Investigation of Anomalous Degradation Tendency of Low-Frequency Noise in Irradiated SOI-NMOSFETs

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www.mdpi.com, Mar. 01, 2023 – 

Abstract

In this work, we present new evidence of the physical mechanism behind the generation of low-frequency noise with high interface-trap density by measuring the low-frequency noise magnitudes of partially depleted (PD) silicon-on-insulator (SOI) NMOSFETs as a function of irradiation dose. We measure the DC electrical characteristics of the devices at different irradiation doses and separate the threshold-voltage shifts caused by the oxide-trap charge and interface-trap charge. Moreover, the increased densities of the oxide-trap charge projected to the Si/SiO2 interface and interface-trap charge are calculated. The results of our experiment suggest that the magnitudes of low-frequency noise do not necessarily increase with the increase in border-trap density. A novel physical explanation for the low-frequency noise in SOI-NMOSFETs with high interface-trap density is proposed. We reveal that the presence of high-density interface traps after irradiation has a repressing effect on the generation of low-frequency noise. Furthermore, the exchange of some carriers between border traps and interface traps can cause a decrease in the magnitude of low-frequency noise when the interface-trap density is high.

Keywords: low-frequency noise; ionizing radiation; radiation effects; partially depleted silicon-on-insulator (PDSOI); MOSFET; border trap; interface trap

1. Introduction

Recently, silicon-on-insulator (SOI) technology has become increasingly important in radiation-exposed environments such as space and military electronics due to its greater immunity to single-event effect (SEE) and suppression of the latch-up effect [1,2,3]. Furthermore, due to their strong anti-interference properties, high integration density, and fast operation speed, SOI MOSFETs have been found to be applicable to signal processing circuits, especially in radiation-exposed settings [4,5]. It is well known that low-frequency noise plays a substantial role in the performance of low-noise signal processing circuits [6]. However, as devices become increasingly scaled down, noise emerges as a crucial factor affecting device and circuit performance. Thus, exploring the electrical characteristics and low-frequency noise of SOI MOSFETs under irradiation, in addition to the underlying physical mechanisms, is of critical importance. As devices are miniaturized, it has been observed that thinner gate oxide becomes less susceptible to the total ionizing dose (TID) effect, leading to decreased threshold-voltage shifts and leakage current [7]. Despite this, TID still affects the low-frequency noise of smaller devices, related to interface traps at the Si/SiO₂ interface and border traps in oxide near the interface (2–3 nm). Some studies have focused on the low-frequency noise of MOSFETs under irradiation [7,8,9,10,11,12,13,14,15]. However, most of these studies only compared the low-frequency noise of devices before and after irradiation with a certain dose, and few have studied the degradation trend of the low-frequency noise of devices in a wider range of radiation doses.

Two mechanisms are mainly responsible for the low-frequency noise in semiconductor devices: the mobility fluctuation model and the carrier number fluctuation model. The low-frequency noise due to fluctuation of carrier mobility in the channel caused by various scattering processes, referred to as fundamental 1/f noise, and the low-frequency noise due to fluctuation of the carrier number caused by defects traps, referred to as nonfundamental 1/f noise, are observed in MOSFETs. Nonfundamental 1/f noise usually has a much larger magnitude than fundamental 1/f noise. Moreover, fundamental 1/f noise dominates only when the defect density is reduced enough to ignore nonfundamental 1/f noise [16]. Therefore, it is generally assumed that the low-frequency noise in MOSFETs is caused by the fluctuation of carrier number. The theoretical sources of the fluctuation in carrier number mainly comprise the following [17]. McWhorter suggested that tunneling exchange of carriers between the channel and border traps of the same energy level is the primary generator of low-frequency noise [18]. Sah et al. then proposed an alternative tunneling model, which utilized interface traps as intermediate states during the tunneling process [19]. Apart from the tunneling model, Dutta and Horn determined that low-frequency noise also could be attributed to the trapping of carriers thermally activated by border traps [20]. However, tunneling and Dutta-Horn models both fail to explain low-frequency noise of a clean silicon surface in a vacuum environment [21]. To explain the low-frequency noise generated without an oxide layer or suitable traps inside the oxide layer, Jäntsch suggested that low-frequency noise is generated through the random walk of electrons at the interface [22]. As mentioned above, both border traps and interface traps contribute to low-frequency noise in MOSFETs. Moreover, the tunneling theory remains widely accepted.

The work of Sah et al. demonstrates that carriers liberated by interface traps can be caught by border traps [19]. Additionally, the random walk theory exhibits that the time constants of interface traps can be modulated to be close to that of border traps by carriers randomly walking at the interface [22]. These previous investigations have given the physical basis of carriers exchanging between interface traps and border traps. However, few research addresses the effect of increasing interface traps on the low-frequency noise of MOSFETs after irradiation.

In this paper, the measurements of low-frequency noise behavior and DC electrical characteristics of SOI NMOSFETs of different sizes and gate oxide thicknesses at irradiation doses are presented, serving as a valuable reference for circuit design applications that necessitate good noise performance in radiation conditions. Furthermore, analysis of the low-frequency noise magnitudes post-irradiation is conducted. Additionally, a novel physical mechanism is suggested for low-frequency noise in NMOSFETs with a high density of interface traps.

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