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INTERMEDIATE FREQUENCY CIRCUIT COMPONENTS FORINTEGRATION OF ON-CHIP AMPLIFIER WITH THz DETECTORS
R. Yadav 1,∗ , S. Preu, Terahertz Devices and Systems, TU Darmstadt, Darmstadt, Germany
A. Penirschke, High Frequency Tech., Mittelhessen Univ. of Appl. Sci., Friedberg (Hesse), Germany
1
also at High Frequency Tech., Mittelhessen Univ. of Appl. Sci., Friedberg (Hesse), Germany
Abstract
The demand for THz detectors for beam diagnosis andalignment at THz generating accelerator facilities increases
continuously especially for room temperature applications.
The Zero-Bias Schottky Diode (ZBSD) and field effect transistor (TeraFET) based Terahertz (THz) detectors are well
suited for both, signal power detection at DC as well as Pulse
shape diagnostics by down-conversion at intermediate frequencies (IF). The limited signal strength due to the roll-off
factor of the low pass filter characteristic of the detectors
at higher THz frequencies requires wide-band amplifiers toenhance the IF signal from a few µW to nW well above thenoise floor of the subsequent post detection electronics. Using external amplifiers would enhance the signal losses evenfurther due to additional connectors and rf-cable losses anddegrade the signal to noise ratio (SNR). In order to maximize
the SNR, we propose to have an on-chip amplifier integrated
in the detectors intermediate frequency (IF) circuit in thesame housing. In this work, we present the design and parametric analysis of components for transition to an IF circuit,which will be integrated in the ZBSD and TeraFET on chipwith amplifier. A rigorous design analysis has been done
to find the optimal parameters for wide-band operation inorder to enhance the detector’s resolution to capture pulses
in the pico-second range with the help of fast post detection
electronics.

INTRODUCTION
Research and development of materials and components
in the Terahertz (THz) domain has been done rigorously
since last couple of decades in order to explore this part ofelectromagnetic spectrum [1–3]. Due its excellent characteristics, the THz domain has numerous applications such as
in imaging, spectroscopy, quality control, security, medicalindustry, astronomy, communications, energy and matter research, beam diagnostics and alignment at accelerator facilities, etc. [3]. The increasing demand for using this frequency
spectrum has led the scientists to develop THz sources anddetectors in this operable range. Room-temperature THz
detectors developed in Refs. [4–6] are proved to be a cuttingedge technology for detector applications in THz domain.
The detector itself is composed of both active and passive
part. The active part consists of a semiconductor device suchas Field Effect Transistors (TeraFET), Zero-Bias Schottky
Diode (ZBSD), etc. The passive part is composed of all
passive components which are used for connecting active to∗ rahul.yadav@iem.thm.deRF connector and later on post-detection electronics. TheTeraFET or ZBSD rectifies the THz signal coupled through asilicon lens and is further down-converted from THz to millimeter frequency range [1]. The received signal is typicallyfeed to the post detection electronics by using the intermediate frequency circuit (IF). For the detection of pico-second
range pulses, it is necessary to increase the IF bandwidthcomponents in the IF circuit as well as impedance matchedcircuitry to harness the full potential of the detectors [7].
Considering the important requirement for the optimizationof the detector IF circuitry, in this paper we present the transition and ultra-broadband planar power divider (UBPPD)
components which will be used for integrating the amplifier
on-chip together with active devices in a same housing. The
transition structures and UBPPD are investigated using 3Delectromagnetic field simulation software (CST).

Figure 1: Block diagram of the THz detector.

TRANSMISSION LINESAS TRANSITION STRUCTURES
The block diagram of THz detector circuit is shown inFig. 1. The blocks inside the dotted box represents the intermediate frequency (IF) circuit. The THz signal incident
on the silicon lens is coupled to Zero-Bias Schottky Diode(ZBSD), which is mounted on the back side of the siliconlens (depicted as blue colour in Fig. 1.). A planar power
divider along with a broadband phase shifter will be used
to convert the rectified signal from single-ended to the differential input of the Trans-impedance amplifier (TIA). The
output of the IF circuit is fetched to the read-out electronics
via an RF connector (K-connector in this case as our target
is to optimize detector initially until 40 GHz).
Figure 2 shows the transmission lines from TeraFET orZBSD to the IF circuit. The dimensions of these structures are calculated according to Refs. [8, 9]. Four different
types of transmission lines such as coupled micro-strip lines(Fig. 2 (a)), co-planar wave-guide with only signal on top(Fig. 2 (b)), co-planar wave-guide (Fig. 2 (c)) and micro-strip
line (Fig. 2 (d)) is investigated. All transmission line structures are simulated on a thin-film substrate with a thickness

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ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 1 0other variable (𝜃). While observing the phase (Fig. 3 (b)), itvaries linearly along the frequency for coupled micro-strip
line.

