1.
Introduction

Due to its excellent material properties, silicon carbide (SiC) offers some unique advantages like high switching speed and high operating temperature, making it an attractive candidate for high temperature, high frequency, and high power density applications^[1]. By taking advantage of SiC, some researchers have made certain progresses in power integration and reported several integrated circuits based on different technologies like CMOS, JFET, and bipolar. For example, based on 4H-SiC BJT, Refs. [2–7] reported several integrated circuits, which can operate normally up to 500 °C. However, most of the BJT structures used in ICs are a vertical structure, which is not really an optimal solution for ICs. Refs. [8–10] reported several integrated circuits using 6H-SiC depletion-mode JFET, and one of them can function with high performance at the highest temperature, up to 961 °C. In addition, Zhang reported the first power integration sample based on 4H-SiC lateral JFET in 2008^[11]. However, the JFET is a depletion-mode device, and its reliability may need extra attention. Except Ref. [11], there has been no report on the one-chip power integration, especially the integration of ICs and vertical power devices, because there are rarely good solutions to solve the device isolation issues and compatibility of fabrication processes between the power devices and signal devices.



In today’s silicon, GaAs and SiC ICs, dielectric and p/n junction are the main solutions for device isolation. Meanwhile, separation by implanted oxygen (SIMOX) and wafer bonding techniques have been extensively studied for future Si ultra large scale ICs and silicon-on-insulator (SOI) structures. However, SIMOX may cause severe lattice damage and oxygen incorporation. Refs. [12–14] implanted vanadium ion into SiC and succeeded in the selective formation of a semi-insulating layer in SiC. Based on this technique, the isolation problem can be solved and it will greatly help the integration of SiC power ICs.



Considering the various device options, SiC BJTs are arguably the most promising choice for high temperature applications since the SiC MOSFETs are hindered by gate oxide issues^[15], the MESFETs always suffer from increasing gate-to-channel leakage current at elevated temperature^[16]. Among all demonstrated integrated circuits, the JFET circuits provided the highest temperature durability but the SiC JFETs usually operate in depletion-mode, which may induce reliability issues. The BJT circuits can provide significantly better electrical performance in a wide temperature range.



To compromise the fabrication processes of the vertical power BJT and improve the performance of the lateral signal BJT, in this paper, we propose a novel lateral BJT structure that is suitable for monolithic power integration with vertical power BJT. Section 2 will illustrate the device structure. Section 3 will show the simulation results and some discussions. Section 4 will conclude this paper.

2.
Device concept

Fig. 1(a) shows the conventional lateral SiC BJT structure, and Fig. 1(b) shows the cross-sectional view of the proposed power integration concept. The signal BJT can be fabricated in the same processes of power BJT, except the formation of the semi-insulator layer. Therefore, the lateral signal BJT and vertical power BJT can be monolithically integrated at the cost of one extra mask. Different from the traditional lateral BJT, the emitter and collector regions of the signal BJT in the power integration structure have the same doping concentration. Besides, the base fingers are no longer placed between the collector and emitter fingers as the traditional structure but are fabricated between the collector fingers to improve the base transport factor. In the forward conduction mode, both the emitter and collector will emit electrons to the base when the collector-emitter voltage is relatively small, thus the electron injection efficiency will not decrease, caused by the wider external base, thus its current gain will not decrease too much. In the forward blocking mode, as the doping concentration of the collector is high, the voltage is mainly applied on the base region. Since the lateral BJT is only targeted for low voltage and/or logic applications where the breakdown voltage of devices is usually lower than 50 V, it is within the capability of the base region for 1200 V voltage rating power BJT. Therefore, the proposed structure is very promising for achieving power integration. In this paper, the performance of the lateral BJT is comprehensively studied by simulation, which will help demonstrate the feasibility of power integration using this method.






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Figure1.
Cross-sectional view of (a) traditional lateral BJT and (b) proposed power integration concept.

3.
Simulation results and discussion

Two-dimension numerical simulations were done to investigate the performance of the proposed structure. Physical models used for TCAD simulations include the parallel electric field dependence mobility model, the concentration and temperature dependent mobility model, the concentration dependent recombination model, the bandgap narrowing model, the impact ionization model, and the lattice self-heating model. The model parameters are deprived from some published papers where they have been calibrated^{[7, 17–20]}. The lateral BJT shares the same epilayers with the power BJT, and some key parameters are listed in Table 1.

