On the time-dependent transport mechanism between surface traps and the 2DEG in AlGaN/GaN devices

The physical mechanisms involved in the trapping and de-trapping processes associated to surface donor traps in GaN transistors are discussed in this work. The paper challenges the conventional transient techniques adopted for extrapolating the trap energy level via experiments and TCAD simulations. Transient TCAD simulations were employed to reproduce the time-dependent electrical behavior of a Metal-on-Insulator Field-Effect-Transistor (MISFET) and explain the influence of the electric field and energy barrier on the transient time associated to the trapping and de-trapping mechanisms of surface traps. The comparison between three test-structures and the relative variation of the trapping and de-trapping times with the bias and trap parameters leads to the suggestion of a proposed test-structure and bias configuration to accurately extrapolate the energy level of surface traps in GaN transistors

True Material Limit of Power Devices - Applied to 2-D Superjunction MOSFET

Predictions on limits of silicon in power devices have failed spectacularly in the past, but in spite of that, the theory that generated them is still used today and adapted for wide bandgap materials to justify their superior standing against silicon. The superjunction MOSFET was the first device to break by more than one order of magnitude the so-called ‘limit of silicon’ above 600V. The current theory of superjunction seems however to define a new technology based limit rather than a material based only limit. This implies that by scaling down the dimensions, in particular the cell pitch, the on-state resistances can continually decrease by several orders of magnitude, without a boundary. This paper shows that the down scaling of the cell dimensions cannot happen indefinitely and there is a material dependent intrinsic limit for any power device, which no longer is limited by the geometry or the technology available. Using an analytical approach, and backed up by advanced numerical simulations, we show that the minimum cell pitch is 0.18 μm for silicon and 0.05 μm for 4H-silicon-carbide, and further reduction in the cell pitch would result in an increase in the specific resistance. Finally, a new figure of merit for a superjunction MOSFET based on a rigorous 2D analysis is define

Static and Dynamic Effects of the Incomplete Ionization in Superjunction Devices

The incomplete ionization of impurity atoms affects the free carrier concentration of several wide bandgap semiconductor materials even at room temperature, thus modifying the electrical properties of power devices. In this paper, the influence of the partial ionization of the dopants on the static and dynamic behavior of wide bandgap semiconductor based SuperJunction devices has been numerically investigated through extensive 2D finite element simulations. Whereas this physical effect has only a minor impact on the static device’ characteristics, if a reverse bias pulse with a rise time comparable or smaller than the ionization time constant is applied to the structure, a “dynamic ionization” phenomenon can take place. The onset of this time dependent ionization is the cause of charge unbalance effects in the device structure, due to temporal dependence of the activated number of dopants. Electro-thermal simulations, which have been carried out for a 4H-SiC SuperJunction diode, show a non-uniform temperature distribution during the transients which leads, in turn, to the self-heating of the device. Through an accurate redesign of the cross-sectional view of the device, the drawbacks of the incomplete ionization have been mitigated and the device’ performance enhanced.EP/M506485/1 Engineering and Physical Sciences Research Counci

Reverse-Conducting Insulated Gate Bipolar Transistor: A Review of Current Technologies

The Reverse Conducting IGBT has several benefits over a separate IGBT and diode solution and has the potential to become the dominant device within many power electronic applications; including, but not limited to, motor control, resonant converters, and switch mode power supplies. However, the device inherently suffers from many undesirable design trade-offs which have prevented its widespread use. One of the most critical issues is the snapback seen in the forward conduction characteristic which can prevent full turn-on of the device and result in the device becoming unsuitable for parallel operation (required in many high voltage modules). This phenomenon can be suppressed but at the expense of the reverse conduction performance. This paper provides an overview of the technical design challenges presented by the RC-IGBT structure and reviews alternative device concepts which have been proposed in literature. Analysis shows that these alternate concepts either present a trade-off in performance characteristics, an inability to be manufactured, or a requirement for a custom gate drive.EPSRC Doctoral Training Partnership scheme (grant RG75686) UK Innovate Project Number 10287

