I. INTRODUCTION

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 ^(1-3). 4H-SiC devices have been considered superior replacements for existing silicon technologies ^(4,5). 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 ^(6-8). 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 ^(9,10). 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 ⁽¹¹⁾. 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 ⁽¹²⁾. 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. DEVICE DESCRIPTION

The cross-section of the conventional 4H-SiC LIGBT and dual-emitter 4H-SiC LIGBT are shown in Fig. 1(a) and (b), respectively. Fig. 2 show the proposed dual-emitter 4H-SiC LIGBT equivalent circuit. The dual-emitter 4H-SiC LIGBT structure is identical to the conventional 4H-SiC LIGBT structure except it has an additional emitter region.

Fig. 1. Cross sectional view of (a) the conventional 4H-SIC LIGBT (b) and the proposed dual-emitter 4H-SIC LIGBT.

Fig. 2. The proposed dual-emitter 4H-SiC LIGBT equivalent circuit.

Fig. 3 show the layout of the conventional 4H-SiC LIGBT and dual-emitter 4H-SiC LIGBT. These devices were fabricated on the N-epitaxial layer on top of the Si face of a 4H-SiC wafer. Both devices have the same geometry, which is $L_{ch}$ = 1 µm, Ln-drift = 16 µm, gate oxide thickness = 500 Å and W = 223 µm. The drift region was formed through the N-epitaxial layer. The N-type emitter and the P-type collector were formed by implanting nitrogen and aluminum, respectively. Generally, in order to form the N-type emitter in 4H-SiC, nitrogen is used rather than phosphorous because of the higher ionization energy and lower atomic weight. Thus it is possible to spread more quickly board deeper in substrate.

Fig. 3. The layout the layout of (a) the conventional 4H-SiC LIGBT, (b) 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. Fig. 4 shows magnified image of the fabricated proposed LIGBT. The properties of the device are summarized in Table 1.

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

The operating mechanism in the dual-emitter 4H-SiC LIGBT is the same as in the conventional 4H-SiC LIGBT; the operating system involves the injection of high density holes from the collector into the additional emitter. Forward current did not occur in the minority carrier recombination due to the shorter length of the low doped N-epitaxial layer than the minority carrier diffusion length. The increased electron current from the additional emitter decreased the on-state voltage drop due to the conductivity modulation. Also, the dual-emitter LIGBT has a higher current density (768 mA) than the conventional LIGBT (128 mA) at the collector voltage of 20 V, due to its additional emitter region. As a results the dual-emitter 4H-SiC LIGBT has a lower on-state voltage drop (0.86 Ω) than the conventional LIGBT (6.67 Ω) ^(13,14).

III. EXPERIMENTAL RESULTS

Fig. 5. (a) Measured on-state characteristics of the 4H-SiC conventional LIGBT and proposed LIGBT for gate bias 15 V, 20 V, (b) comparison of the on-state voltage drop when the gate bias is 20 V.

Fig. 6. Measured electron and hole mobility of the 4H-SiC conventional LIGBT and proposed LIGBT with varying channel length.

The electrical characteristics of the dual-emitter 4H-SiC LIGBT and conventional 4H-SiC LIGBT were investigated by using the 4145 semiconductor parameter analyzer and 370A curve tracer. Fig. 5(a) shows the on-state characteristics of conventional LIGBT and dual-emitter LIGBT. Both devices have the same geometry. The knee voltage is about 4 V due to high concentration of the P-well. The collector driving current of the 4H-SiC dual-emitter LIGBT is above 18 mA when the gate bias and collector bias are at 20 V, which is nearly six times higher than that in the conventional 4H-SiC LIGBT.

Fig. 5(b) shows a comparison of the on-state characteristics when the gate bias is 20 V. The on-state voltage drop of the dual-emitter LIGBT is 7.5 V at the collector current of 1 mA, which is lower than that of the conventional 4H-SiC LIGBT. Thus, the dual-emitter LIGBT improves the on-state voltage drop by about 22%. The dual-emitter 4H-SiC LIGBT contains an additional emitter, and has a higher electron and hole mobility than the conventional 4H-SiC LIGBT. The dual-emitter 4H-SiC LIGBT has a higher electron/hole current (IE/IH) than the conventional 4H-SiC LIGBT, due to the increase in the electron and hole mobility. Fig. 6 shows a comparison of the electron and hole mobility simulation characteristics with changing channel length. The on-state voltage drop, $V_{CE}$ is the sum of the emitter-base voltage drop ($V_{EB}$) of the PNP transistor and the collector-base voltage drop across this transistor ($V_{CB}$), or

The voltage, $V_{CB}$, is the drain-source voltage ($V_{DS}$) across the MOSFET. Because IB= IMOSFET and the MOSFET operates in the linear region, we obtain

where,

$C_{OX}$ is the oxide capacitance per unit area, $μ_{s}$ represents the surface electron mobility, $Z$ is the channel width, and $L_{ch}$ is the channel length. Thus, the LIGBT on-state voltage ($V_{CE}$) is explicitly expressed in terms of the collector-emitter current, $I_{CE}$ = $I_{B}$ + $I_{C}$, and gate voltage, $V_{GE}$ = $V_{GS}$, assuming $V_{GB}$ = 0. The equation for $V_{CE}$ is another initial condition necessary for studying the switching transient ⁽¹⁵⁾. As the dual-emitter LIGBT has two N-channels due to the additional emitter region, the value of ($Z$/$L_{ch}$) is increased and $V_{CB}$ becomes smaller due to the larger $K$ value. Therefore, the voltage drop, $V_{CE}$, is decreased and the increasing electron injection into the N-drift region enhances conductivity modulation. As more electron current is injected, more hole current is injected from the collector to the N-drift region in order to maintain charge neutrality. Therefore, the proposed dual-emitter LIGBT has a higher on-state current capability and a lower on-state voltage drop than the conventional LIGBT. The on-resistance, shown in Fig. 7, is about seven times higher. Fig. 7 shows the collector saturation current $I_{c,sat}$ and saturation trans conductance $g_{m,sat}$ as a function of the gate bias when $V_{CE}$=30 V. The threshold voltage is 5 V. When the gate voltage is 20 V, the conventional LIGBT has a saturation current of 3.8 mA and $g_{m}$ of 0.09 mS. On the other hands, the dual-emitter 4H-SiC LIGBT has a saturation current of 23.5 mA and $g_{m}$ of 0.58 mS which is nearly six times higher than in the conventional 4H-SiC LIGBT. This is because the conductivity. Fig. 8 shows the breakdown voltage for drift lengths of 8 µm, 11 µm, 13 µm, and 16 µm and analyzes the differences between two structures when the breakdown voltage is the largest at 16 µm drift length.

Fig. 7. (a) Collector current, (b) $g_{m}$ as a function of the gate bias for the conventional LIGBT and the proposed dual-emitter LIGBT when $V_{CE}$ = 30 V.

Fig. 8. Breakdown voltage characteristics of the conventional LIGBT and the proposed dual-emitter LIGBT for drift lengths of 8 µm, 11 µm, 13 µm, 16 µm (a) Leaner scale, (b) Log scale.

IV. CONCLUSIONS

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 $g_{m}$ 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 250 V.