RESEARCH

Wide Band-Gap Semiconductor

Wide-bandgap semiconductors (also known as WBG semiconductors or WBGSs) are semiconductor materials which have a relatively large band gap compared to conventional semiconductors. Conventional semiconductors like silicon have a bandgap in the range of 1 - 1.5 electronvolt (eV), whereas wide-bandgap materials have bandgaps in the range of 2 - 4 eV. Generally, wide-bandgap semiconductors have electronic properties which fall in between those of conventional semiconductors and insulators. Wide-bandgap semiconductors permit devices to operate at much higher voltages, frequencies and temperatures than conventional semiconductor materials like silicon and gallium arsenide. They are the key component used to make green and blue LEDs and lasers, and are also used in certain radio frequency applications, notably military radars. Their intrinsic qualities make them suitable for a wide range of other applications, and they are one of the leading contenders for next-generation devices for general semiconductor use. The wider bandgap is particularly important for allowing devices that use them to operate at much higher temperatures, on the order of 300 °C. This makes them highly attractive for military applications, where they have seen a fair amount of use. The high temperature tolerance also means that these devices can be operated at much higher power levels under normal conditions. Additionally, most wide bandgap materials also have a much higher critical electrical field density, on the order of ten times that of conventional semiconductors. Combined, these properties allow them to operate at much higher voltages and currents, which makes them highly valuable in military, radio and energy conversion settings. The US Department of Energy believes they will be a foundational technology in new electrical grid and alternative energy devices, as well as the robust and efficient power components used in high energy vehicles from electric trains to plug-in electric vehicles. Most wide-bandgap materials also have high free-electron velocities, which allows them to work at higher switching speeds, which adds to their value in radio applications. A single WBG device can be used to make a complete radio system, eliminating the need for separate signal and radio frequency components, while operating at higher frequencies and power levels.

Research Example 1

I. A New 4HSiC GGNMOS Based ESD Protection Circuit

Conventional gate floating NMOS (GFNMOS)

Conventional floating body NMOS (FBNMOS)

Proposed ESD protection circuit :
gate-body floating NMOS (GBFNMOS)

The structures of the general GGNMOS and FBNMOS as well as the proposed ESD protection circuit are shown in Fig. The FBNMOS, which further improves the GFNMOS, has a smaller leakage current and improved triggering voltage characteristics in a small ESD state than the general GGNMOS but still retains a high triggering voltage because of the very high breakdown voltage in the 4H-SiC material. The proposed 4H-SiC ESD protection circuit is composed of two NMOFETs, and the body and gate of the main NMOS (MN3) are connected to the source and drain of the floating NMOS (FN3), respectively. Furthermore, the drain area of MN3 is connected to the anode, and the source of MN3 and the gate and body areas of FN3 are connected to the cathode. Under normal operating conditions, the body and gate voltage of MN3 are zero and MN3 is turned off. Therefore, the proposed ESD protection circuit will not interfere with the normal functions of the input circuits. The ESD current flowing from the anode terminal, the body, and gate of MN3 is floating and the base potential of the NPN parasitic bipolar transistor in MN3 is increased. It helps to apply a forward bias to the base-emitter junction of the NPN parasitic bipolar transistor of MN3. Consequently, the proposed structure has a lower triggering voltage than the general GGNMOS and FBNMOS as a result of the simultaneous application of the floating technology to the gate and body of the GGNMOS

II. Process Description & Comparison with Conventional ESD Protection Circuits

(Left) Layout and (Right) magnified image

TLP I–V characteristic curves

uThe devices have the same geometry, a gate oxide thickness of 500 Å. The N-type source and the P-type body were formed by implanting nitrogen and aluminum respectively, in general, to form the N-type in 4H-SiC is used to nitrogen than phosphorous because of the higher ionization energy and low atomic weight. respectively. A silicide metal processing method is used to form ohmic contacts such as N+ and P+, and Ni metal is used for the silicide process of DC sputtering that proceeds from about 800 to 1000 A. Silicide is formed at 1050 °C by heat treatment for 30 s in an environment using an RTP process equipment. Besides, the P-body step was carried out at 650 °C using an aluminum source. To form ohmic contacts well, the N+ implant concentration was set high.
Which discharge the ESD current of the components, were all designed with a 3-multifinger structure. Compared with 209.4 V of GGNMOS and 182.7 V of FBNMOS, the proposed ESD protection circuit had a smaller triggering voltage of 121.8 V.

