MRS Spring Meeting 2024

Magnetic Field-Effect Transistors based on The Topological Semimetal NbP


In the ever-evolving landscape of electronics and semiconductor technology, the quest for faster and more efficient transistors has driven researchers to explore unconventional materials and device architectures. Topological semimetals, like Weyl semimetals (WSMs), are promising candidates for redefining transistor technology in this context$^1$. Their unique transport properties, such as linear electronic bands, chiral fermions, and topologically protected surface states, can enable innovative device architectures, potentially outperforming standard complementary metal-oxide-semiconductor (CMOS) technology. Linear energy dispersion, in particular, yields low carrier mass and ultra-high mobility, which translates to the extreme magnetoresistance, up to $10^6$ %, exhibited by these materials, such as $Cd_3As_2$, NbP, $WP_2$, and many others$^2$. In this work, we demonstrate a new type of transistor that operates through a magnetoresistive coupling between a WSM and a superconductor. The active material is a topological WSM, NbP, whose resistivity is modulated via a magnetic field generated by the nearby superconductor. By using magnetic fields as a control quantity, the performance of the realized device is not reliant on ultra-thin channels, such as is the case for semiconductor electric-field devices. More importantly, this type of device allows to modulate the resistivity of a semi-metallic channel material with extremely low resistivity, ρ, and high carrier density (up to $10^{20} cm^{-3}$), which is also not possible in a traditional semiconductor field-effect transistor. A full characterization of the NbP crystallite, performed with an externally generated magnetic field (up to 9 T), reveals very high values of MR, consistent with the ones reported in the literature. The fit of quantum oscillation data according to the Lifshitz–Kosevich formula allows the extraction of the main transport-related quantities, such as electron mobility, carrier density, and effective mass. The superconductive gate is made of NbN, and it is implemented to eliminate the contribution of self-heating, caused by the power dissipated in the gate electrode, to the resistivity modulation. The SC transition of the NbN gate occurs in a temperature range of 13.1 to 13.4 K, above the operating point of the device (5 K). The device characterization was done superposing the locally generated field with an external field offset which is able to shift the operating point of the device towards a region where the MR coupling, dρ/dB, is maximized. Furthermore, we were able to accurately reproduce the resistivity variation of the external magnetic field with the local magnetic gate using point-wise measurements. Due to the exceptionally large electron mobility of this material, which reaches over $10^6 cm^2$/Vs, and the strong magnetoresistive coupling, we show that the realized magnetic Weyl transistor operates with very high transconductance gain at nanowatt levels of power dissipation. Transconductance gain, together with overall low parasitic RC delay indicates the potential for improvements over standard cryogenic amplifier technologies. We foresee qubit readout signal amplification as a highly attractive area of application for this device due to the increasing need for low-power cryogenic amplifiers, operating at power budgets that will continue to shrink as quantum computers advance. Moreover, our device presents a generalized scheme to control the transport properties of Weyl semimetals, many of which are sensitive to magnetic fields. Overall, the obtained results indicate a promising path forward for integrated Weyl semimetal electronics that can leverage the often extreme transport properties of such materials.