Figure 2: Investigated transition structures from active device (ZBSD) to IF circuit (a) Microstrip coupled lines,(b) Co-planar wave guide (CPW) as signal on top, (c) Coplanar wave guide (CPW) as all three ground-signal-groundon top and (d) Micro-strip line.
of 4.5 µm and a dielectric constant of |𝜀| of 2.8. Table 1
shows the abbreviations meaning and dimensions of thestructures shown in Fig. 2.
Table 1: Dimensions of the Transition Structures
Abbreviation

Full form

Dimension[mm]

Gap between co-planar
0.011
micro-strip lines
GCPWA
Gap for CPW type A
0.050
GCPWB
Gap for CPW type B
0.012
SubH
Substrate height0.0045SubL
Substrate length
0.25
SubW
Substrate width1WCML Width of co-planar micro-strip line 0.011
WCPWA
Width of CPW A
0.012
WCPWB
Width of CPW B
0.012
W1MS
Width of micro-strip line
0.020
W2MS Width of micro-strip line tapered
0.012
GCML

Figure 3: Study of varying 𝜃 on performance of coupledmicro-strip line (a) Insertion loss over the frequency and (b)Variation of phase over the frequency.

The flexibility of having more degree of freedom while
having transition from ZBSD or TeraFET to IF circuit gives
an upper limit for the orientation of the other components.
Therefore to understand the effect of bending (𝜃) the transition structures, we studied this effect on coupled micro-strip
lines as shown in Fig. 3. The return loss (RL) is given as [8]:𝑅𝐿(𝑑𝐵) = −20 ⋅ 𝑙𝑜𝑔|Γ| ,(1)where |Γ| is voltage reflection coefficient. The transmission
coefficient (T) and insertion loss (IL) is given by [8]:𝑇 =1+Γ(2)

𝐼𝐿(𝑑𝐵) = −20 ⋅ 𝑙𝑜𝑔|𝑇 | .

(3)

Theta (𝜃) is varied from 0 to 90°. Figure3 (a) shows thetransmission variation versus frequency. It is observed thatat lower frequencies (below 10 GHz) the variation of (𝜃)doesn’t have a significant effect, however between 30 to40 GHz, (𝜃 = 0°) have less transmission as compared to theFigure 4: Comparison of different types of transition for abending angle 𝜃 is 0°.
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Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3IBIC2023, Saskatoon, Canada
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ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 1 0Figure 4 shows the comparison of all four transition structures. The CPW based structures A and B have −1 dB IL
over the whole frequency range (Fig. 4 (a)) but having a zerophase crossing at ∼25 GHz (Fig. 4 (b)). For micro-strip andcoupled micro-strip lines no zero-phase crossing is observed
and at the same time IL is −0.5 dB. The results propose thatmicro-strip lines are the optimum solution as a transition
structures in this specific case.

ULTRA-BROADBAND PLANAR POWER
DIVIDER (UBPPD)
A power divider sub-divides the power either equally or
unequally [8, 10, 11]. For the development of compact THz
detectors we need an ultra-broadband planar power divider(UBPPD) in order to divide the power equally in two arms
(represented by block single-ended to differential output inFig. 1), which will be further phase shifted by using a 180°
phase shifter.
Figure 5 shows the investigated ultra-broadband planar
power divider (UBPPD). The capacitively or inductively
loaded concept of transmission line has been used (Fig. 5 (a))
here to reduce the circuit size and increase bandwidth. Thislead to increase of capacitance or inductance per unit length,
which ultimately lead to slower wave propagation becauseof decrease in phase velocity. Figure5 (b) shows the sketchof two sections of UBPPD.

