1
Characterization of High-Electron-Mobility
Transistors as THz Detector for Communications
AIT AMAR Hafssa
IES, PhoThera, Univ Montpellier
Faculty of Science – Montpellier, France
Abstract— The wireless communications followed an increase
more and more fulgurating, the objective is always the same to the
transmition of the highest data rates and with great speed.
However more and more application use the wireless
communication, the radio spectrum is saturated it will not be able
to follow this evolution The solution thus will turn therefore
towards the exploitation of domain THz.
II.
THE TERAHERTS DOMAIN
The Terahertz domain represented in figure 1, is the domain
of electromagnetic waves whose spectral band is located
between the infrared and the microwaves, it extends from
frequencies of the order of a hundred GHz up to a few tens of
THz.
Nevertheless, the implementation of systems for the generation
and the detection of THz radiation is one of the main obstacles that
restrain the expansion of this domain.
The transistors HEMT are of particular interest as THz
detectors for communications due to their low output impedance
and low manufacturing cost.
Keywords—HEMT, the wireless communications, detector, THz
radiation.
I. INTRODUCTION
The wireless telecommunications sector is in permanent
evolution. however, the electromagnetic spectrum is saturated
and it will no longer be able to respond to this evolution. The
solution to this saturation of the spectrum is directed to the
sector of THz systems, and which represents a great interest for
the scientific community and the industry.
However, the absence of adequate sources and detectors in this
domain still one of the main obstacles that limits its expansion.
Schottky diode detection is the most used because of their high
sensitivity (a few kV/W) [1], but they are limited by their output
impedance which is very high compared to 50 Ohm of wireless
telecom transmission line. However, transistors HEMT
represent an interesting solution for the detection of THz waves,
in particular for THz communications. a study was initiated by
Dyakonov and Shur in the early 90s [2], demonstrate that a
transistor can function as a terahertz detector or source when
the gate length is nanometric in size.
This project therefore aims to study the characteristics of a
HEMT transistor which represents an interesting solution for
the detection of THz waves, in particular for the realization of
very high-speed wireless communications.
XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE
Figure 1:The terahertz domain within the electromagnetic spectrum
THz domain represents a great scientific interest justified by
the properties of these waves.
THz waves have a strong power to penetrate matter, where
visible and infrared do not reach, they are non-ionizing and
low-energy radiation (1 THz corresponds to a photon energy of
4.1 meV), this propriety allowing many new applications in the
security, biology, medicine and telecommunications sectors.
III. TERAHERTZ SOURCES AND DETECTORS
The fundamental elements of THz systems are the
sources and the detectors. Their main characteristics are their
output power, sensitivity, frequency and tunability.
The manufacture of sources and detectors adapted to the
THZ domain, still one of the main obstacles which limits his
technological evolution.
The THz domain is located at the
border between microwaves and optics and for this reason we
find optical, electronic or optoelectronic THz sources.
A. Sources:
The most used terahertz sources are divided into two
categories electronic and optical:
2

Frequency multiplication electronic sources:
The principle is based on the generation of harmonic
frequencies in a non-linear device. Several systems
use a varicap diode, which has the particularity of
behaving like a capacitor whose capacitance value
varies with the reverse voltage applied to its terminals.
These diodes are commonly used in frequency
multipliers, and make it possible to obtain signals up
to 2 THz [3].

Quantum Cascade Laser:
It is a semiconductor laser whose band is divided into
several sub-bands, The QCL is based on an optical
transition of an electron between quantized levels of
the conduction band, this transition of the carriers is at
the origin of THz radiation, occurs between these
energy levels.
heterojunction that is to say a junction between two different
types of semiconductor materials with different energy bands,
this to increase the mobility of electrons by reducing the
distribution of ionized impurities. [9].
We dispose a pHEMT type transistor, where "p" in pHEMT
means pseudomorphic, it differs from the HEMT in the
structure as shown in the figure, the pHEMT has a thin layer of
𝐼𝑛𝑋 𝐺𝑎1−𝑋 𝐴𝑠 deposited on the undoped GaAs layer.
