Physics and performance of high temperature superconducting vortex flow transistors

The physics and performance of high temperature superconducting vortex flow transistors is analyzed both theoretically and experimentally. Vortex flow transistors based on Josephson vortices are found to be superior to those based on Abrikosov vortices with respect to sensitivity, speed and impedance level. For Josephson vortex flow transistors based on single long junctions and parallel arrays of a large number of short junctions, expressions for the current gain, the transresistance and the response time are derived as a function of the various device parameters. Our analysis shows that Josephson junctions with high products of their normal resistance and critical current are required in order to obtain large current gain, high transresistance and small response time. Both symmetric and asymmetric Josephson vortex flow transistors based on YBa₂Cu₃O_{7 - δ} bicrystal grain boundary junctions have been fabricated. Our experimental results agree well with the model predictions. Transresistance values of several Ω have been achieved for devices based on YBa₂Cu₃O_{7 - δ} grain boundary Josephson junctions. The current gain could be increased by using an asymmetric bias current injection. For asymmetric devices a current gain above 20 has been obtained at temperatures below 60 K.