Josephson Field Effect Transistor Research Articles

Density-dependent critical currents in quantum-well-coupled weak links

We investigate a Josephson field-effect transistor in which the electron gas in an InAs quantum well serves as the weak link between superconducting Nb electrodes. We modulate the density Ns of electrons and the critical current Ic in the weak link by applying a voltage to an insulated gate. Measurements of the dependence of Ic on Ns are in good agreement with a model in which each occupied subband of the InAs quantum well makes an independent contribution to Ic. Inclusion of the nonparabolic band structure in InAs is crucial to this agreement.

Spectroscopic measurements on the Andreev reflection probability as a function of temperature

The temperature dependence of the Andreev reflection coefficient A(E,T) at a superconductor/normal-metal interface is a key issue for the critical current in a Josephson field-effect transistor at finite temperature. In this letter, we discuss our experimental observations of A(E,T) as a function of temperature determined by point contact spectroscopy. In addition, we point out major discrepancies between our findings and predictions from different theoretical models.

Transport properties of Nb/InAs(2DEG)/Nb Josephson field-effect transistors

We investigate transport properties of mesoscopic semiconductor-superconductor weak links. The superconducting Nb electrodes of our junctions are coupled by the two-dimensional electron gas of an InAs heterostructure grown on a GaAs substrate. We report on the properties of Josephson field-effect transistors utilizing these junctions.

Controlling Josephson transport by manipulation of Andreev levels in ballistic mesoscopic junctions

We discuss how to control dc Josephson current by influencing the structure and nonequilibrium population of Andreev levels via external electrostatic gates, current injection and electromagnetic radiation. In particular we will consider the ‘giant’ Josephson critical current in ‘long’ SIS tunnel junctions and the regular and anomalous nonequilibrium Josepson currents in three terminal SNS junctions. We will briefly discuss applications to the Josephson field effect transistor (JOFET) and to the newly invented Josephson interference transistor (JOINT).

On the nature of the electric-field effect on YBa2Cu3O7−δ grain boundary junctions employing epitaxial SrTiO3 gate insulators

We have fabricated Josephson field-effect transistors based on YBa2Cu3O7−δ bicrystal grain-boundary junctions (GBJs) and epitaxial SrTiO3 films as gate insulators. The SrTiO3 gate insulator shows high products of the breakdown field Ebd and the dielectric constant εr up to Ebdεr=1.3×1010 V/m allowing measurements over a wide range of applied gate electric-field Eg. The critical current Ic of the GBJs is found to depend highly nonlinear on Eg. Remarkably, the measured Ic(Eg) are very similar to the εr(Eg) curves. This strongly suggests that the observed electric-field effect is not due to a field-induced change in carrier concentration but is related to the dielectric properties of the SrTiO3 gate insulator.

Properties of doped-YBCO bicrystal grain-boundary junctions for Josephson field effect transistor

We report the properties of hole-overdoped and underdoped YBa2Cu3O7−x (YBCO) grain-boundary junctions (GBJ) on the 24° tilt SrTiO3 (100) bicrystals as a function of temperature and junction width. Epitaxial thin films of YBCO, Y0.7Ca0.3Ba2Cu3O7−y (Co-YBCO) were deposited by pulsed laser deposition, and the GBJs were fabricated by standard photolithography with argon ion-milling. We measured the IcRn products of ∼ 1.2 mV for Ca-YBCO and ∼ 0.3 mV for Co-YBCO GBJ at 27 K. Critical current densities(Jc) of the junctions were 1.4×103 and 3.0x104 A/cm2 at 27 K for the Ca- and Co-YBCO GBJs, respectively. This is explained in terms of a weak-link behavior at the grain-boundaries in comparison with the bulk properties of the doped YBCO films.

Ginzburg–Landau theory of Josephson field effect transistors

A theoretical model of high-Tc Josephson field effect transistors (JoFETs) based on a Ginzburg–Landau free energy expression whose parameters are field- and spatially-dependent is developed. This model is used to explain experimental data on JoFETs made by the hole-overdoped Ca-SBCO bicrystal junctions (three terminal devices). The measurements showed a large modulation of the critical current as a function of the applied voltage due to charge modulation in the bicrystal junction. The experimental data agree with the solutions of the theoretical model. This provides an explanation of the large field effect, based on the strong suppression of the carrier density near the grain boundary junction in the absence of applied field, and the subsequent modulation of the density by the field.

Superconducting transistors using InAs-inserted-channel InAlAs/InGaAs inverted HEMTs

A newly fabricated Josephson field effect transistor (JOFET) is coupled with a two-dimensional electron gas (2DEG) in a strained InAs quantum well inserted into an inverted modulation-doped structure with an HEMT-type gate. We indicate the improved characteristics of the JOFET with the HEMT-type gate, instead of the MIS-type gate. The superconducting critical current as well as the junction's normal resistance can be completely controlled via a gate voltage of about -1 V; this provides voltage gain over unity, the first time for a JOFET.

