Effects of interface roughness scattering on RF performance of nanowire transistors

Abstract

As the metal oxide semiconductor field effect transistor (MOSFET) has been scaled down to the nano-meter regime, it has become difficult to further scale the planar type MOSFET [1]. As an alternative device structure, a nanowire transistor has been proposed to the scientific community. Not only that it can suppress the short channel effects in a logic device, it is also a good candidate for a RF device due to its high transconductance and high cut-off frequency [2]. Because of the volume inversion effects, the mobility of electrons in nanowire structures is improved [3], which results in a high transconductance. The capacitance of a nanowire transistor becomes smaller due to a larger effective oxide thickness caused by the volume inversion phenomenon [3]. This paper addresses the effects of interface roughness scattering in a nanowire transistor especially on the cut-off frequency through a full band quantum mechanical simulation [4] of atomistically generated nanowire structure as depicted in Fig. 1. Fig. 3(a) shows that the cut-off frequency of a smooth nanowire increases as the diameter decreases, however, it is greatly reduced due to the interface roughness scattering as the diameter of the nanowire decreases down to 2 nm. This phenomenon is studied by investigating the behavior of the transconductance (G m,on ) and the total gate capacitance (C g,tot ). They are related to the cut-off frequency throughHere, the ON-state is defined by the gate voltage V G =V TH +2/3V DD where V TH is the threshold voltage and V DD (=0.6 V) is the supply voltage. The Threshold voltage is defined through a constant-current method using threshold current = diameter of NW × 1e-7 (A). The transconductance in Fig 3(b) shows a similar trend to the cut-off frequency and shows a peak at around 3 nm when interface roughness scattering is included. On the contrary, the total gate capacitance shown in Fig. 3(c) is not affected much by the interface roughness scattering since carriers are strongly confined in the middle of the channel at the ON-state and the interface roughness has relatively small effects on the gate capacitance than when they are drawn near the surface at the ON-state as in a planar MOSFET or a nanowire transistor with a relatively large diameter.For a further investigation, the transmission spectrum in energy space (Fig. 2) is studied. In a nanowire sample with the lowest mobility, the transmission is reduced more than the nanowire sample with the highest mobility. The transmission curve shows resonance peaks at certain energies caused by resonant states sitting inside the channel [4]. The interface roughness (IR)-mobility is directly related to the transconductance and it is degraded as diameter is reduced. Traditionally at a small electron density, the IR-limited mobility decreases as D Si 6 . However, at high electron density, the decreasing rate is mitigated due to screening effects.In the spatially resolved electron velocity plot Fig. 3(e), it can be clearly identified that the electron velocity is reduced signifi...