Parth Pratim Sarkar

Scientific
Department of Engineering and Sr.
Technological Studies, University of Officer
Kalyani, Kalyani, Nadia, West
Bengal, India

Collaboration Partners
Research CenterintheTHz
Antenna
Integration
Simulation

Hiroshi Inokawa
Hiroaki SatohResearch Institute of Electronics, Shizouka University, Japan

Research Results (Including Purpose, Results, Figures, etc.)

Optimized design parameters such as thickness and doping densities of different layers of n+-n-pp+ structured double-drift region GaN IMPATT diode designed to operate at 1.0 THz is alreadyreported by the authors. Comprehensive simulation studies have already provided the estimationof power delivery capability, parasitic series resistance and noise performance of the optimized
source at 1.0 THz. However, it is well known fact that the realization of the optimized structure isa tricky job by considering the state-of-the-art GaN fabrication technology. Especially, achieving
the absolute value of doping density of n- and p-layers is almost unrealistic task. Therefore, it isvery important to acquire the knowledge about how much extent the power output, series resistanceand noise measure of the source are affected due to the change in doping level of both n- and players. In this present study, the sensitivities of the above-mentioned parameters with respect tothe change in the doping densities of n- and p-layers.
1. Structure and Fabrication Issues
The realization of wide bandgap semiconductor material based avalanche transit time sources is avery attractive area of research to the researchers working since last two decades in order to find ahigh power, low noise solid-state source at terahertz frequency regime (0.30 – 10.0 THz). Among
different wide bandgap materials like SiC, GaN, diamond, etc., GaN is the most potential material
for realizing high frequency, high power, low noise semiconductor devices. The proposed structureof the DDR GaN IMPATT diode is shown in Figure 1 (Biswas et al., 2018; Chakraborty et al.,2019). It is well known fact that the realization of the optimized structure is a tricky job byconsidering the state-of-the-art GaN fabrication technology. Especially, achieving the absolutevalue of doping density of n- and p-layers is almost unrealistic task. Therefore, it isvery important to acquire the knowledge about how much extent the power output,
series resistance and noise measure of the source are affected due to the change in
doping level of both n- and p-layers.

Fig. 1. GaN IMPATT structure for 1.0 THz operation.

2. Results
The doping concentrations of both n- and p-layers are assumed to be varied between ΔN = ±10%of the optimized values, i.e. ±10% of 6.8×1023 and 7.0×1023 m-3 respectively, and the structure is
re-simulated in order to study the aforementioned effect. The Fig. 2 illistrates the sensitivity
analysis results. Maximum 21.9% of change in power output is observed. It is observed that thepower output is increased with the decrease of doping levels. Sharp increase in power output isobserved just after the device enters to the punch-through region. Sharp decrease in series
resistance is observed after the device become punch-through for lower doping density of n- andp-layers. Noise measure is slightly increased with the decrease of doping density of n- and p-layers.

Fig. 2. Variations of percentages of change in power output, series resistance and noise measurewith the percentage of change in doping density of n- and p-layers.

List of Publications Related to the Collaboration ResearchNIL

Li st of Presentations ( Conference, Meeting, etc)
1)Arindam Biswas, Aritra Acharyya, Kazuki Hirai, Hiroaki Satoh, Hiroshi Inokawa, “1.0 THz
GaN IMPATT Source: Influence of Change in Epitaxial Doping Concentrations,” Annual Meetingfor 2019 Cooperative Research at Research Center of Biomedical Engineering (Communicated,Mar. 13, 2020).
2) Arindam Biswas, Aritra Acharyya, Hiroaki Satoh and Hiroshi Inokawa, "Study on high-power terahertz
sources based on GaN IMPATT diode," Symposium on Frontier of Terahertz Science VI, Con6, p. 43 (TokyoInstitute of Technology, Japan, Nov. 28-30, 2019) [in Japanese]
List of Awards
NIL

Registration of research-theme continuation for next year
Prior consent from the collaboration partner in the Research Center is necessary.

YesNoResearch plan for the next year (from April 1, 2020 to March 31, 2021), if the collaboration research is continued.

