Low noise transistor microwave amplifiers


Diverse experiments depend on microwave signal processing technology, ranging from searching for the origin of fast radio bursts to investigating the nature of dark matter to the development of near-term quantum computers. While this technology is now relatively mature, present transistor amplifiers remain around a factor of 5 above the standard noise limit set by quantum mechanics. Although superconducting amplifiers operating at the quantum limit are available and in widespread use, they have a number of practical limitations that prohibit their use in many applications.


Our group is studying how to realize transistor microwave amplifiers with noise figure approaching the standard quantum limit, which would find transformative applications in radio telescopes, quantum computers, and other technologies. A particular focus is on understanding the physical origin of hot electron noise that is associated with the heating of the electron gas in the channel of a transistor. To address these and related questions, we work with collaborators to fabricate custom devices, which are then characterized in a custom cryogenic probe station to determine their microwave noise properties.


Microscopic image of a discrete GaAs HEMT in our cryogenic probe station that enables measurements of microwave noise and S parameters. We use this approach to determine the physical origin of microwave noise in the devices and how it may be mitigated by materials science considerations and new nanofabrication processes.

Measured noise temperature versus physical temperature (black line) and that predicted assuming the drain (output) noise temperature is fixed at the room temperature value. The measurements indicate the drain noise generator must exhibit a temperature dependence, constraining the physical origin of the drain noise.

Transport and fluctuation phenomena in semiconductors


Charge transport phenomena in semiconductors play a fundamental role in the noise performance of transistor microwave amplifiers. We develop and use ab-initio methods to understand the microscopic origin of transport and noise properties of semiconductors. These methods permit the prediction of these properties without any adjustable parameters and thereby provide a strict test of the theory of electron-phonon interactions as well as facilitate the study of novel materials that have not yet been synthesized. Present work focuses on elucidating the role of higher-order phonon scattering in semiconductors and exploring the charge transport properties of a class of emerging semiconductors, Boron-V compounds.


Drift velocity versus electric field and current power spectral density (PSD) versus electric field (left and right) for GaAs at 300 K. The various calculations are the typical one-phonon theory (dashed black line), one-phonon theory with electronic structure adjusted to match experiment (dash-dotted yellow line), and two-phonon scattering (solid blue line). Two-phonon scattering is required to explain the experimental results, indicating the important role of higher-order phonon scattering. Further, the calculated PSD remains in poor agreement with experiment, indicating other electron-phonon scattering processes are still missing from the theory.

Novel nanofabrication processes - atomic layer etching

Detectors and emerging quantum hardware rely on nanofabrication processes to transform a uniform substrate into a complex spatial architecture that endows it with its function. Despite the capabilities of these processes, it is now clear that the chemical and structural quality of the resulting structures are insufficient to meet future scientific needs. The core problem is the precision gap between material growth and removal: while methods exist to synthesize materials with near-perfect atomic quality, analogous methods to remove material do not. Typical etching methods such as wet etches or reactive ion etching leave up to nanometer-scale surface roughness and a poor quality surface region that extends tens of Angstroms into the bulk. Thus, the initial Angstrom-scale quality of the substrate is degraded by nearly two orders of magnitude, indicating a fundamental gap in precision between material growth and removal. This precision gap leads to a variety of deleterious effects that are now the primary limiting mechanism for diverse figures of merit for devices such as electromagnetic resonators.


Atomic layer etching (ALE) is a method that has potential to mitigate the nanofabrication precision gap. ALE can be roughly viewed as the inverse of atomic layer deposition in that it consists of two or more self-terminating reactions at the surface of a substrate. Unlike in ALD, though, ALE leads to the removal, rather than deposition, of a monolayer. In the simplest two-step process, the first step weakens the bonds at the top monolayer of  material, for instance by fluorination. The second step  serves to remove this modified surface compound and can be accomplished by various schemes. In directional ALE, ions in a low energy plasma are directed to the surface, dislodging primarily the modified monolayer. In isotropic ALE, another reactant volatilizes the modified surface which is subsequently purged.


Schematic of an ALE process.  The surface is fluorinated with an SF6 plasma, and the resulting modified surface products are removed by ligand exchange.


EDS image from cross-sectional TEM samples of an Al film (left) and ALE/ALD treated  Al. In the processed sample, the native oxide has been removed and replaced with AlF3.

Quantum simulation


Quantum computers based on platforms like superconducting qubits are now a topic of intense interest. In the near-term, the most promising prospects for their application is in quantum simulation, where they may enable the study of highly entangled quantum systems that are out of reach of classical computers. Our group is exploring the capabilities of these near-term quantum computers. A particular interest is the use of mid-circuit measurements as a resource for quantum simulation. We have recently exploited advances in mid-circuit measurement capability to obtain the first observation of a measurement-induced quantum phase transition on superconducting quantum devices.


Schematic of the hybrid random quantum circuit used to study measurement-induced entanglement phase transitions. Each time step consists of two layers (dark blue and light pink) of randomized 2-qubit gates in a brickwork pattern and a layer of measurements randomly placed on each qubit with probability p. The measurements can be projective, or weak with strength, achieved through coupling with an ancillary qubit. Each 2-qubit gate comprises random single-qubit rotations and a randomly-directed CX.


Von Neumann entanglement entropies versus measurement rate rescaled using a scale-invariant function. The measured entanglement entropies collapse onto a single curve, highlighting the critical nature of the entanglement phase transition.