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High-Efficiency Metal-Base Transistor Prepared by MBE or LPCVD Using a Composite Epitaxial Base

IP.com Disclosure Number: IPCOM000039601D
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 16K

Publishing Venue

IBM

Related People

Delage, S: AUTHOR [+2]

Abstract

The main advantages of metal-base transistors (MBTs) are that only majority carriers participate in the ballistic current transport through the base thus eliminating the carrier storage time present in minority carrier devices and the resistivity of epitaxial metal bases is minimal thus allowing the use of ultrathin layers which reduces the carrier transit time, both yielding very fast response times of the device. Another very important aspect is its integrability in monolithic structures. The very high frequencies that should be realizable with MBTs can make this kind of device a serious competitor with GaAs- based devices. To reduce scattering of electrons in the metal base, the base material has to be made as perfect as possible, i.e., epitaxial.

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High-Efficiency Metal-Base Transistor Prepared by MBE or LPCVD Using a Composite Epitaxial Base

The main advantages of metal-base transistors (MBTs) are that only majority carriers participate in the ballistic current transport through the base thus eliminating the carrier storage time present in minority carrier devices and the resistivity of epitaxial metal bases is minimal thus allowing the use of ultrathin layers which reduces the carrier transit time, both yielding very fast response times of the device. Another very important aspect is its integrability in monolithic structures. The very high frequencies that should be realizable with MBTs can make this kind of device a serious competitor with GaAs- based devices. To reduce scattering of electrons in the metal base, the base material has to be made as perfect as possible, i.e., epitaxial. The choice of the metals for effective use in MBTs is therefore limited to those which grow epitaxially on semiconductors. The efficiency of actual exploratory epitaxial MBTs is limited by the symmetry of the two Schottky barriers that compose this device. The electron current crossing the metal base is limited by reflections at both interfaces (quantum mechanical matching of the wave functions and optical phonon backscattering essentially in the collector) and by scattering in the metal layer. The image force lowering of the barrier at the collector side plays an essential role in the efficiency of electron collection in a symmetric MBT. On the other hand, an asymmetric transistor with a lower barrier at the collector side increases the potential energy of the incoming electron at the collector-base interface and also increases considerably the number of states available to accept an incoming electron. This decreases the reflection at the collector interface and thereby should increase the efficiency. The electrons lose energy during their passage through the base by electron-electron or electron-phonon scattering. In a symmetric MBT the electrons, having lost energy in the basis, must tunnel through part of the Schottky barrier at the collector side. This increases considerably their reflection coefficient at this interface. In return, an asymmetric barrier should increase largely the number of inelastically scattered electrons collected. This article describes a technique to achieve asymmetrization by using either a dual or a graded metal basis in the MBT. Such composite epitaxial metal-based transistors (CEMBTs) are producible with standard techniques at the present state of technology. The collection efficiency of a CEMBT is expected to exceed 50% with properly chosen materials. A special embodiment of the idea of a CEMBT is the growth of a graded or dual epitaxial silicide on Si(111) surfaces. On the Si(111) surface continuous epitaxial silicide layers as thin as 5 nm are available and can be produced either by Molecular Beam Epitaxy (MBE) or by Solid-Phase Epitaxy (SPE). In the case...