A Josephson field effect transistor using an InAs-inserted-channel In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure

A Josephson field effect transistor (JOFET) was coupled with a two-dimensional electron gas in a strained InAs quantum well inserted into an In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure. The characteristics of this JOFET are much improved over previous devices by using a high electron mobility transistor (HEMT)-type gate instead of the usual metal-insulator- semiconductor (MIS)-type gate. The superconducting critical current as well as the junction normal resistance are completely controlled via a gate voltage of about −1 V; this provides voltage gain over 1 for a JOFET.

Submicron Gate-Fitted Superconducting Junction Using a Two-Dimensional Electron Gas

We fabricated submicron-gate superconducting junctions which are coupled with an InAs channel inserted in an InAlAs/InGaAs heterostructure. Electron beam lithography, and chemical and rf sputter etching techniques are used to fabricate a junction in the submicron range. The fabrication process of the junction is described in detail. The fabricated gate configuration shows high controllability of both the superconducting critical current and normal resistance of the junction using gate voltage. This provides a voltage gain of over 1 and enables the first demonstration as a Josephson field effect transistor. Moreover, new quantum phenomena, e.g., Fabry-Pérot interference and quantization of critical current as well as focusing of Andreev-reflected holes in a quantum point contact, were observed. Junction characteristics from the viewpoints of both three-terminal operation and the new quantum phenomena are reported.

Critical currents and supercurrent oscillations in Josephson field-effect transistors

The dc Josephson effect of a superconductor/two-dimensional electron gas/superconductor (S/2DEG/S junction in the clean limit is investigated with emphasis on the field-effect dependence of the critical current. Calculation of the Josephson current is based on solving the Bogoliubov--de Gennes equations for a steplike variation of the pair potential. In the normal conducting region, the motion of electrons is quantized in one direction by means of a triangular-well potential. The 2DEG is contained in a semiconductor treated within the effective-mass approximation. We assume an abrupt band-edge jump at the interfaces, and additional scattering is taken into account by \ensuremath{\delta}-function potential barriers. Normal scattering leads to the formation of resonant states, which are visible in critical current oscillations. The ratio of Fermi velocities in the 2DEG and the S regions determines the rates of Andreev and normal scattering and has a large influence on the field-effect dependence of the critical current. We take an effective mass suitable for the inversion layer on p-type InAs and consider Nb for the superconducting contacts. Typical magnitudes of the calculated critical current are some \ensuremath{\mu}A-per-\ensuremath{\mu}m junction width for experimentally accessible values of junction length, temperature, and surface carrier density of the 2DEG.

Noise properties of SQUIDs with Superconductor-Semiconductor-Superconductor proximity effect Josephson junctions

SQUIDs using Superconductor-Semiconductor-Superconductor (SSmS) proximity effect Josephson junctions were prepared and noise measurements were carried out. Since SSmS junctions are basic elements of Josephson field effect transistors (JoFETs), information about dynamic properties of JoFETs can be gained in this way. A planar geometry was used for the SSmS junctions, with a single crystalline silicon wafer acting as both, substrate and proximity layer. Rf- and dc-SQUIDs could be realized. When the SQUIDs were operated in a flux locked loop, flux noise values comparable to conventional tunnel junction SQUIDs were measured.

Critical currents of semiconductor-coupled Josephson weak links

Critical currents of semiconductor-coupled Josephson weak links, including Josephson field effect transistors, are discussed with emphasis on experimental results. Phenomenological expressions are used illuminate outstanding issues. Recent experiments are described which could lead to improved understanding of the superconductor-semiconductor interfaces of key importance in these devices.

Prospects for proximity effect superconducting FETs

The authors address the question of how a JOFET (Josephson field effect transistor) works and whether or not a useful device of this type is feasible. They show that JOFETs represent a limiting case of normal FETs in which superconducting source and drain contacts result in zero channel resistance. This results in improved device characteristics, but does not have a major effect on speed or power dissipation. With conventional superconductors, the input and output voltage scales are incompatible, but the authors describe the extent to which this can be overcome by using high-temperature superconductors, making possible voltage gain exceeding unity. Device length and operating temperature are constrained by the natural length scale for the penetration of superconductivity into normal materials, so that submicrometer device dimensions and operating temperatures less than approximately 77 K are required. Silicon is effectively ruled out in favor of III-V materials for device operation above approximately 20 K.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Feasibility of hybrid Josephson field effect transistors

We consider the feasibility of fabricating planar superconductor-semiconductor-superconductor Josephson junctions in which the junction supercurrent is controlled by a gate electrode isolated from the junction by either a dielectric film (MOS-JOFET) or a Schottky barrier (MES-JOFET). We find that device critical currents between ∼1 and 100 μA and critical temperatures approximately a few K appear possible. We discuss the circuit applications of such devices.