Effects of 5 MeV proton irradiation on 4H-SiC lateral pMOSFETs on-state characteristics

Abstract

Silicon carbide (SiC) MOSFETs are often used in harsh environments where they are exposed to ionizing radiation. This paper investigates the effects of high-energy proton irradiation on SiC MOSFETs, with a particular emphasis on the on-state characteristics of lateral pMOSFETs. By comparing with the lateral 4H-SiC nMOSFET after irradiation in our prior work, we observed the deviations of important electrical parameters. As the irradiation dose increases, the threshold voltage (${\mathrm{V}}_{\mathrm{TH}}$) of pMOSFETs is negatively shifted and has more negative offsets than that of nMOSFETs. This is due to two factors which affect ${\mathrm{V}}_{\mathrm{TH}}$ after irradiation: the near interface oxide traps (NIOTs) and the interface traps (${\mathrm{N}}_{\mathrm{it}}$). These two factors compensate each other for nMOSFETs, but strengthen each other for pMOSFETs. It is found that an increase in the density of interface traps (Dit) in both types of MOSFETs after irradiation. However, unlike the nMOSFET, the pMOSFET has a decreasing field effect mobility (${\mathrm{\mu }}_{\mathrm{FE}}$) with the increase of irradiation dose. We believed that besides the effect of Dit on ${\mathrm{\mu }}_{\mathrm{FE}}$, NIOTs have a more profound influence on ${\mathrm{\mu }}_{\mathrm{FE}}$, resulting in different changes between nMOSFETs and pMOSFETs’ ${\mathrm{\mu }}_{\mathrm{FE}}$.

Introduction

Silicon carbide (SiC) is an attractive material for realizing high-frequency, high-power and high-temperature electronic devices, owing to its superior properties such as wide bandgap, high breakdown field, high thermal conductivity, and high saturation electron drift velocity [1]. The benefits of CMOS circuits, which comprise n-channel and p-channel Metal-Oxide-Semiconductor Field Effect Transistors (MOSFET), include full voltage output swing, temperature independent logic level, low power consumption, and high noise tolerance. By combining the exceptional material properties of SiC with these advantages, SiC CMOS integrated circuits have the potential for even more attractive development. With the growing use of advanced semiconductor devices in space and nuclear technology, reliability and lifetime expectations are becoming increasingly rigorous in harsh working environments characterized by high temperatures and strong radiation. Though the growth technology of large-size 4H-SiC materials has laid a good foundation for the development of its semiconductor electronic devices, so far, their high reliability has not yet been achieved, and in terms of 4H-SiC MOSFETs, there are still problems of low oxide quality and interface states. These defects will lead to low channel mobility and threshold voltage instability [2]. Furthermore, SiC MOSFETs often undergo multifarious radiation exposure that alters their electrical performance or even leads to permanent faults, which puts more pressure on the reliability of device gate oxides [3]. Therefore, it is necessary to study the influence of radiation induced defects.

Theoretical research on the irradiation effects of SiC MOSFETs remains inadequate thus far. Previous studies have focused on the total dose radiation effects of $\gamma$-rays and X-rays, which only have a simple ionization effect [4], [5], [6], [7]. Refs. [8], [9], [10], [11] have investigated the effects of proton irradiation on the radiation hardness of vertical 4H-SiC power MOSFETs. The negative shift of the threshold voltage was observed after irradiation and was caused by the total ionizing dose effect. As for lateral MOSFETs, M. Alexandru et al. [12], M. Florentin et al. [13], and M. Cabello et al. [14] all have studied the effects of different proton energy and proton fluence ranging from $5\times 10^{12}\mathrm{p}/{\mathrm{cm}}^2$ to $5\times 10^{15}\mathrm{p}/{\mathrm{cm}}^2$ on the electrical properties of lateral n-channel 4H-SiC MOSFET devices. The results of these experiments have been similar, with the threshold voltage of nMOSFETs shifting negatively, the ${\mu }_{\mathrm{FE}}$ increasing, and the threshold voltage stability improving with the increase in irradiation dose [15]. These improvement has been attributed to the diffusion of nitrogen caused by irradiation, which increased the quality of the interface by combining with the hanging bonds of silicon and carbon near the interface. Although the above prior reports are mainly concentrated on the irradiation effects on the threshold voltage and ${\mu }_{\mathrm{FE}}$ of the lateral nMOSFET device, the intrinsic mechanism of their changes remains poorly understood. And these studies did not involve the irradiation effects on the electrical characteristics of lateral p-channel MOSFET. It should be noted that due to the negative drift of threshold voltage, the ${\mu }_{\mathrm{FE}}$ extracted under the same gate voltage may correspond to different electric fields. In addition, the ${\mu }_{\mathrm{FE}}$ of device is not only related to interface traps, but also related to the near interface oxide traps [16], and irradiation effect on the interface traps is not directly characterized. Therefore, the composition and electrical characteristics of the near interface oxide traps need to be revealed, and then the change mechanism of threshold voltage and ${\mu }_{\mathrm{FE}}$ can be obtained.

In this paper, the effects of 5 MeV proton irradiation on the on-state characteristics of lateral 4H-SiC pMOSFET are studied in detail. By comparing with the lateral 4H-SiC nMOSFET after irradiation in our prior work [17], we elucidated the relative mechanism of the proton irradiation effects on MOSFETs’ important electrical parameters. It is found that the threshold voltage of nMOSFET and pMOSFET have an obvious negative shift after irradiation, but the negative shift of pMOSFET is larger. The components of trapped charges near 4H-SiC/SiO₂ interface are introduced firstly. Then the reasons for the threshold voltage shift with increasing irradiation dose and the underlying mechanism behind the negative offset difference between nMOSFET and pMOSFET are analyzed based on the trapped charges near the interface. On the other hand, the ${\mu }_{\mathrm{FE}}$ of nMOSFET increases after irradiation, while that of pMOSFET decreases. The effect of irradiation on the device ${\mu }_{\mathrm{FE}}$ is confirmed by analyzing the influence of traps around the interface on the number of channel carriers in the conduction state. The results offer a meaningful reference to improve the hardness of devices against proton irradiation and radiation failure analysis.