As a consequence, the quantum well height is increased
without the need of too strongly doped layers by using a
semiconductor material with a lower band gap than GaAs, such
as InGaAs. This difference in crystallographic and physical
structure leads to specific phenomena such as increased
mobility.
In effect, thanks to the high mobility of the electrons, these
transistors allow an operation in high frequencies, which
candidates them as being THz detectors
B. Detectors:
Like sources, THz detectors still a great technological
challenge, because of the low output power levels of THz
sources. A good detector must be sufficiently sensitive,
capable of detecting the presence of THz radiation even in the
case of very low powers. The detection systems in THz
spectral ranges can be divided into two categories [5]:

incoherent detection systems (with direct detection
sensors), which allow only signal amplitude detection
and which, as a rule, are broadband detection systems,
 coherent detection systems, which allow detecting not
only the amplitude of the signal, but also its phase.
The most used terahertz detector is Schottky barrier diodes,
once component is considered to be the most effective solution
for detecting THz at room temperature. The physical principle
of Schottky barrier diodes is based on the passage of electrons
through a Schottky between metal-semiconductor.
Various studies have been done for years on Schottky diodes
that have undergone various improvements over time in order
to develop a wide band Schottky diode, with high sensitivity (a
few kV/W) [7]. However, this component has several
disadvantages especially a high manufacturing cost and a very
high output impedance [8].
For this purpose, theoretical studies made by Dyakonov and
Shur have shown that the transistor with high electron
mobility HEMT can also bring to detect THz radiations [9].
C. Transistor pHEMT:
The HEMT (High Electron Mobility Transistors) are
transistors from the family of field effect transistors. They have
a structure like the MESFET. Its principle is based on the
IV.
TERAHERTZ COMMUNICATION
The electromagnetic spectrum is saturated, and with the
increased capacity of wireless communication networks, it is
necessary to implement up systems capable of transmitting
data more and more quickly and at high speed.
Figure 2:structure et diagramme d’enrgie d’un pHEMT
It is provided by Shannon's theorem that the increase in capacity
first passes through an increase in the bandwidth B;
𝐶 = 𝐵 𝑙𝑜𝑔2 (1 +
𝑆
)
𝑁
where C is the capacity in bit/s, B the bandwidth in Hz, and S/N
the signal to noise ratio).
For this reason, the scientist envisioned The THz
communications are as a key technology for future generations.
because it offers the possibility of increasing the transmission of
speeds of the order of Tbits/s, however, the lack of components
adaptable for this frequency range still a real challenge
3
This project therefore aims to study the characteristics of a
HEMT transistor which represents an interesting solution for
the detection of THz waves, in particular for the realization of
very high-speed wireless communications.
𝑻𝑯𝒛, 𝝎𝑻𝑯
V. LE BANC DE TEST :
We begin to describe the experimental bench that will allow
us to characterize the transistor to the detection of THz waves.
Figure 4:THz detection principle by a HMET transistor
Figure 3: Experimental setup using a pHEMT detector
As shown in the figure, the system consists of two
synthesizers, the first delivers a carrier frequency signal OL of
13. 5 GHz with an amplitude of 10 dBm, multiplied by a factor
of 24 and the second delivers a signal of frequency IF fixed at
1 GHz and amplitude of -5 dBm, a mixer consists in mixing the
carrier signal of frequency 𝑓𝑜𝑙 with the modulating signal of
frequency 𝑓𝑖𝑓 .
The principle of detection with pHEMT transistors is
illustrated in the figure, is based on the properties of nonlinearity of the transistor, when a THz wave is applied to the
transistor, an AC voltage Uac is formed between the gate and
the source, this voltage is due to THz radiation, consequently a
DC voltage appears between the drain and the source ∆U and
proportional to the incident THz power, the voltage Vgs is used
to polarize the transistor
VI.
CARACTERISTIQUES DES TRANSISTORS MWT-LN300
We have at our disposal a transistor MWT-LN300 type
pHMET. the figure 4 presented the dimensions of the
component:
The modulated THz signal is emitted by the source and
collimated with the help of the first Teflon lens and then
focused towards the transistor with the help of the second lens
and observed on the electrical spectrum analyzer.
