M. Sakowicz

Broadband terahertz imaging with highly sensitive silicon CMOS detectors

This paper investigates terahertz detectors fabricated in a low-cost 130 nm silicon CMOS technology. We show that the detectors consisting of a nMOS field effect transistor as rectifying element and an integrated bow-tie coupling antenna achieve a record responsivity above 5 kV/W and a noise equivalent power below 10 pW/Hz(0.5) in the important atmospheric window around 300 GHz and at room temperature. We demonstrate furthermore that the same detectors are efficient for imaging in a very wide frequency range from ~0.27 THz up to 1.05 THz. These results pave the way towards high sensitivity focal plane arrays in silicon for terahertz imaging.

Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications

Resonant frequencies of the two-dimensional plasma in FETs increase with the reduction of the channel dimensions and can reach the THz range for sub-micron gate lengths. Nonlinear properties of the electron plasma in the transistor channel can be used for the detection and mixing of THz frequencies. At cryogenic temperatures resonant and gate voltage tunable detection related to plasma waves resonances is observed. At room temperature, when plasma oscillations are overdamped, the FET can operate as an efficient broadband THz detector. We present the main theoretical and experimental results on THz detection by FETs in the context of their possible application for THz imaging.

Terahertz responsivity of field effect transistors versus their static channel conductivity and loading effects

We study the broadband photovoltaic response of field effect transistors on terahertz radiation. A simple physical analytical model of the response is developed. It is based on plasma density perturbation in the transistor channel by the incoming terahertz radiation. The model shows how the non-resonant detection signal is related to static (dc) transistor characteristics. We analyze loading effects related to capacitive, inductive, and resistive coupling of the detector to the read-out circuit as a function of modulation frequencies and loading resistors. As we show, the proposed physical model completed by loading effects fully describes the experimental results on the non-resonant sub-terahertz detection by all studied III-V (GaAs, GaN) and silicon based transistors. Field effect transistors were recently proposed as the best terahertz detecting pixels for fabrication of low cost focal plane arrays for terahertz imaging. This article gives prospects for electrical simulation of these transistors and th...

A broadband THz imager in a low-cost CMOS technology

Terahertz (THz) technology has become of large interest over the last 10 years. THz rays are an alternative to X-rays for imaging through thin materials and their non-ionizing character makes them inherently health-safe. The THz domain is also suitable for heterodyne detection and the use of radar techniques to perform 3D imaging. Commercial applications range from non-destructive testing, security screening of objects or persons, and medical imaging to secure communications.

Polarization sensitive detection of 100 GHz radiation by high mobility field-effect transistors

Detection of 100 GHz electromagnetic radiation by a GaAs/AlGaAs high electron mobility field-effect transistor was investigated at 300 K as a function of the angle α between the direction of linear polarization of the radiation and the symmetry axis of the transistor. The angular dependence of the detected signal was found to be A0 cos2(α−α0)+C with A0, α0, and C dependent on the electrical polarization of the transistor gate. This dependence is interpreted as due to excitation of two crossed phase-shifted oscillators. A response of the transistor chip (including bonding wires and the substrate) to 100 GHz radiation was numerically simulated. Results of calculations confirmed experimentally observed dependencies and showed that the two oscillators result from an interplay of 100 GHz currents defined by the transistor impedance together with bonding wires and substrate related modes.

Terahertz radiation detection by field effect transistor in magnetic field

We report on terahertz radiation detection with InGaAs/InAlAs field effect transistors in quantizing magnetic field. The photovoltaic detection signal was investigated as a function of the gate voltage and magnetic field. Oscillations analogous to Shubnikov–de Haas oscillations as well as their strong enhancement at the cyclotron resonance were observed. The results are quantitatively described by a recent theory, showing that the detection is due to rectification of the terahertz radiation by plasma waves related nonlinearities in the gated part of the channel.

Quantum and transport lifetimes of two-dimensional electrons gas in AlGaN/GaN heterostructures

The transport and quantum lifetimes were respectively deduced from low-temperature mobility and Shubnikov–de Haas measurements as a function of carrier density in metal organic vapor phase epitaxy-grown AlGaN∕GaN∕sapphire heterostructures. We show experimentally that the lifetime ratio varies as a bell curve, qualitatively confirming a recent theoretical prediction. However the experimental ratio varied much less than was theoretically predicted: From 9 to 19 for carrier densities in 1–9×1012cm−2 range. Moreover, we show the variation of quantum time with carrier density presents some discrepancy with the theoretical study. We also show that transport to quantum lifetime ratio cannot be used alone as a clear figure of merit from AlGaN∕GaN heterojunctions.

Imaging above 1 THz limit with Si-MOSFET detectors

We demonstrate that a proper antenna and transistor design can provide high responsivity for Terahertz radiation and imaging capability even above the 1 THz limit with a low-cost 130 nm CMOS technology. This result opens the way to CMOS THz imagers working at high frequencies and therefore exhibiting high a spatial resolution — down to ~300 µm.

Terahertz detection by two dimensional plasma field effect transistors in quantizing magnetic fields

Detection of terahertz radiation by two dimensional electron plasma in high electron mobility GaAs∕GaAlAs transistors was investigated at cryogenic temperatures in quantizing magnetic fields. Shubnikov–de Haas oscillations of the detection signal were observed. A double (optical and electrical) modulation technique applied allowed us to study the influence of gated and ungated parts of the transistor channel on the detection. Our results provide a direct experimental evidence that both the gated and ungated plasma participate in the detection and clearly show the necessity to improve theoretical models that usually take into account only the gated part of the channel.

Low electron mobility of field-effect transistor determined by modulated magnetoresistance

Room temperature magnetotransport experiments were carried out on field-effect transistors in magnetic fields up to 10 T. It is shown that measurements of the transistor magnetoresistance and its first derivative with respect to the gate voltage allow the derivation of the electron mobility in the gated part of the transistor channel, while the access/contact resistances and the transistor gate length need not be known. We demonstrate the potential of this method using GaN and Si field-effect transistors and discuss its importance for mobility measurements in transistors with nanometer gate length.