The entire project can be divided into six primary developmental steps; those are given by(A) Design and Fabrication of the Seed IMPATT diode Structure,(B) Bonding and Packaging,(C) Resonant-Cap Cavity for THz IMPATT Source,(D) Broadband Oscillator Realization,(E) Source-Antenna Integration, and(F) Power Combining.
All the important issues related to each of the abovementioned steps are discussed below.
(A) Design and Fabrication of the Seed IMPATT diode Structure
Design and simulation of GaN based DDR IMPATT seed structure (Figure 1) for 1.0 THz
frequency generation has already been carried out. Doping and thickness of different layersof the DDR structure are already chosen subject to obtain maximum DC to RF conversion
efficiency. The metal contacts for both anode and cathode have been confirmed via technicaldiscussion with our research group as well as by acquiring knowledge from the publishedliterature. All details regarding the proposed DDR structure and its large-signal performance
have been already published [1]. However, after several discussions with the Japan researchgroup, some issues have risen regarding the proposed structure; those are point-wise briefed
here.
1. Instead of using Sapphire [c(1000)-Al2O3] as the substrate for growing the entire DDRstructure, it can be grown on GaN substrate. Primary advantage of homo-epitaxial
growth over the hetero-epitaxial growth is reduction of dislocation at the interface of
substrate and grown layer. Moreover, better thermal conductivity of GaN thanSapphire enables the GaN substrate to act as an internal heat sink.
2. Since the efficiency of the THz diode is expected to be smaller than 10%; therefore
large amount of heat energy supposed to be dissipated within the diode during its
continuous wave steady-state operation. This will lead to thermal runway of the diode.
In order to avoid this thermal issue, an external heat sink (having cylindrical shape)
preferably made of type-IIa diamond (thermal conductivity ~1200 W m-1 K-1) has tobe attached below the substrate layer (Figure 2 (a)) of the diode chip by using
appropriate adhesive substance (having high thermal conductivity) [2-4]. The
temperature distribution inside the type-IIa diamond heat sink is shown in Figure 2(b).
3. Proposed structure may face electric field crowding effect which may lead to the edge(local) breakdown or premature breakdown. We have to incorporate necessary
structural modification in the device structure in order to avoid such premature
breakdown.

Figure 1: Schematic diagram showing the vertical section of the 1.0-THz GaN DDR IMPATT structure grown on
sapphire substrate.

Figure 2: (a) Diode chip attached on a cylindrical shaped type-IIa diamond heat sink, and (b) temperature
distribution inside the heat sink for steady-state thermal operation at 500 K [2].

After finding the appropriate solutions for the abovementioned three issues, thebonding and packaging issues will have to be taken into consideration.
(B) Bonding and Packaging
Wire bonding of anode and cathode terminals of the diode has to be done with the S4 package.
Wire bonding of the anode can be done by following the conventional wire edge boding
technique. Middle portion of a 5 – 10 μm diameter gold wire can be bonded on the top of thediode chip (anode) by thermal sonic compressor and both ends of the gold wire can beconnected with gold coated ring-cap (package anode) of the S4 package by using silver epoxy
baked at 1500C for half an hour. The diode chip attached with external type-IIa diamond heatsink has to be die bonded to gold coated copper cylinder at the lower surface of the S4package; it will act as integral heat sink along with the diamond heat sink. However, thebonding of the cathode with the gold coated copper cylinder (package cathode) is a tricky
job. We have to find out the probable procedure for this. Figure 3 shows the bonding andpackaging of the diode chip in a S4 package. Both anode and cathode of the diode chip haveto be bonded with gold coated cap and gold plated copper cylinder respectively by usingmultiple numbers of gold wires in order to reduce effective parasitic series resistance;
however, only single wire bonding is shown in Figure 3. After packaging, the overall
equivalent circuit of the packaged diode is shown in Figure 4. Here, Lp and Cp are the package
inductance and capacitance, -RD, CD and RS are the diode negative resistance, capacitance andparasitic series resistance (all are functions of frequency).

Figure 3: Schematic illustrating the bonding and S4 packaging of the diode chip.

Figure 4: Equivalent circuit of the S4 packaged IMPATT chip.

At THz regime stud-type package may be the better option as compared to the S4package [5]. We are still searching for the best package for THz IMPATT diode.
(C) Resonant-Cap cavity for THz IMPATT Source

The packaged diode has to be embedded in an appropriately designed rectangular waveguidecavity resonator as shown in Figure 5. The diode has to be reverse-biased and may beembedded inside the cavity via a bias post as shown in Figure 5. The packaged diode mounted
inside the suitable cavity resonator (circuit) leads to device-circuit interaction which resultsoscillation. The magnitude of the overall negative resistance of the packaged diode designed
to operate at 1.0 THz is very small, in the orders of 0.1 – 1.0 Ω [1]. On the other hand, the realpart of the circuit impedance of the cavity resonator (resonant frequency fr = 1.0 THz)remains in the order of 100 Ω. Therefore, a huge impedance mismatch is expected at thispoint and very inefficient power transfer can occur from diode to the resonator. The
impedance matching between device and circuit can be achieved by using two possible
methods. Those are
1. By using reduced height waveguide cavity, and2. By using resonant-cap cavity.

Figure 5: Schematic of the packaged diode embedded inside a rectangular waveguide cavity resonator via a bias
post.

