Diamond Superconducting FETs and High Current Vertical FETs Using Heavily Boron Doping

Recently, gate-controlled supercurrent (GCS) transistors have attracted attention as major components in superconducting circuits. In the transistors, the supercurrent is modulated by directly applying an electric field to metallic superconducting nanobridges or nanowires of metals such as Nb, Ti, V etc. [1] However, these thin metal superconductors device are based on side gate structure not suitable for device integration and degrade their performance by oxidation which is crucial for realistic application. Here, we report on a top-gated superconducting thin film transistor using a superconducting boron (B) -doped diamond for the first time. The B concentration was around 10²¹cm^-3 and the superconducting transition temperature was 5 K. The large superconducting critical current (<i>I_c</i>) exceeding 100 µA was successfully controlled by a top-gated structure at 1.4 K with a channel width of 25 µm. It is relatively high compared with other metal GCS transistors [2]. The <i>I_c</i>is completely suppressed and a phase transition from the superconducting to the normal state is observed at <i>V</i>_GS around −20 V. But the suppression does not occur at positive <i>V</i>_GS above +20V. This asymmetric nature is different from the symmetric gate control of metal GCS transistor. The <i>I_c</i> control by the gate has been observed up to 3 K. The top-gated structure enabled the suppression of <i>I_c</i> without using nanostructures such as nanobridges, which require precision miniaturization techniques. As <i>I</i>_c suppression is possible with field-emitted current of 1 nA at the same channel (25 µm in width), the mechanism may be heating the superconductor by the field-emitted electrons from the gate to the superconductor. However, many other mechanisms are also discussed in metal GCS transistors [2].