A. Principe de detéction :
We have shown that a HEMT transistor can work as an
electromagnetic radiation detector in the Terahertz band,
Figure 5:dimensions of the MTW-LN300 transistor
The chip is soldered on a 2 cm long and 1 cm wide wafer
with microstrip lines, terminated by a high frequency K-type
connector. The contacts of the transistor are connected to the
tracks of the wafer by bonding.
4
The carrier frequency is set at 300 GHz for a power of 10 dBm,
the modulation frequency varies from 1MHz to 10 GHz using
the automated measurement under LabView.
The signal detected by the MWT transistors is observed on the
electrical spectrum analyzer.
Figure 6:The transistor placed in the middle of the wafer is bonded to
the tracks.
We want to characterize the detection of the HEMT
according to different values of carrier frequency and
modulating.
In a first step we set the modulation frequency and using a
LabVIEW application we make an automatized measuring by
varying 𝑓𝑜𝑙 from 270 to 340 GHz and we read the detected
signal.
Figure 7: signal detected and measured by the spectrum analyzer. for
a sinusoidal amplitude modulation of 1 GHz.
we can see that the cut-off frequency of the transistor is about
2.8 GHz.
We observe that the detected signal decreases when the
frequency 𝑓𝐼𝐹 increases, so that the transistor works correctly
between 1MHz and 2GHz, beyond that we notice a dysfunction
of the transistor.
therefore, the transistor is able to detect modulation frequencies
up to 10 GHz.
VII. CHARACTERIZATION OF THE OUPUT
WITHOUT POLARIZING THE TRANSISTOR LN300.
Figure 7:Figure 6: shows the dependence of the signal detected by
the transistor on the carrier frequency, for a modulation frequency of
1GHz
The aim is to determine the output impedance of the
transistor Zs= R+jX with R the real part of Zs and X the
imaginary part, in order to evaluate the adaptation of the
pHMET.
The measurements were made for an OL power of 10 dBm,
and an IF power of - 5dBm, on the spectrum analyzer, we read
the signal detected at the output of the transistor.
As we can see that we have a maximum detection between 295
GHz and f = 305 GHz.
We measure the detected signal as a function of the
modulation frequency.
IMPEDANCE
A. Methodology :
5
The figure 8 shows the measurement methodology used
We started to calibrate with a calibration kit, then we connect
the drain of the transistor with the network analyzer to measure
the reflection coefficient in a frequency band from 60MHz to
10GHz and without biasing the transistor.
Figure 8 : methode de measure de coefficient de réflexion
The component is soldered on a board so it is nece²ssary to
take into account the line connecting the SMA and the
component. For that it is necessary to make the measurement of
the line alone and the impedance + line separately. The VNA
measurements obtained are treated on a MATLAB program
B. Results :
The results of the line correction for the measurement of
the output impedance of the transistor shown in figure.
Figure10: the variation of the output impedance as a
function of the modulation frequency
VIII. CONCLUSION:
The use of terahertz frequencies has been limited by the
complexity of developing adequate and sufficiently powerful
THz radiation sources, and on the other hand by the absence of
detectors sensitive to low power and low cost.
Figure 9: the line phase correction of the output
impedance of the transistor
We can observe from the results obtained presented on figure
10, a strong variation of the real part and the imaginary part
according to the frequency, which maladapts our system.
We observe that from a frequency of 2.8 GHz which
corresponds to the cut-off frequency observed on the
characteristic of the transistor figure 7, we also contact that
from the output impedance starts to increase
The work carried out in this project focused on the detector
part of the measurement setup, and more precisely the
characterization of a GaAs high electron mobility transistor
(HEMT) as a TH detector. We were able to demonstrate the
impact of the carrier frequency and modulation on the detected
signal and that the transistor is capable of detecting modulated
terahertz signals up to 10 GHz on the one hand, and on the other
hand the measurement of the output impedance showed us the
variation of the real and imaginary part of the impact as a
function of frequency impacting the adaptation of our system.
However, many improvements are applicable, in particular
the polarization of transistor, the data transmission with the help
of bench of measurement or to go far the characterization of a
transistor DHBT which represents a sufficient interest for the
detections THZ thanks to its sensitivity.
REFERENCES