Parameter	Proposed lateral BJT	Conventional lateral BJT
Base doping (1017 cm?3)	1	1
Emitter doping (1019 cm?3)	2	2
Collector doping (cm?3)	2 × 1019	4.8 × 1015
Collector width (μm)	4	4
Emitter width (μm)	5	5
Base width (μm)	5	5
Distance between C and B	2	2
Distance between C and E	1.5	/
Lifetime （τn = 2τp, μs）	0.6	0.6

Table1.
Key parameters used in the simulations.



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Parameter	Proposed lateral BJT	Conventional lateral BJT
Base doping (1017 cm?3)	1	1
Emitter doping (1019 cm?3)	2	2
Collector doping (cm?3)	2 × 1019	4.8 × 1015
Collector width (μm)	4	4
Emitter width (μm)	5	5
Base width (μm)	5	5
Distance between C and B	2	2
Distance between C and E	1.5	/
Lifetime （τn = 2τp, μs）	0.6	0.6

3.1
Output characteristic

Fig. 2 compares the forward I–V characteristics of the proposed lateral BJT with that of the conventional lateral BJT. Compared to the conventional BJT, the proposed lateral BJT has a longer current path in the base region, which means that the base transport factor is lower than that of conventional lateral BJT. However, the on-state resistance is lower because the doping concentration of the collector region in the proposed lateral BJT is much higher.






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Figure2.
(Color?online) Comparison of I–V between (a) the traditional lateral BJT and (b) the novel lateral BJT structure.

3.2
Current gain

Fig. 3(a) shows the common-emitter current gain varying with the collector current density. The current gain increases with the collector current at first, and then decreases with the collector current. The reason is that the electron injection efficiency increases with the collector current in low-level injection in the base, but in high-level injection, the injected minority (electron) carrier concentration in the base region exceeds its doping concentration. To satisfy charge neutrality, the majority (hole) carrier concentration in the base region will increase as well. Then the injection of holes from the base region to the emitter region is enhanced, inducing a reduction of the injection efficiency and current gain of the lateral BJT. It can be seen that the maximum current gain in the simulation is still higher than 130 at room temperature.



Fig. 3(b) shows the temperature dependent current gain of the proposed lateral BJT at J_B = 564 mA/cm² and V_CE = 4 V. When the temperature increases, the carrier concentration increases, leading to a reduction of mobility and the injection efficiency. As a result, the current gain of the proposed lateral BJT decreases when the temperature increases. According to Fig. 3(b), the current gain decreases from 133 to 50 when the temperature increases from 25 to 325 °C.






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Figure3.
(a) Current gain varying with the collector current and (b) temperature-dependent current gain.

3.3
Forward-blocking characteristic

Fig. 4(a) shows the simulated forward-blocking I–V characteristic of the proposed lateral BJT. When the collector leakage current density is 0.1 mA/cm², the breakdown voltage is 107 V. Considering the compatibility with the vertical power BJT, the collector region of this proposed lateral BJT is heavy doped. Therefore, the space charge region of the collector junction mainly expands to the base region under the forward blocking mode, which means that the base region will bear the most forward-blocking voltage. Fig. 4(b) depicts the electric field distribution along the orthogonal direction of the collector junction. It is observed that the space charge region is mainly expanded in the p-base region.






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Figure4.
(a) Breakdown characteristic of the lateral NPN BJT and (b) electric field along the orthogonal direction of the collector junction.

3.4
Switching characteristics

The signal BJTs are usually used in high-speed, high-current drivers. Here, a simple circuit with a resistive load is used to investigate the lateral BJT’s dynamic performances. The supplied voltage source is 5 V, the resistive load is 5 Ω, the stray inductance is 0.1 nH, and the base drive resistance is 5 Ω, as shown in Fig. 5. In Fig. 6, it can be observed that the rising time and fall time are 200 and 275 ns, respectively.






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Figure6.
The simulated waveforms of the inverter.






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Figure5.
Circuit configuration of the inverter basic logic gate.

4.
Conclusion

This paper proposes a lateral BJT structure, which is suitable for power integration based on 4H-SiC bipolar technology. Simulation results show that the maximum common emitter current gain of the proposed lateral BJT are 133 and 52 at 25 and 300 °C, respectively. Its breakdown voltage is higher than 100 V at J_B = 0.1 mA/cm². The Mix-Mode simulation results show that the rising and fall times are 200 and 275 ns, respectively. Therefore, the proposed lateral BJT is a good choice to achieve power integration based on 4H-SiC technology.