New vortex flow transistors made of YBa/sub 2/Cu/sub 3/O/sub y/ thin films

We have investigated the performances of several types of vortex flow transistors including nanobridge vortex flow transistors (NBVFTs) based on a parallel array of nanobridges, planar Josephson vortex flow transistors (planar JVFTs) based on a parallel array of grain boundary Josephson junctions, and new JVFTs with a stacked structure (stacked JVFTs). Considering the integration and the reduction of the L/R time constant, the areas of the transistors were restricted to less than 350 /spl mu/m/sup 2/. A NBVFT showed a flux-to-voltage transfer function of 2.6 mV//spl Phi//sub 0/, which was one order of magnitude larger than that of the other transistors. In contrast, the NBVFTs showed a very small current gain due to a large kinetic inductance of a nanobridge, while the NBVFT had the smallest area among the three. A planar JVFT with asymmetric geometry was easy to fabricate and showed a current gain of 2.2 at 4.2 K. However, the planar JVFT requires a large area, leading to a long response time other than the internal delay time. A stacked JVFT also showed a current gain of 2.5 at 4.2 K. A layered structure yielded a strong coupling between the body of the JVFT and the control line. Due to this strong coupling, the response time of the stacked JVFT was considerably improved compared to that of the planar JVFT.