III. Electrical Characteristics and Thermal Reliability Measurement Results

TLP I–V characteristic curves of the GBFNMOS according to changes in design variables

Electrical characteristic at high temperature (300–500 K).

uGBFNMOS has structurally low the triggering voltage of GGNMOS (MN3), and it can adjust the trigger voltage by changing the design variable D1, As L increases, the holding voltage increases from 64.8 to 81.6 V. Furthermore, as D1 increases, the trigger voltage decreases from 165.4 to 121.8 V. The heat-loss rate of the secondary current at 500 K of the ESD protection circuit is only 4.89% compared with that at room temperature (300 K). Furthermore the heat-loss rate of holding voltage and current are very low (< 2%). Thus, we demonstrate the excellent high-temperature characteristics and thermal reliability of the fabricated ESD protection circuit.

Research Example 2

I. INTRODUCTION

The conventional 4H-SIC LIGBT

Proposed dual-emitter 4H-SIC LIGBT

The proposed dual-emitter 4H-SiC LIGBT equivalent circuit

Silicon carbide (SiC) is one of the wide band gap semiconductors for achieving high critical electric fields, high thermal conductivity and high saturated electron drift velocity. 4H-SiC devices have been considered superior replacements for existing silicon technologies. On the research level, much research has been conducted on the design and integration of driver circuits together with the 4H-SiC power devices in a power converter system. And 4H-SiC devices will make power electronic systems compatible with high temperature environments. The insulated gate bipolar transistor (IGBT) has become a popular switching device in electronic power applications. On-resistance of the lateral IGBT is smaller than that of a lateral MOSFET, implying partial conductivity modulation in the N-epitaxial layer. It combines the good features of both bipolar and MOSFET structures. Therefore, the 4H-SiC LIGBT is a promising power device for use in high voltage power integrated circuit applications, due not only to its superior isolation performance but also to the good characteristics of lateral power devices. Many of the advantageous characteristics of the 4H-SiC LIGBT device, such as high frequency, improved efficiency, and high-temperature operation, have been reported. In addition, applications exist that require the monolithic integration of high voltage and low voltage devices, and in these cases, lateral high-voltage devices are reasonable options. For this reason, 4H-SiC lateral devices have been extensively studied in the past. The purpose of this paper is to describe a 4H-SiC LIGBT with dual-emitter structure, which has improved forward voltage drop, on-resistance and current driving capabilities compared to the conventional 4H-SiC LIGBT.

II. Process Description & Comparison with Conventional ESD Protection Circuits

The layout the layout of the conventional 4H-SiC LIGBT

The layout the layout of Dual-emitter 4H-SiC LIGBT

Magnified image of the fabricated proposed dual-emitter 4H-SiC LIGBT

Thickness and doping concentration of the P+ layer are approximately 0.2 μm and 3E18 cm-3 and for the N+ layer are 0.2 μm and 5E18 cm-3, respectively. The thickness and doping concentration of the P-well layer are 0.7 μm and 1E18 cm-3, respectively. The process step for the N+ implant and P+ implant were carried out at 650 °C using a nitrogen and aluminum source, respectively. And, Silicide metal processing method to form ohmic contact such as N + and P +, silicide process includes using Ni metal was used Th. = 800 ~ 1000Å was Deposition in DC sputter method, silicide is formed 1050 °C for 30s heat treatment in an environment using an RTP process equipment. Also, the P well step was carried out at 650°C using an aluminum source. The punch through problem between the P well bottom and N+ substrate did not occur because of a sufficiently high saturation of the P well concentration. Figure 4 shows magnified image of the fabricated proposed LIGBT.

III. Electrical Characteristics and Thermal Reliability Measurement Results

Measured on-state characteristics of the 4H-SiC conventional LIGBT and proposed LIGBT for gate bias 15V, 20V

Measured on-state characteristics of comparison of the on-state voltage drop when the gate bias is 20V

Measured electron and hole mobility of the 4H-SiC conventional LIGBT and proposed LIGBT with varying channel length

This paper compares a dual-emitter 4H-SiC LIGBT to the conventional 4H-SiC LIGBT. The dual-emitter 4H-SiC LIGBT leads to a significant improvement in on-state performance. The additional emitter between the collector and gate regions in the dual-emitter 4H-SiC LIGBT provides an additional current path that leads to a lower on-state voltage drop, and higher current density than a conventional 4H-SiC LIGBT. In terms of the on-state voltage drop, the dual-emitter 4H-SiC LIGBT has superior electrical characteristics when compared to the conventional 4H-SiC LIGBT. Experimental results show that the dual-emitter 4H-SiC LIGBT has a saturation current of 23.5 mA and gm of 0.58 mS, which is nearly six times higher than that of a conventional 4H-SiC LIGBT. Furthermore, the off-state characteristic of the dual-emitter 4H-SiC LIGBT and conventional 4H-SiC LIGBT are equal to 250V.

​단국대학교 반도체 설계 연구실

RESEARCH

단국대학교 반도체 설계 연구실