A New Structure for a Superconducting Field Effect Transistor
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A NEW STRUCTURE FOR A SUPERCONDUCTING FIELD EFFECT TRANSISTOR STEPHANE TYC, AND ALAIN SCHUHL Laboratoire Central de Recherches, Thomson-CSF, 91404 Orsay Cedex, France ABSTRACT A new structure is proposed and described which can solve the most severe drawbacks of current architectures for Josephson FETs. Its advantages are discussed, and several realizations are suggested. INTRODUCTION Logic operation based on the Josephson effect is very attractive because of its high speed and low dissipation. However, it is difficult to build all logic functions using only Josephson Junctions (JJ). For that reason, there have been numerous attempts to make transistors that utilize superconductivity in one way or another so that they can be interfaced with JJs and bring the essential three terminal functions to JJ circuits. Of the many proposals (see e.g. [1] [2]for a review of the more classic ones, and [3]for an introduction to the single flux quantum logic familly) perhaps the most straightforward is the JOFET (Josephson Field Effect Transistor) [4]. In this paper, we first introduce the JOFET and recall oft quoted arguments [5]which lead to the conclusion that it has little future as a useful device. Secondly, we propose a sister design: the SUperconducting PERcolating Field Effect Transistor (SUPERFET), that has the potential of becoming a useful device, and we discuss several possible realizations of the SUPERFET. All the discussion could be translated advantageously in terms of high temperature superconductors but we restrict ourselves to conventional ones. THE JOFET A JOFET has essentially the same design as a normal FET, the only difference being that the source and drain are made of a superconductor (lead and niobium are frequent choices). A supercurrent can then flow between source and drain (see Fig. 1.a) through the semiconductor because Mat. Res. Soc. Symp. Proc. Vol. 241. ©1992 Materials Research Society
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of the proximity effect. Cooper pairs leak from the superconductor into the adjacent semiconductor where they have a finite lifetime r = h/27rkBT [6]. These Cooper pairs travel in the semiconductor at the velocity v of quasiparticle excitations. This velocity depends on the particular semiconductor used. One associates with the lifetime and the velocity, a characteristic length 6 for the exponential decay of the number of Cooper pairs, into the semiconductor and away from the superconducting interface. One usually discerns two cases. In the clean limit, the Cooper pairs die before they collide, and the characteristic length is given by 6c = yr. In the dirty limit, the Cooper pairs experience many collisions before they collapse, and the length is 6D = ýv'iU where D is the diffusion coefficient. The transistor effect is obtained by imposing a gate voltage VG which decreases or increases the number of electrons in the channel and thereby acts upon the velocity v, and on the decay length E(Vc) (7]. The maximum supercurrent I1 which can flow from source to drain is given by I,
=
A exp (-L/l(Vg)),
(1)
where L
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