Investigation of GaAs-Based Large Area FET for Direct Picosecond Pulse Detection using Heterodyne System

Mr. Rahul Yadav1,2, Mr. Florian Bek1, Dr. Fahd Rushd Faridi1, Dr. Sergey Kovalev3, Dr. Gulloo Lal Prajapati3, Dr. J. Michael Klopf3, Prof. Andreas Penirschke2, Prof. Sascha Preu1
1Terahertz Devices and Systems, Technical University of Darmstadt, Darmstadt, Germany. 2High Frequency Technology, Mittelhessen University of Applied Sciences, Friedberg (Hesse), Germany. 3Institute of Radiation Physics,Helmholtz-Zentrum Dresden Rossendorf, Dresden, Germany


GaAs based high electron mobility field effect transistors (HEMTs) are excellent detectors for beam alignment and synchronization of coherent picosecond (ps) scale Terahertz (THz) pulses generated at high power free electron laser (FEL) facilities. In this paper, we show measurement results of investigations performed to test detector limitations in the IF domain. We used a 1 kHz Ti:sapphire oscillator-amplifier laser system (Astrella, Coherent) to generate THz signals using the second order non-linear coefficient of a LiNbO3 crystal. A 110 GHz Keysight Infinium oscilloscope was used as IF post detection electronics. The LAFET used has dimensions of  0.3 mm × 0.3 mm with the AC capacitive source-gate coupling with a gate length of 1.5 µm and a channel length of 4.5 µm.  Fig. 1 (a) shows the DC transconductance of the detector. As the optimum responsivity is in the vicinity of the transconductance peak we determine the operational point for detector at a UGS bias of  – 0.3 V. Fig. 1 (b) shows the recorded time trace. The ringing comes from the impedance mismatch between the active device and post detection circuitry. The FWHM of ~ 40 ps is observed in Fig.1 (c), which is equivalent to ~ 25 GHz bandwidth in frequency domain. Fig. 1 (d) shows fourier transform of the measured data. The IF bandwidth is limited by the IF circuit, bonding techniques, RF connectors (SMA used in this work) and losses in cables along with the adapters. Therefore, we are working towards optimizing IF circuitry and detector in the ongoing project.