The Physics of Metal Base Transistors
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THE PHYSICS OF METAL BASE TRANSISTORS
E. Rosencher, F. Arnaud d'Avitaya, P.A. Badoz, C. d'Anterroches, G. Glastre, G. Vincent and J.C. Pfister. Centre National d'Etudes des T616communications - BP : 98 - Chemin du Vieux Ch~ne - 38243 MEYLAN CEDEX - FRANCE
Abstract Epitaxial Si/CoSi 2/Si structures can be grown under ultra-high vacuum conditions. The metallic CoSi 2 films can be extremely thin typically between 1 nm and 20 nm. The electrical properties of these heterostructures are presented, mainly the transport of electrons in the metallic films parallel to the interfaces and the transfer of electrons through the metal film. The influence of pinholes in the CoSi 2 layers will be discussed.
I. Introduction Thanks to advances in ultra-high vacuum technology, it has recently become possible to realize epitaxial Semiconductor/Metal/Semiconductor (SMS) structures [1, 2] using a Si/CoSi 2 /Si sandwich. The rather small lattice mismatch (1.2 %) between Si and CoSi 2 crystals as well as their similar cubic structures allow the production of monocrystalline Si/CoSi 2 and Si/CoSi 2 /Si heterostructures. These SMS structures open the way to promising ultra-low base resistance devices for millimeter wave applications. In this paper, we intend to review the main results on the physics of metal base transistors (MBT). This latter denomination adresses two different kinds of structures : the SMS-Transistor where the CoSi film is intended to be continuous and Permeable Base Transistors (PBT3 where discontinuities in the metallic film are intentionally introduced by nanolithography techniques. Though this paper is mainly focused on the electrical properties of these structures, some informations on the morphological quality of these sandwiches are given in Sec. II, which are necessary for the understanding of the transport properties. In Sec. III, parallel transport in the ultra-thin metal films will be addressed while the perpendicular transport through the SMS structure will be developed in Sec. IV. We shall describe the current status of the Si/CoSi 2 /Si permeable base transistor in Sec. V. A brief discussion is given in Sec. VI which tentatively describes the general trends in the future research on metal base transistors.
II. Morphology of Si/CoSi2 /Si heterostructures This section is intended to describe the results necessary as a background for the understanding of transport properties rather than to provide an explanation to the growth mechanisms, which is still clearly lacking. The morphology observations are made using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron Microscopy (HRTEM). One of the key points for the growth mechanisms is the cleaning of the silicon surface before CoSi 2 formation. Up to these days, we have used the Shiraki process, the details of which are described in Ref. [3] and [4]. This process is based on chemical oxidation-etching cycles, leaving a very thin protective SiOx film which evaporates when heated above 750°C. The SM
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