0.1- mu m gate-length superconducting FET

Abstract

A superconducting field-effect transistor (FET) with a 0.1pm-length gate electrode was fabricated and tested at liquid-helium temperature. Two superconducting electrodes (source and drain) were formed on the same Si substrate surface with an oxide-insulated gate electrode by a self-aligned fabrication process. Superconducting current flowing through the semiconductor (Si) between the two superconducting electrodes (Nb) was controlled by a gate-bias voltage.superconducting device with three terminals using dc A power is desired for development of superconducting integrated circuits. Several types of three-terminal superconducting devices have been proposed and experimentally tested [ 11-[5]. Superconducting field-effect transistors (FET's) have excellent input/output isolation compared with others. The first experimental superconducting FET was realized by the authors [6]. The gate electrode was formed on the opposite side of the Si single-crystal film from the two superconducting electrodes, so these devices were not suitable for integrated circuits. Takayanagi and Kawakami [7] and Ivanov et al. [8],[9] reported on superconducting FET's using compound semiconductors. The gate lengths of their devices were about 0.5 and 0.4 pm, respectively, much larger than that of shortchannel Si MOSFET's. A smaller gate length is necessary to improve the switching speed of the superconducting FET's and to realize highly integrated circuits, because a small gatelength superconducting FET has a larger superconducting current to drive the next stage. However, planar devices with a short gate length have been difficult to fabricate on one side of a Si single crystal because the two superconducting electrodes are less than 0.2 pm apart in a Si semiconductor substrate and the gate electrodes must lie between the two.In this paper we describe the fabrication and operation of a superconducting FET with a gate length as short as 0.1 pm. This FET has a planar structure with a gate electrode between the two superconducting electrodes on the same side of a Si single crystal. The electric characteristics are measured at 4.2 K and the static characteristics are discussed.A cross-sectional view is shown in Fig. 1. The device has a single-crystal Si substrate, two As+ ion-implanted areas, a gate oxide, a poly-Si gate electrode, and a superconducting source and drain. The original aspect of this structure is the 0.1 -pm-length gate electrode with an overhang and insulating sidewalls of very thin Si3N4 film. A top view of the device and a scanning-electron micrograph of the gate structure are shown in Fig. 2(a) and (b), respectively. Fig. 1. Cross-sectional view of the superconducting FET with 0.1-pm gate \ , / Sdurce electrode(Nb1 Drain electrode(Nb) (a) -150nm (b) scanning electron micrograph of the gate structure (cross-sectional view).Fig. 2. (a) Micrograph of the superconducting FET (top view) and (b)A multilayer of gate oxide, doped poly-Si, and Si3N4 was formed on a surface of the p-Si single-crystal substrate. The gate oxide was a therma...

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