Study of inversion capacitances and drive currents for MOSFETs made of high-mobility semiconductors.

摘要

Over the past 50 years, silicon-based MOSFETs have been scaled according to Moore's Law. Traditional methods of scaling involved reduction of channel length and gate oxide thickness. In recent years, as silicon MOSFETs decreased in size to the nanoscale regime, new scaling methods such as hybrid orientation and strain engineering were introduced. But even these methods may not be sufficient to deliver the increased switching speed needed in future MOSFETs. High-mobility channel materials are being considered as a promising alternative to silicon.;In this work, the four high-mobility channel materials studied are germanium (Ge), gallium arsenide (GaAs), indium gallium arsenide (InGaAs), and indium arsenide (InAs). The two material-specific components of the drive current equation, mobility and inversion capacitance (Cinv), are used to evaluate the improvement in drive current achieved from use of high-mobility channel materials. In the past, drive current was assumed proportional to mobility but, in fact, the effect of quantum confinement on the Cinv value changes this relationship. This study has found that, for a gate dielectric of EOT≤1 nm, the advantages of higher mobilities for the aforementioned semiconductors are substantially downgraded due to their lower values. Quantum confinement of inversion carriers occurs in all MOSFETs biased into inversion. But, as MOSFETs are reduced in size and gate oxides become very thin, the quantum confinement effect becomes more noticeable in determining Cinv.;Quantum confinement shifts the centroid of the inversion charge away from the oxide-semiconductor interface, which reduces the Cinv value. Therefore, in order to determine an accurate Cinv value for high-mobility channel materials, a CV simulator incorporating quantum confinement effects was used in this study. The simulation of carrier concentration distributions shows that the amount by which the inversion charge is shifted is determined by the density of states of the material. For materials with lower density of states, the centroid of charge was shifted further from the interface and found to correspond to lower Cinv.;Once Cinv values were obtained, an Id ratio was calculated by taking the product of Cinv and mobility for the high-mobility channel material and dividing it by the same product for silicon. This investigation has found that high-mobility channel materials are indeed a promising alternative to silicon in delivering higher drive current in nMOSFETs, although the current gain is much less than the mobility gain for each semiconductor studied. For the InAs nMOS device, the Id was 8-9x higher than in Si. InGaAs, GaAs, and Ge had Id ratios of ∼4.5, ∼3.5 and ∼2.5, respectively. For pMOSFETs, the advantage of using high-mobility channel materials is much less pronounced. The one exception was Ge, which due to its high hole mobility, had an Id ratio of about 4. The study also revealed that for ultra-short channel nMOSFETs, where channel length is close to electron mean free path, the ballistic transport of electrons diminishes the advantage of using high-mobility channel materials over silicon.