Terahertz Detection using on chip Patch and Dipole Antenna-Coupled GaAs High Electron Mobility Transistors l l l 2 12 1 Shamsun Nahar , Alexey Gutin , Andrey Muraviev , Ingrid Wilke , Michael Shur , and Mona M. Rella 1 Department
of Electrical, Computer and Systems Engineering,
2Department of Physics, Applied Physics & Astronomy Rensselaer Polytechnic Institute, Troy, NY 12180 Email: nahars,
[email protected]
Abstract-This paper presents high responsivity plasmonic
E
terahertz (THz) power detectors operating at 0.3 THz. The detectors are implemented using 130 nm depletion mode In GaAs/GaAs pseudomorphic High Electron Mobility Transistors
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(pHEMT) with on chip patch and dipole antennas connected to the gate terminal. The measured absolute responsivity at room temperature is 7V
/W,
while the normalized responsivity with
respect to the THz beam and physical antenna area is 8kV
/W,
with a noise equivalent power (NEP) of 9.1
The
pW/ j(H z).
paper also addresses the bias dependency of the signal to noise ratio (SNR), THz detector input impedance and the matching requirement for the connection between the antenna and input to the gate of the THz detector.
Index Terms-Plasmonic THz detector, GaAs technology, patch antenna, dipole antenna, THz imaging.
I. INTRODUCTION
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Fig. 1: THz detectors with on chip (a) patch (b) dipole antenna. Device dimensions are given in J-lm.
Terahertz and sub millimeter-wave imaging have emerged into an active research area with exciting applications in
II. FABRICATED DETECTORS
security, industrial quality control and biomedical imaging. Various architectures based on coherent and non-coherent
The THz detectors are fabricated using Triquint TQPI3-n
detection have been proposed targeting increased sensitivity
InGaAs/GaAs pHEMT process with the active device con
and noise performance. Unlike conventional radio receiver
nected to an on chip patch/dipole antenna at the gate terminal
architecture, plasmonic THz detection is not limited by the
for efficient coupling of THz radiations. A single device with
cut-off frequency of the used field effect transistors (FET) and
a gate length and width of 130 nm and 10 f-Lm respectively is
thus can be considered as a low power, low form factor, and
connected to the single ended patch antenna feed line whereas
fully integrable alternative in THz imaging and communication
two identical devices are connected to a differential dipole
systems.
antenna. The gate of the active device is also connected to
Dyakonov and Shur explained the operation of plasmonic
a bias pad via a high resistance line to minimize the THz
detectors using the shallow water analogy to the two dimen
radiation coupling through the pads. The pads are wirebonded
sional 2-D electron fluid in the FET channel [1]. A complete
to side-brazed Dual In-Line (SBDIP) ceramic packages for
analytical expression valid in all regions of FET operation
external bias connections. The device/antenna dimensions are
(sub-threshold, linear, saturation) has been proposed in [2].
illustrated in Fig. 1 for the patch and dipole antennas.
Several approaches including FET biased at saturation, on chip
The on chip antennas have been realized by merging the
antenna ([3], [4]), integrated lens, the use of on-chip voltage
two metal layers available in the Triquint TQP13-n process
amplifiers both in silicon and heterojunction technologies
(one local and one global) into one thick metal layer. The
have been proposed for enhanced responsivity over a wide
GaAs substrate of 85 f-Lm height is used as the dielectric
frequency range covering 0.2 THz to tens of THz. In this
medium and the back metal as the ground plane. The antenna
paper, we present the design and performance summary of
structures are simulated in 3D EM simulator HFSS. Antenna
THz detectors based on on-chip patch and dipole antennas
simulation results show ",(295
coupled InGaAs/GaAs HEMT.
GHz
The paper is organized as follows: section II describes the
- 305
GHz)
GHz
- 304
GHz)
and ",(291
bandwidth for patch and dipole antennas
respectively as shown in Fig. 2. The simulated directivity for
detector and antenna design, the measurement setup, results
the patch antenna is 3.8 and 3.2 dB for the dipole antenna.
and corresponding analysis are included in section III and
To assess the antenna effect, the detectors are fabricated with
conclusions are drawn in section IV.
and without antenna using the same bias pad connections. The
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-- Dipole Antenna ,-" ........... ," "
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Reference Signal from chopper Wire Wire Grid Grid Polarizer Polarizer
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Fig. 2: Simulated return loss of the on chip patch and dipole antennas showing a bandwidth of 9 & 14 GHz respectively. The bandwidth is defined as the frequency range where the return loss < -10 dB while interfacing 50 0 load.
measured open drain voltage response of the detector relative to the applied frequency is given in Fig. 3, showing an order of magnitude improvement in the detector response with the use of the patch/dipole antenna. III. MEA SUREMENT RESULTS The detectors have been characterized using a single arm of a THz Mach-Zehnder interferometer from Microtech in struments, Inc. The measurement setup is shown in Fig. 4. The setup employs a Backward Wave Oscillator (BWO) (QS1370, tunable from 218 to 376 GHz) as THz radiation source. The THz beam is collimated by a polished Tsurupica lens with diameter and focal length of 50 and 60 mm respectively. The beam is modulated by an optical chopper from Scitech Instruments (300D2), it passes through a wire grid polarizer which is set to 100 % transmission and two other Tsurupica lenses of same diameters but with focal lengths of 120 mm. Subsequently, the THz beam is reflected from a metal mirror to a wire grid polarizer set to 100 % reflection. The beam is focused on the detector with a 120 nun focal length lens. The final polarizer in the beam path is used to align the electric field polarization with the antenna for maximum response. The packaged detectors are mounted on an XYZ translation stage, which is used to align the detector in the beam focal point. The detector response is measured using a lock-in amplifier (Stanford Research Systems SR-830) with an input impedance of 10 MO, at 150 Hz of chopper frequency.
70 �----� 60 - With Dipole antenna - Without Antenna :> 50 E -;40