On the robustness of ultra-high voltage 4H-SiC IGBTs with an optimized retrograde p-well

The robustness of ultra-high voltage (>10kV) SiC IGBTs comprising of an optimized retrograde p-well is investigated. Under extensive TCAD simulations, we show that in addition to offering a robust control on threshold voltage and eliminating punch-through, the retrograde is highly effective in terms of reducing the stress on the gate oxide of ultra-high voltage SiC IGBTs. We show that a 10 kV SiC IGBT comprising of the retrograde p-well exhibits a much-reduced peak electric field in the gate oxide when compared with the counterpart comprising of a conventional p-well. Using an optimized retrograde p-well with depth as shallow as 1 μm, the peak electric field in the gate oxide of a 10kV rated SiC IGBT can be reduced to below 2 MV.cm -1 , a prerequisite to achieve a high-degree of reliability in high-voltage power devices. We therefore propose that the retrograde p-well is highly promising for the development of>10kV SiC IGBTs

Investigation of the Dual Implant Reverse-Conducting SuperJunction Insulated-Gate Bipolar Transistor

This letter presents the Dual Implant SuperJunction (SJ) trench Reverse-Conducting (RC) Insulated Gate Bipolar Transistor (IGBT) concept with two implanted SJ pillars in the drift region; one from the cathode side and another from the anode side. The proposed device is compatible with current manufacturing processes and enables a full SJ structure to be achieved in a 1.2kV device as alignment between the pillars is not required. Extensive Technology Computer Aided Design (TCAD) simulations have been performed and demonstrated that utilising this dual implantation technique can result in a 77% reduction in turn-off losses for a full SJ structure, compared to a conventional RC-IGBT. The results show that any snapback in the on-state waveform significantly increases the turn-off losses and only a deep SJ device (pillar gap < 10μm) warrants the additional processing expense

Material selection for Micro-Electro-Mechanical-Systems (MEMS) using Ashby's approach

A key aspect in design optimization of a product or a system is the selection of materials that best meet the design needs, ensuring maximum performance and minimum cost. Ashby's approach, originally introduced for macro-systems and products, has been very successfully employed for Micro-Electro-Mechanical-Systems (MEMS)/micromachined sensors, actuators and devices. This paper presents a comprehensive review and critical analysis of MEMS material selection studies using Ashby's approach reported in the literature during the last two decades. Performance and Material Indices derived for various microsystems and MEMS devices have been summarized. Moreover, all MEMS materials reported in the literature and the most suitable materials proposed for a variety of MEMS systems and devices have also been consolidated. A material selection case study utilizing micro-scale properties of 51 MEMS compatible materials has been presented to demonstrate that the use of different materials' bulk properties is not the best choice for MEMS materials selection. This paper will serve as a reference guide and useful resource for researchers and engineers engaged in the design and fabrication of various microsystems and MEMS sensors, actuators and devices

Optimal edge termination for high oxide reliability aiming 10kV SiC n-IGBTs

The edge termination design strongly affects the ability of a power device to support the desired voltage and its reliable operation. In this paper we present three appropriate termination designs for 10kV n-IGBTs which achieve the desired blocking requirement without the need for deep and expensive implantations. Thus, they improve the ability to fabricate, minimise the cost and reduce the lattice damage due to the high implantation energy. The edge terminations presented are optimised both for achieving the widest immunity to dopant activation and to minimise the electric field at the oxide. Thus, they ensure the long-term reliability of the device. This work has shown that the optimum design for blocking voltage and widest dose window does not necessarily give the best design for reliability. Further, it has been shown that Hybrid Junction Termination Extension structure with Space Modulated Floating Field Rings can give the best result of very high termination efficiency, as high as 99%, the widest doping variation immunity and the lowest electric field in the oxide