Yasunobu Sumida

GaN-Based Super Heterojunction Field Effect Transistors Using the Polarization Junction Concept

GaN super heterojunction field effect transistors (super HFET) based on the polarization junction concept are demonstrated for the first time. The super HFET has charges of 2-D electron gas and hole gas, respectively induced by positive and negative polarization charges at GaN/AlGaN/GaN heterointerfaces. Analogous to the RESURF concept, these unintentionally doped positive and negative polarization charges compensate each other in the off state condition to enhance the breakdown capability of the super HFET. The super HFETs have been fabricated on sapphire substrates and the electrical measurements show breakdown voltages over 1.1 kV with specific on-resistance of 6.1 mΩ· cm^2.

High Density Two-Dimensional Hole Gas Induced by Negative Polarization at GaN/AlGaN Heterointerface

High density two dimensional hole gas (2DHG) with a charge density of 1.1×1013 cm-2 has been demonstrated for the first time in GaN/AlGaN heterostructures. The 2DHG is induced by negative polarization charges at the GaN/AlGaN interface. The layer structures have been designed based on theoretical simulation results to maximize the 2DHG charge density. The heterostructures have been grown on sapphire substrate by metal organic chemical vapor deposition. Hall mobility of the 2DHG of 16 cm2 V-1 s-1 has been measured at room temperature with sheet resistance of 35 kΩ/sq.

Correlation between deep-level defects and turn-on recovery characteristics in AlGaN/GaN hetero-structures

We report on a correlation between deep-level defects and turn-on recovery characteristics in AlGaN/GaN hetero-structures, employing Schottky barrier diodes. Photo-capacitance spectroscopy measurements reveal three specific deep levels located at ∼2.07, ∼2.80, and ∼3.23 eV below the conduction band, presumably attributable to Ga vacancies and/or impurity C present in the GaN buffer layer. Additionally, from photo-assisted turn-on current recovery measurements, by using 390 and 370 nm long-pass filters, the recovery time becomes significantly faster due to inactivation of their corresponding deep-level traps. Therefore, the ∼2.80 and ∼3.23 eV levels are probably responsible for the carrier-trapping phenomena in the bulk region.

Delay Time Analysis of AlGaN/GaN Heterojunction Field-Effect Transistors with AlN or SiN Surface Passivation

We have carried out delay time analysis for the high-frequency characteristics of AlGaN/GaN heterojunction field-effect transistors (HFETs) on sapphire substrates, focusing on a comparison between the effects of AlN and SiN surface passivation. HFETs with AlN passivation, in comparison with those with SiN passivation, exhibit high current gain cut-off frequencies fT with good flatness, in accordance with high intrinsic transconductance due to the self-heating reduction effect of AlN. From the delay time analysis, we find that the effective saturation electron velocity for AlN and SiN passivation is similar, while the electron mobility under the gate for AlN passivation is higher. This implies that the higher fT is mainly due to the higher electron mobility under the gate owing to the self-heating reduction by AlN passivation. This is consistent with the fact that the saturation velocity has a weak temperature dependence, while the mobility has a strong dependence.

GaN-based bidirectional Super HFETs Using polarization junction concept on insulator substrate

GaN based bidirectional Super Heterojunction Field Effect Transistors (BiSHFETs) using the polarization junction (PJ) concept are demonstrated for the first time. The fabricated BiSHFETs are arrayed on an insulator substrate of Sapphire and measured isolation voltage between the devices is more than 2 kV. Measured on-resistances of the fabricated BiSHFETs with MES and PN gate structures are 24 Ωmm and 22 Ωmm in the both directions respectively.

GaN based Super HFETs over 700V using the polarization junction concept

GaN Super Heterojunction Field Effect Transistors (Super HFETs) based on the polarization junction (PJ) concept are demonstrated on Sapphire substrates. These Super HFETs were fabricated from a GaN/Al<inf>0.23</inf>Ga<inf>0.77</inf>N/GaN hetero structure with 2D hole and electron gas densities of 1.1×10¹³ and 9.7×10¹² cm⁻² at the respective hetero-interfaces. The Super HFETs show breakdown voltage above 700 V with on-resistances of 15 Ω·mm. In addition, the super HFETs have inherent body diodes and its reverse conducting characteristics are demonstrated.

Temperature Dependence and Current Collapse of AlGaN/GaN Super Heterojunction Field Effect Transistor

1. Introduction Conventional AlGaN/GaN hetero-structure field-effect transistors (HFET) have the problems of current collapse which is a phenomenon of the decreasing drain currents. Surface passivation by insulation films and metal field plate designs are widely used as techniques to suppress current collapse in AlGaN/GaN based HFETs. The metal field plate enables reduction of the electric field concentration on the gate edge. As an another way, the electric field concentration can be decreased by the coexistence of two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) generated by AlGaN/GaN polarization junctions [1-4]. The coexistence of high density 2DHG and 2DEG is achieved by optimizing the growth of GaN/AlGaN/GaN double hetero structures. Here, we report the results of our investigation on the DC characteristics of the fabricated super HFETs.

Polarization Junction based Super HFETs and derivatives in GaN

Gallium Nitride (GaN) is a wide band gap semiconductor, which can be grown on Silicon, Sapphire, Silicon Carbide or Diamond. Its electric field strength is 10 times higher than that of Silicon. High density two dimensional electron gas (2DEG), arising due to polarization properties of the AlGaN/GaN hetero structure provides extremely low on-state resistance. The conventional GaN Hetero-junction Field Effect Transistors (GaN-HFET) available today are based on the use of the 2DEG with metal field plates to reduce the electric field crowding at the gate and drain ends to increase the breakdown voltage. Such approaches are non-ideal. In this paper, we will highlight the basics of Polarization Super Junction (PSJ) concept in GaN for power switching applications and highlight some of the device structures and electrical results. GaN PSJ-FETs are significantly superior to the GaN-HFETs for the following reasons: • GaN PSJ-FET requires a significantly smaller area in comparison to a conventional GaN FET plus its external anti-parallel diode. • The PSJ approach is the (only) viable approach for power devices to go beyond GaN’s onedimensional material limit in terms of its specific on-state resistance versus breakdown voltage. Simulation results show that, beyond 300 V, PSJ-FET can offer 1/10th of resistance of an equivalent conventional GaN-HFET, which increases to more than 1/100th beyond 2 kV. • Unlike other wide band-gap materials, PSJ-FETs can operate from cryogenic temperatures to in excess of 250 C, as the polarization charges are temperature independent. GaN PSJ devices such as diodes, FETs and bidirectional switches are ideal in applications from inverters in home appliances, automotive and aerospace applications, motor drives and harsh environment applications for more efficient, power dense and high temperature compliant electric power conversion and rad-hard space applications. From a manufacturing perspective, fabrication of GaN PSJ devices is simpler than the super-junction devices in Silicon – an important consideration. The system level reward is a step-change in electrical power conversion performance, with a clear reduction in energy consumption and reduction in CO2 emission.