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Electronic states for two possible high-pressure phases (with the ThSi 2 structure and with the AlB 2 structure) of CaSi 2 are determined by means of a local density approximation band-structure calculation. We confirm that these polymorphs can be regarded as doped sp 2 networks of Si. However, a simple picture in terms of π-orbitals is not adequate for explaining the conduction bands, because (i) an s-like band, which is anti-bonding in sp 2 connections but bonding between segments of the network, appears and (ii) there is π -d hybridization between Si and Ca.

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A new technique that integrates the metal gate multifin field effect transistor (multi-FinFET) and the conventional polycrystalline silicon (poly-Si) gate planar FET is proposed. It solves the problems of the previous scheme, such as the complicated process integration due to the coexistence of TiN gate FinFETs and poly-Si gate planar FETs, the fin width consumption by multiple gate oxidation, the large fin pitch limited by the resolution of lithography, and the gap-filling ability of shallow trench isolation (STI). The newly proposed technique forms multifin structures by spacer patterning through the gate poly-Si electrode for planar FETs. The drain current gain due to an increase in effective channel width is estimated, and the basic electrical characteristics of a multi-FinFET are evaluated.

A three series-connected transistor model is introduced to understand the electrical characteristics of conventional recess-channel-array transistors (RCATs) and modified RCATs. An RCAT is considered to be a serial connection of three transistors consisting of one bottom transistor and two vertical transistors. The electrical characteristics of a cell transistor are explained by a balance of those transistors. A newly modified fin-RCAT which has a fin structure at the bottom of a silicon recess is proposed to improve cell transistor characteristics. This design improves cell current by 70% while maintaining retention characteristics.

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