention in the last decade. These HFETs, however, are still quite limited by their inherently depletion-mode (D-mode: negative pinch-off voltage) nature, relatively poor gate-leakage, and questionable long-terms reliability. In addition, since AlGaN/GaN HFETs operate at extremely high-power densities, performance of these devices has so far remained quite limited by self-heating effects. While a number of techniques have already been developed for realization of enhancement-mode (E-mode: positive pinch-off voltage) AlGaN/GaN HFETs, these techniques in addition to having a number of difficulties in achieving enhancement-/depletion-mode pairs, fall short of satisfying requirements such as low leakage-current, drain-current stability, and pinch-off voltage stability at the high operating temperatures and at elevated electric-fields. Among these techniques, fluoride-based plasma treatment is the most widely accepted. As an alternative to this mainstream technique, polarization-engineering of AlGaN/GaN HFETs through exploring the impacts of the mesa geometry is studied as a possible avenue for selective transformation of the D-mode nature of AlGaN/GaN HFETs to an E-mode character. Whereas limited experimental studies on the pinch-off voltage of HFETs realized on different isolation-feature geometries have indicated the presence of a certain correlation between the two, such observations lack the required depth to accurately identify the true culprit. This technique is expected to be ultimately capable of producing enhancement-/depletion-mode pairs without adding any extra steps to the microfabrication process. In light of this requirement, microfabrication of AlGaN/GaN HFETs using a number of alternative isolation-feature geometries is explored in this study. In addition to developing an in-house microfabrication process, transistors designed according to these novel isolation-feature geometries have been fabricated through the services offered by Canadian Microelectronics Corporation (CMC). Investigation of the variation of pinch-off voltage among the devices fabricated through this latter means has conclusively indicated that the pinch-off voltage shift, rather than exclusively being caused by the surrounding-field effect, is also correlated to the perimeter-to-area ratio of the isolation-features. In addition, through characterization and thermal modeling of these groups of devices, in this study a new approach is unveiled for reducing self-heating in AlGaN/GaN HFETs. According to finite element analysis (FEA) and electrical measurement of average channel temperature, an improved heat-dissipation was observed in HFETs enjoying a more distributed nature of the two-dimensional electron gas (2DEG) channel. This is observed to be the case especially for isolation features which offered the center of the channel a smaller distance to the side walls. Observations also indicate a more distinct gain in thermal management with reduction of the gate-length and also the surface area of the isolation pattern. Results suggest that self-heating in AlGaN/GaN HFETs can be substantially nullified by reducing the island-width below a certain threshold value, while maintaining the total width of the transistor constant. In addition to exploring these alternatives on AlGaN/GaN HFET structures, in-house microfabrication of AlN/GaN MISFETs is also studied. The results of DC characterization of these novel transistors are also presented

Development of A1GaN/GaN HBTs and HFETs for high power and high frequency operation.

GaN-based semiconductors show promise for the fabrication of electronic components capable of high power, high frequency operation. This has lead to the development of both AlGaN/GaN heterostructure field effect and bipolar junction transistors. In order to increase the efficiency of these devices novel fabrication technologies are examined. Contact resistances to buried p-type layers, exposed using a novel wet etch, were found to be superior when compared to those formed on dry etched structures. Utilising this technology in conjunction with a novel inverted n-p-n AlGaN/GaN HBT is found to increase the emitter-base heterojunction quality and also reduces growth complexity. Replacing the n-type collector with a Schottky diode was found to significantly reduce leakage in the component and enable normal operation under common emitter bias conditions. Bulk and surface trapping effects in AIGaN/GaN heterostructures are independently identified. Bulk traps are found to be located close to or at the me~-semiconductor interface which results in a modification to the HFET band structure. Gate leakage current along the AIGaN surface is found to be controlled by injection from the gate and a surface hopping conduction mechanism which dominates at high and low temperature respectively. Passivation of the structure using SiN reduces current flow at the AIGaN surface. Employing a plasma pre-treatment, contamination at the surface of the device can be removed allowing intimate contact between the passivation and AlGaN films. Using a CF4 plasma treatment is found to remove these contaminants and deposits a thin film of AlF3 on the AlGaN surface which reduces current collapse in AlGaN/GaN HFETs to negligible levels. GaN capping layers on HFETs can reduce both parasitic contact resistances and also reduce current collapse. Methods of selectively etching through the GaN capping layer are presented in order to develop a suitable self-aligned gate recess process

Anisotropic Dielectric Breakdown Strength of Single Crystal Hexagonal Boron Nitride

