Mihaela Alexandru

Design of logic gates for high temperature and harsh radiation environment made of 4H-SiC MESFET

Silicon carbide MESFETs are very attractive devices for high frequency applications, and communications. Progresses in the manufacturing of high quality SiC substrates open the possibility to new circuit applications. SiC unipolar transistors, such as JFETs and MESFETs have also a promising potential for digital integrated circuits operating at high temperature (HT) and/or in harsh environments. An increasing demand for HT compliant circuits comes from intelligent power management, automotive industry, and intelligent sensors for harsh environment, space and aerospace as well. The present work is a demonstration of logic gates design with normally-on 4H-SiC MESFET devices using HT Spice models extracted from experimental measurements. A complete library of functional HT logic gates allows the implementation of complex logic embedded in power management circuitry.

Design of Digital Electronics for High Temperature Using Basic Logic Gates Made of 4H-SiC MESFETs

– 4H-SiC MESFET transistors are very attractive devices for high temperature application and communications. The JFET and MESFET transistors have a promising potential for integrated circuits able to operate at high temperature and harsh radiation environments, due to the superior electrical, mechanical and chemical proprieties of 4H-SiC. Progresses in the manufacturing of high quality SiC substrates open the possibility to new circuit applications.

4H-SiC MESFET specially designed and fabricated for high temperature integrated circuits

Due to its wide bandgap, 4H-SiC is a potential candidate for developing devices capable to operate at elevated temperatures. Nowadays there is an increasing demand for high temperature circuits for drivers of SiC switches applicable to intelligent power management, automotive industry, intelligent sensors for harsh environment, space and aerospace among others. This paper is presenting high temperature experimental results in the 25°C-300°C temperature range of the 4H-SiC planar-MESFET specially designed and fabricated for high density SiC integrated circuits implementation.

High temperature-low temperature coefficient analog voltage reference integrated circuit implemented with SiC MESFETs

A high temperature compensated voltage reference integrated circuit (IC), was designed and for the first time fabricated on silicon carbide (SiC) material, using MESFET devices. A planar finger type MESFET was developed for this purpose. The schematic and the principle of the presented circuit is based on a new concept that replace the typical bandgap reference and avoid the necessity of using an operational amplifier (OpAmp), which is not yet developed on SiC. The experimental temperature coefficient (TC) is significantly better than a Zener diode and comparable to the normal bandgap voltage references on silicon, but the present circuit is able to work up to 300°C. The circuit contains also a linearized temperature sensor.

4H-SiC Digital Logic Circuitry Based on P+ Implanted Isolation Walls MESFET Technology

The design and development of SiC integrated circuits (ICs) nowadays is a necessity due to the increasing demand for high temperature intelligent power applications and intelligent sensors. Due to the superior electrical, mechanical and chemical proprieties of 4H-SiC poly-type, 4H-SiC MESFET transistor is a good compromise for ICs on SiC able to work at higher temperatures (HT) than on Si. This paper presents new experimental results of approaching embedded logic gates with SiC MESFETs and resistors, built in junction-isolated tubs. The P+ implantation isolation technology offers important perspectives regarding the integration density of devices per unit area and wafer surface, being able to use far more complex design geometry for modeling ICs on SiC.

SiC Schottky Diode surge current analysis and application design using behavioral SPICE models

This work presents thermal analysis results of surge current test performed on pressed-pack encapsulated SiC Schottky Diodes. An original method for temperature evaluation during high current pulses, based on behavioural SPICE models, was used to approach the analysis. Silicon Carbide (SiC) is one of the most adequate wide bandgap (WBG) material for manufacturing high temperature and high power electronics. However, the actual generation of commercially available SiC power diodes (Schottky and JBS) shows a maximum junction temperature of only 175°C. This important derating of the SiC devices, which theoretically are capable to sustain much higher temperatures, is due to the packaging limitation. The aim of our investigations is to overcome the actual limitations of SiC device packaging and to obtain reliable SiC devices able to operate at temperatures over 300°C.

Comparison between mesa isolation and p+ implantation isolation for 4H-SiC MESFET transistors

Silicon Carbide (SiC) is considered the wide band gap semiconductor material that can presently compete with silicon (Si) material for power switching devices. Progresses in the manufacturing of high quality SiC substrates open the possibility to new circuit applications. SiC unipolar transistors, such as JFETs and MESFETs have also a promising potential for digital integrated circuits operating at high temperature and/or in harsh environments. An increasing demand for high temperature compliant circuits comes from intelligent power management, automotive industry, and intelligent sensors for harsh environment, space and aerospace as well. Mesa isolation is a widely used isolation technique for the definition of individual devices due to its simplicity as fabrication process [1]. It is mostly used for the protection of high power devices. The junction isolation is widely used for bipolar, Bi-CMOS integrated circuits (IC) and can be achieved by diffusion or implantation process. The present work is presenting the experimental comparison between the mesa isolation process and p+ implantation junction isolation for 4H-SiC MESFET transistors.

Monolithic Integration of Power MESFET for High Temperature SiC Integrated Circuits

This work provides experimental result on fabricated 4H-SiC lateral power MESFET intended to be used in further development of high temperature integrated circuits for power application. The power SiC MESFET device was developed using a planar technology on silicon carbide and P implant isolation technique. Its destination to monolithic integration demands a lateral layout connection topology. The use of quite high doped N type epitaxial layer (1017cm-3) typical for the integrated circuits raises difficulties to keep the leakage current of the Schottky gate in a decent range. Therefore, a hexagonal close loop gate in conjunction with three metal interconnection levels was adopted, thus obtaining a compact lateral MESFET device and avoiding any drain to source parasitic leakage path. Using the tungsten gate MESFETS, the first generation of monolithic integrated lateral power MESFET device was integrated on the same wafer with digital circuits and a voltage reference analog circuit able to operate up to 250C. The temperature range can be next improved by using higher barrier for the gate contact.

Study on the Feasibility of SiC Bandgap Voltage Reference for High Temperature Applications

This work demonstrates that a stable voltage reference with temperature, in the 25°C-300°C range is possible using SiC bipolar diodes. In a previous work, we have been demonstrated both theoretical and experimentally, the feasibility of SiC bandgap voltage reference using SiC Schottky diodes [1]. The present work completes the investigation on SiC bandgap reference by the using of SiC bipolar diodes. Simulated and experimental results for two different SiC devices: Schottky and bipolar diodes showed that the principles that govern the bandgap voltage references for Si are also valid for the SiC. A comparison between the output voltage levels of the two types of bandgap reference is also presented.

10 MeV Proton Irradiation Effect on 4H-SiC nMOSFET Electrical Parameters

This work presents the 10 MeV protons irradiation effects on 4H-SiC MOSFETs at different fluences. MOSFETs main electrical parameters, such as the channel mobility (µEFF), threshold voltage (VTH), transconductance (gm) and subthreshold current, were analyzed using the time bias stress instability (BSI) technique. Applying this method allowed us to study the effect of carriers interaction with generated interface traps, whether in the bulk or at the interface. Improvements, such as VTH stabilization in time and a significant increase of the µEFF at high fluencies, have been noticed. We assume that this behavior is connected with the atomic diffusion from the SiO2/SiC interface, towards the epilayer during proton irradiation. These atoms, in majority Nitrogen, may create other bonds by occupying various vacancies coming from Silicon and Carbon’s dangling bond. Therefore, by enhancing the passivated Carbon atoms number, we show that high irradiation proton could be a way to improve the SiO2/SiC interface quality.