Hot Carrier Instability Mechanism in Accumulation-Mode Normally-off SOI nMOSFETs and Their Reliability Advantage

The Accumulation-Mode SOI MOSFETs, in comparison to the Inversion-Mode FD SOI MOSFETs, have several advantageous characteristics, such as higher current drivability, lower 1/f noise and suppressed kink effect, and it has been proposed that this device gives analog, digital and their mixed circuits better performances [1-3]. These advantages become larger as dopant concentration of SOI (NSOI) is increased [2,3]. However, there have been only small amount of works reported on the reliability issues in this device [4,5]. Then, this work studies the degradation mechanism of the hot carrier instability in normally-off Accumulatin-Mode SOI nMOSFETs. The schematic view of the device used in the experiment, when working in the saturation region, is shown in Fig.1. From the experimental examination of degradations in transistor transfer characteristics and flicker noise level at various transistor working region shown in Fig.2, it is revealed that the major degradation in the Accumulation-Mode nMOSFET is the positive charge and interface trap generation at the front gate Si/Insulator interface at the Drain side due to the injection of hot holes that are generated by the impact ionization caused by channel hot electrons and drained toward the front interface by the vertical electrical field in the locally depleted region as shown in Fig.3. The degradation mechanism originates from the unique current conduction mechanism in Accumulation-Mode MOSFETs in the saturation working region, that the pinch off occurs near the back Si/Insulator interface at the Drain side as schematically explained in Fig.1. It is experimentally shown that the Accumulation-Mode nMOSFETs have higher hot carrier immunity than the Inversion-Mode FD SOI nMOSFETs as shown in Fig.4. Also, the effect of the hot carrier stress to the transistor transfer characteristics such as subthreshold slope and current drivability is shown to be smaller for devices with higher dopant concentration of SOI (NSOI). It is because of the smaller influence of the front gate Si/Insulator interface to the total current drivability in higher NSOI devices in this device structure. Thus, when utilizing higher dopant concentration of SOI to this device, trade-offs do not arise between the hot carrier reliability concern and transistor performance acceleration, such as higher current drivability, smaller flicker noise level and smaller kink effect. Consequently the Accumulation-Mode MOSFET is an advantageous device structure that can suppress the effect of degradations of front gate insulator film to the transistor performance. In addition, the flicker noise analysis would be a useful tool to detect localized defects at Si/Insulator interfaces especially for devices that have multiple current conduction paths such as FinFETs, Multi-channel FETs, gate all around transistors and so on. References [1] J.Colinge, T-ED, pp.718, Vol.37, 1990 [2] W.Cheng et al., JJAP, 3110, Vol.45, 2006 [3] W.Cheng et al., to be published at ICSICT, 2006 [4] F.Duan et al., T-ED, pp.972, Vol.6, 1997 [5] A.Acovic et al., SOI Conference, pp.134, 1994 G