Adam Rämer

Terahertz rectification by plasmons and hot carriers in gated 2D electron gases

Here we discuss on detection using antenna-coupled field effect transistors. We show, that gate-to-channel separation plays strong role for the dispersion of plasmons excited within the channel changing from gated 2D to ungated 2D plasmon. This change also strongly affects the impedance and the efficiency of rectification. We present experimental data which clearly indicates on with plasmonic rectification competing physical phenomenon. There are strong indications, that this additional signal originates from the inhomogeneous heating of charge carriers i.e., diffusion of “warm” electrons.

Efficient detection of short-pulse THz radiation with field effect transistors

Here we report on the efficient detection of THz radiation generated from Ti:sapphire femtosecond laser system. The large-area interdigitated antenna is excited by optical pulses with pulse duration from 15 to 47 fs. We have tested two AlGaN/GaN HEMT-based THz detectors with integrated broadband antennas. Detectors are comparable in sensitivity but differ in applied parasitic signals elimination techniques. Despite being sensitive, the pick-up of the first detector without strong parasitic elimination exceeded the theoretically expected noise level about 100 times. For a second detector we have implemented more stringent interference avoiding techniques, such as gate shunting with a low resistance value resistor, and have managed to significantly improve systems detection capabilities. We have lowered a pick-up near to the thermal noise level and have extended signal/noise ratio to 26 dB.

Thermal noise-limited sensitivity of FET-based terahertz detectors

Here we present a detailed study on estimation of noise-dependent parameters, such as signal-to-noise ratio (SNR) or noise equivalent power (NEP), of field-effect-transistor based terahertz detectors (TeraFETs). Commonly, these parameters are estimated from a well-known assumption, that detectors performance is limited by the thermal noise of transistors channel. However, practice shows that the influence of other noise sources or transient effects is considerable. We summarize TeraFET noise measurements performed on different material systems based transistors, such as AlGaN/GaN, AlGaAs/GaAs, silicon CMOS, and monolayer graphene. We have achieved a good agreement between thermal noise and measured data. However, attention has to be paid to gate leakage currents and slow defect charging and discharging effects, which can strongly influence TeraFETs performance estimation.

Optimization of the Design of Terahertz Detectors Based on Si CMOS and AlGaN/GaN Field-Effect Transistors

TeraFETs are THz power detectors based on field-effect transistors (FETs) integrated with antennas. The first part of this paper discusses the design of Si CMOS TeraFETs leading to an optimized noise-equivalent power close to the room-temperature limit. The impact of the choice of the gate width and gate length, the role of the parasitic effects associated with the technology node, and the conjugate matching of antenna and FET impedance – which is possible over narrow THz frequency bands because of the frequency dependence of the channel impedance resulting from plasmonic effects – are highlighted. Taking these aspects into account, we implement narrow-band detectors of two different designs. Using a 90-nm and a 65-nm CMOS technology, we reach a room-temperature cross-sectional NEP of 10 pW/√Hz at 0.63 THz. We then explore the optimization of AlGaN/GaN TeraFETs equipped with broadband antennae. A room-temperature optical NEP of 26 pW/√Hz is achieved around 0.5 THz despite the fact that the existence of pr...