Progress of the study on carrier scattering mechanisms of silicon/germanium field effect transistors

Abstract

As the feature size of Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) continues to decrease, large numbers of new problems appear. Techniques such as strain project, new channel materials and new device structures are considered by academics and industry to be effective ways to continue to improve device performance. In this paper, the scattering mechanism of carriers in the device channel is studied from three aspects: strain technique, new channel material and new structure device: (1) strain technique: Biaxial tensile strain can change carrier distribution among different energy levels, which affects Coulomb scattering and Coulomb mobility. Furthermore, from the TEM image, it is found that biaxial tensile reduces the channel surface roughness for Si nMOSFET, leading an enhancement of electron mobility. However, no such similar phenomena were observed in pMOSFET. Based on this, a new method for calculating the mobility of MOSFET surface roughness scattering using TEM image has been proposed. (2) New channel material: In the germanium (Ge) transistors with different crystal faces, the scattering mechanisms of electron under high field conditions are different. The phonon scattering dominates the Ge(100) transistor, while the surface roughness scattering dominates the Ge(110), (111) transistors. This result is quite different from Si MOSFET. Therefore, a unified model for the scattering mechanism of electron in Ge nMOSFET has been proposed. In SiGe transistors, alloy scattering mainly play a role in the region with relatively small effective electric field (<i>E</i><sub>eff</sub>). The strength of alloy scattering would be weakened in high field and could be decreased by reducing the thickness of SiGe layer. (3) New structure devices: In ultra-thin body germanium (GeOI) transistors, carrier transport is influenced by high- k /channel interfaces, as well as Ge channel/buried oxide (BOX) interface. As the Ge layer thickness decreases, carrier distribution is closer to the interfaces, which intensifies Coulomb scattering and surface roughness scattering. As a result, the mobility in GeOI transistor decreases as the thickness scaling. In addition, the distribution of electron in different energy valleys changes with the thickness decrease in Ge layer, which affects the scattering of electrons. When the Ge film is lower than 10 nm, a part of electron in the L valley will move to the Γ valley, causing the decrease of electron effective mass and increasing the electron mobility.