How to achieve a homogeneous
sensitivity in
THz detector arrays
M. Sakhno, J. Gumenjuk-Sichevska, F. Sizov
Institute of Semiconductor Physics NASU,
Kiev, Ukraine,
e-mail: sakhno.m@gmail.com
THz CMOS FPA principle
Antenna
FET
• Advantages of Si FET THz Detectors
Based on standard silicon technology with high level of integration
Un-cooled
Can be assembled into arrays for real time THz/mm wave imaging;
Mechanically robust;
Low costs at high volumes
2
Detector characterization
NEP – noise equivalent power. Minimal power which can be
detected by detector
'
Goal
S
NEPmeas  NEPel
2
1. Uniform NEP for different

G A
elements of the array
4
2. Minimal NEP
• NEPel electrical NEP of detector
itself
• G the antenna gain
1. Maximal G and ηa
• ηa matching between the
2. Uniform G and ηa
antenna and the detector
3
System photograph (silicon FET array
implementation)
Printed antennas on finite electrically thick substrate
Modelled system
4
System parameters
1 mm
1mm
10 mm
a1 ,μm
10
a2 ,μ m
75.8
r,μm
164
d ,μm
20
φ,deg
104
The modeled system design: 8 antennas on a substrate of finite
size. Antennas are positioned symmetrically relative to the
substrate center
Modeling using EMSS FEKO
5
Cut-off frequency of the first mode fc1 for
infinite substrate
f c1 
c
4h  r  1
f  f c1
,
Pozar, D.: Considerations for millimeter wave printed antennas. IEEE
Trans. Antennas Propag. 31, 740–747 (1983)
hcritical  0.25diel
h, µm
εr=2
εr=7
εr =12
50
1.5THz
0.612 THz
0.452 THz
140
0.536 THz
0.219 THz
0.162 THz
650
0.116 THz
0.047 THz
0.035 THz
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Linear gain diagram for substrate thickness h=50 μm,
f=300GHz
f c1  f
r  2
f c1  f
r  7
f c1  f
 r  12
h
diel  0.071
h
diel  0.132
h
diel  0.173
Each antenna was simulated and the results were combined on one picture to
facilitate the comparison of different elements
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Linear gain diagram for substrate thickness h=140 μm,
f=300GHz
f c1  f
r  2
f c1  f
r  7
f c1  f  r  12
h
diel  0.197
d
diel  0.37
d
diel  0.484
8
Linear gain diagram for substrate thickness h=650 μm,
f=300GHz
f c1  f
f c1  f
f c1  f
r  2
h
diel  0.920
h
diel  1.72
r  7
 r  12
h
diel  2.25
9
Antenna pattern for different substrate
relative permittivities

Substrate
thickness is
h=140 μm
f c1  f
f c1  f
10
Dependence of the calculated total antenna gain G
in the normal direction on the substrate
permittivity
2
1
5
4
3
7
6
8
G1
G2
G3
G4
10
5
G, dBi
0
-5
-10
-15
-20
-25
2
4
6
r
8
10
12
11
Calculated gain for normal direction for 1st
th
and 4 elements1 2 3 4 5 6 7 8
10
5
0
G, dBi
-5
-10
-15
1, =2
4, =2
1, =12
4, =12
-20
-25
-30
280
290
300
f, GHz
310
320
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Antenna – transistor matching
Antenna
FET
1-μm Si MOSFET
W/L = 20/2 (μm)
RG = 150 Ω,
RS = 50 Ω,
Cp= 4 fF
Ztr= (200 – j130) Ω
at f = 300 GHz
P
Re Z ant Re Z tr
a 
4
2
Pmax
Z tr  Z ant
1
Z tr  RS  RG 
Z GS ,int
jC p
Sakhno, M., Golenkov, A., & Sizov, F. (2013). Uncooled detector
challenges: Millimeter-wave and terahertz long channel field
effect transistor and Schottky barrier diode detectors. Journal
of Applied Physics, 114(16), 164503. doi:10.1063/1.4826364
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Antenna-detector matching for different
substrate thickness
1
2
3
4
5
6
7
8
• Optimal matching is not for
electrically thinnest substrate
• Matching coefficient variation is
less than gain variation
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System with the lens
hl
4
3
2
1
al
Angle for maximum gain
25
1
Modelled points
Linear fit
20
2
15
3
10
4
5
0
1
2
3
4
Element number
The angle of maximum gain versus the element position for the system with the lens (only the first four elements are
shown because of the mirror symmetry). The substrate parameters are h=50 μm, r=2, the incident radiation
frequency is 300 GHz
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Conclusions
• The substrate electric thickness in THz FPAs plays a crucial role in the
frequency characteristics of the system
• Electrically thick substrate makes NEP of elements non-uniform
• Degradation of antenna pattern can be explained by excitation of
substrate modes.
• Critical substrate thickness is approximately 0.25 wavelength in
dielectric
• Simulation shows that Si CMOS system (substrate thickness h = 50μm
and εr = 2) with the lens can operate as FPA
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Acknowledgements
• This work is partly supported by the SPS:NUKR.SFP 984544 Project
and a joint grant 01-02-2012 from the National Academy of Sciences
of Ukraine and Russian Academy of Sciences.
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Thank You !
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