Dielectric breakdown has historically been of great interest from the perspectives of fundamental physics and electrical reliability. However, to date, the anisotropy in the dielectric breakdown has not been discussed. Here, we report an anisotropic dielectric breakdown strength (EBD) for h-BN, which is used as an ideal substrate for two-dimensional (2D) material devices. Under a well-controlled relative humidity, EBD values in the directions both normal and parallel to the c axis (EBD+c & EBD//c) were measured to be 3 and 12 MV/cm, respectively. When the crystal structure is changed from sp3 of cubic-BN (c-BN) to sp2 of h-BN, EBD+c for h-BN becomes smaller than that for c-BN, while EBD//c for h-BN drastically increases. Therefore, h-BN can possess a relatively high EBD concentrated only in the direction parallel to the c axis by conceding a weak bonding direction in the highly anisotropic crystal structure. This explains why the EBD//c for h-BN is higher than that for diamond. Moreover, the presented EBD value obtained from the high quality bulk h-BN crystal can be regarded as the standard for qualifying the crystallinity of h-BN layers grown via chemical vapor deposition for future electronic applications

Applications of GaN HFETs in UV detection and Power electronics

Gallium nitride (GaN) has some unique material properties including direct band gap, ability to form a heterostructure resulting in two dimensional electron gas (2DEG) formation and a wide band gap (3.4eV) to offer high breakdown voltage. Such material properties make GaN extremely attractive for optoelectronics and power electronics applications. In this thesis, GaN HFETs applications as an Ultraviolet light detector and for power electronics sector are explored. In comparison to other GaN based UV detectors, the AlGaN/GaN HFET is found to be ultra sensitive to UV illumination. A very high dc responsivity (~4.3×107A/W) value is reported and gain mechanisms in the devices are shown to be due to a photo voltage effect in both the AlGaN barrier layer and the GaN buffer region. Understanding of the gain mechanisms from this work will help optimise the design of the future UV photo detectors. For power electronics applications, GaN HFETs grown on a Si substrate are characterized. To reduce buffer leakage both Iron (Fe) and Carbon (C)-doped structures are considered. The vertical leakage mechanism is identified as a Poole Frenkel emission process for both the Fe and C-doped structures. A novel method to reduce the gate leakage current in GaN HFETs is established by using surface chemical treatments. Sulfuric acid works by oxidizing the surface which has a strong passivating effect on the gate leakage current. The surface leakage mechanism is explained by a combination of Mott hopping and Poole Frenkel models. The fluorine ion implant technique is used in GaN HFETs for the development of enhancement mode transistors required in power switching applications. The requirement for a +3V threshold voltage in the power electronics sector is met by combining the fluorine implant with a deposited dielectric layer under the gate. More efficient fluorine incorporation is observed in AlInN/GaN HFETs compared to conventional AlGaN/GaN HFETs. The recipe for fluorine implant in AlInN/GaN HFETs is also optimized to maintain high channel conductivity and transconductance

Investigation of Gallium Nitride Based on Power Semiconductor Devices in Polarization Super Junction Technology

Over the last decade, gallium nitride (GaN) has emerged as an excellent material for the next generation of power devices. GaN transistors, switching losses are very low, thanks to the small parasitic capacitances and switching charges. Device scaling and monolithic integration enable a high-frequency operation, with consequent advantages in terms of miniaturization. For high power/high voltage operation, GaN�based Polarization Super-Junction (PSJ) architectures demonstrate great potential. The aim of this thesis is devoted to the development of PSJ technology. Detailed analysis of the on-state behaviour of the fabricated Ohmic Gate (OG) and Schottky Gate (SG) PSJ HFETs is presented. Theoretical models for calculating the sheet densities of 2DEG and 2DHG are proposed and calibrated with numerical simulations and experimental results. To calculate the R (on, sp) of PSJ HFETs, two different gate structures (Ohmic gate and Schottky gate) are considered herein. The scaling tendency of power devices enables the emergence of multi-channel PSJ concepts. Therefore, lateral and vertical multi-channel PSJ devices based on practical implementation are also investigated. Presented calculated and simulated results show that both lateral and vertical multi-channel PSJ technologies can be well suited to break the unipolar one-dimensional material limits of GaN by orders of magnitude and achieve an excellent trade-off between R (on, sp) and voltage blocking capability provided composition and thickness control can be realised. A novel multi-polarization channel is applied to realize normally-off and high�performance vertical GaN device devices for low voltage applications based on the multi-channel PSJ and vertical MOSFET concepts. This structure is made with 2DHG introduced to realize the enhancement mode channel instead of p-GaN as in conventional vertical GaN MOSFETs. As the 2DHG depends upon growth conditions, p-type doping activation issues can be overcome. The Mg-doped layer is only used to reduce the short-channel effects, as the 2DHG layer is too thin. Two more 2DEG layers P a g e | iv are formed through AlGaN/GaN/AlGaN/GaN polarization structure, which minimizes the on-state resistance. The calculation results show this novel vertical GaN MOSFET – termed SV GaN FET - has the potential to break the GaN material limit in the trade-off between R (on, sp) and breakdown voltage at low voltages. The comprehensive set of development based on the PSJ concept gives a comprehensive overview of next-generation power electronics