Table 2: Dimensions of UBPPD Structure
Abbreviation
Lf
L1
L1a
L2a
L3a
L4aL5aSubH
SubL
SubW
Wf
W1
W1a

Full form

Dimension[mm]

Length of the finger
Length signal coupling pad
Length of transition arm
Length of single section
Length of power divided armfor phase shifter
Length of power divided arm forphase shifter with meander line
Distance between twoout ports P2 and P3
Substrate height
Substrate length
Substrate width
Width of a finger
Width of signal coupling pad
Width of transition arm

0.05
0.45
0.44
0.57
0.95
2.9
1.26
0.25
32.5
0.18
0.50
0.060

Figure 5: Ultra-broadband planar power divider (UBPPD)(a) Block diagram of UBPPD with substrate RO4350B LoPro, (b) Sketch of two section UBPPD and (c) Sketch ofUBPPD with phase delay and shift transmission lines.

Figure 6: (a) Effect of varying (𝜃) on |𝑆11 | and (b) Effect ofvarying (𝜃) on |𝑆21 | and |𝑆31 |.

All the abbreviations in the Fig. 5 are show in Table 2 withthere respective values used for investigation. The structure
was investigated on RO4350B LoPro.
The angle between the two arms (𝜃) plays an importantrole in UBPPD performance. Fig. 6 (a) shows the effect ofvarying the 𝜃 on UBPPD performance. Clearly 𝜃 = 55° ishave better |𝑆11 | results compared two 𝜃 = 35° and 𝜃 = 45°.

Ideal power divider should have |𝑆21 | = |𝑆31 | and as shown inFig. 6 (b), the UBPPD have |𝑆21 | ≈ |𝑆31 | and quite stable over
the whole frequency range. These varying (𝜃) investigation
was performed for two section UBPPD (Fig. 5 (b)).
The proposed UBPPD was investigated for different sections (each section is 0.57 mm long). Figure 7 shows theperformance of 3 different UBPPD having 2, 4 and 6 sections.

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ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 1 0More sections in UBPPD means having a larger transmissionline, which changes the electrical length of the signal and ultimately results in degrading the performance of the component. Amplitude unbalance is calculated by |𝑆31 | − |𝑆21 | andthe phase unbalance is calculated as 𝑑𝑒𝑔(𝑆31 ) − 𝑑𝑒𝑔(𝑆21 ).
Figure 7 (a) shows the amplitude unbalance for 2, 4 and6 section UBPPD. The 2 section structure have ±0.005 dBof unbalance and its comparatively stable over frequency,
while for 4 section it is ±0.01 dB and ±0.02 dB for 6 sections UBPPD. Similar trend is observed for phase unbalance
as well Fig. 7 (b). Therefore, its better to have 2 sections
UBPPD for the IF circuit.

CONCLUSION
The transition from THz detecting device such as TeraFET
and ZBSD to IF electronics is a crucial part for the overall
detector optimization. The parameter study on the variation of bending the transition structures exhibits significant
impact on the propagating signal. From four different investigated structures, we found that the micro-strip line may be
the best option for a transition structures. The UBPPD is
crucial to divide the power equally in two arms which willthan be phase shifted by using a planar phase shifter (the investigation on broadband planar rogers substrate based 180°
phase shifter is still under progress) and finally feed to thebroadband amplifier. The investigated two section UBPPD
is the optimal solution (considering rogers substrates) forthe broadband phase shifter and will be fabricated by using
photo-lithography process and clean room technologies.

Figure 7: (a) Amplitude unbalance comparison for different
sections and (b) Phase unbalance comparison for different
sections.
As a pre-liminary study on using the phase shifter withUBPPD we investigated on meander shaped structures asshown in Fig. 5 (c). The meander shaped quarter-wavelength
structure is used for the phase shifting as meander shaped
structures leads to periodic variation of propagating signal
and slow it down as the phase velocity decreases along it.
The |𝑆11 | parameters are well below -10 dB until 40 GHz(Fig. 8 (a)) while the |𝑆21 | and |𝑆31 | have slightly bigger amplitude unbalance of ±2 dB (Fig. 8 (b)) and the phase unbalance is also significant along with flip at 15 GHz and 25 GHz
as shown in Fig. 8 (c).

Figure 8: Two section UBPDD with meander phase shifter(a) S-parameters, (b) Amplitude unbalance and (c) Phase
unbalance.

ACKNOWLEDGEMENT
This work is supported by the German Federal Ministry of Education and Research (BMBF) under contract
no. 05K22RO1 at Mittelhessen University of Applied Sciences, Friedberg (Hesse), Germany. We are thankful for thecooperation with TU Darmstadt, Darmstadt, Germany andother cooperative partners under this BMBF project.
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ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

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