Current amplification in high-temperature superconductor current injection three-terminal devices

Current injection into superconducting microbridges has been studied for state-of-the-art YBa2Cu3O7−x/SrTiO3/Au (YBCO/STO/Au) planar heterostructures based on c-axis oriented YBCO layers. By injecting a gate current Ig into the YBCO channel of these three-terminal devices the critical current Ic could be reduced up to a gain factor G≡−dIc/dIg∼5 at 77 K. G is found to be roughly proportional to the superconducting quality of the samples as expressed by the critical current density Jc of the YBCO channel. At least for our high-quality samples, this Ic suppression is shown to be mainly due to the perturbation of the electronic superconductive equilibrium state by quasiparticles associated with the injection of the gate current. However, this basic mechanism for the current-injection-based transistor principle was in our experiments always accompanied by substantial parasitic effects, such as current summation and Joule heating due to electric power dissipation in the ultrathin STO barrier. This reduces the efficiency of the transistor mechanism and thus the hopes for THz operation of such devices.