First method may be suitable for microwave/millimetre frequency range up to even 94
GHz. However, to match impedance from reduced height waveguide in which the diode ismounted need to be matched with the full-height waveguide system either with stubmatching or stepped impedance/exponential taper transformer. But for higher millimetrewave frequencies and THz (0.3 – 10 THz) range it is impractical to use post-mounting
technique. Thus at the THz frequency range, resonant-cap cavity using disk-cap resonatorcircuit is the best choice for impedance matching at THz regime [6, 7].
In a resonant-cap cavity type source, the packaged diode is embedded in a high-Q
resonant-cap cavity and two together are mounted in a rectangular waveguide through which
the source is connected to the load as shown in Figure 6 [6]. The disk of the resonant-cap andthe bottom broad-wall of the rectangular waveguide form the cap cavity, which is equivalent
to a radial transmission line causing efficient power transfer from the device to the load [8].
The diameter of the disk must have the dimension D = (λr/4 + m λr/2), where λr is the resonant
wavelength and m = 0, 1, 2, 3, …… [8]. By choosing suitable value of m, appropriate cap-cavity
structure can be designed.

Figure 6: Schematic of a resonant-cap based source with the devices embedded in the resonant-cap cavity [6].

(D) Broadband Oscillation
The disk can be made slotted (Figure 7) in order to increase the bandwidth of the cap-cavity
oscillator [9 -12].

Figure 7: (a) Side and top views of slotted disk structure, and (b) frequency response of slotted and un-slotteddisk structure together with the bottom broad-wall of the waveguide.

(E) Source-Antenna Integration
The radial transmission line structure itself can behave like an integral antenna for the THz
IMPATT source. Its principle of working may be understood in terms of modelling it like a
microstrip antenna. Its major radiation lobe will be along the direction of z-axis as shown in
Figure 8. However, the bias feeding point to the packaged IMPATT will have to be decided(bias post should not interfere with the major lobe) in order to obtain best radiation
efficiency.

Figure 8: Radial transmission line structure for source-antenna integration.

(F) Power Combining
THz power output from a single source may be very small (practically < 10 mW). Therefore,
suitable power combining of multiple sources must be implemented in order to enhance theradiated THz power. Twin-cap IMPATT power combining technique [13], vide Fig. 9, may be
used that may be optimized for phase coherence using some form of Meta-surface too.

PARTS LIST

(1,2,3) Copper blocks, (4) Rectangular
waveguide cavity, (5) Bias filter, (6)Insulating material, (7) Insulating
guard-ring (8) Twin-cap structure, (9)BNC connector, (10) Gold spring, (11)IMPATT diode, (12) Threaded diodeholder.

Figure 9: Twin-cap IMPATT power combiner [13].

References
1.

Arindam Biswas, Sayantan Sinha, Aritra Acharyya, Amit Banerjee, Srikanta Pal, Hiroaki Satoh andHiroshi Inokawa, “1.0 THz GaN IMPATT Source: Effect of Parasitic Series Resistance,” Journal of Infrared,
Millimeter and Terahertz Waves, vol. 39, issue 10, pp. 954-974, 2018.

2.

Aritra Acharyya, Jayanta Mukherjee, Moumita Mukherjee and J. P. Banerjee, “Heat Sink Design for
IMPATT Diode Sources with Different Base Materials Operating at 94 GHz,” Archives of Physics
Research, vol. 2, issue 1, pp. 107-126, 2011.

3.

Aritra Acharyya and J. P. Banerjee, “Heat Sink Design and Temperature Distribution Analysis forMillimeter Wave IMPATT Oscillators using Finite Difference Method,” Archives of Applied Science
Research, vol. 3, issue 2, pp. 107-120, 2011.

4.

Aritra Acharyya, Jit Chakraborty, Kausik Das, Subir Datta, Pritam De, Suranjana Banerjee and J. P.
Banerjee, “Large-Signal Characterization of DDR Silicon IMPATTs Operating up to 0.5 THz,”
International Journal of Microwave and Wireless Technologies, vol. 5, no. 5, pp. 567-578, 2013.

5.

Subal Kar, “Microwave Engineering: Fundamentals, Design and Applications,” Universities Press, 2016,
ISBN: 9788173719899, 8173719896.

6.

Subal Kar, “Modelling and characterization of microwave resonant-cap circuits,” Microwave and Optical
Technology Letters, vol. 16, no. 6, pp. 400-403, 1997.

7.

Subal Kar, “An experimental technique for the design of optimized resonant-cap circuits for microwave
IMPATT oscillators and amplifiers,” Microwave and Optical Technology Letters, vol. 19, no. 2, pp. 81-84,
1998.

8.

Subal Kar, “Microwave Properties of IMPATT Oscillators and Amplifiers with the Device Embedded inNormal and Modified Resonant-Cap Cavities,” Ph.D. dissertation, University of Calcutta, 1989, Chap. 2,
pp. 8-68.

9.

Subal Kar and S.K. Roy, “A modified resonant-cap microwave IMPATT oscillator”, Indian
Patent 157098, March 9, 1983 [Issued on 18.1.86].

10. Subal Kar and S.K. Roy, “A modified resonant-cap microwave IMPATT amplifier”, Indian
Patent 161758, August 24, 1984 [Issued on 30.1.88].
11. Subal kar, and S. K. Roy, “Experimental studies on the improvement of the performance of resonant-cap
type microwave IMPATT oscillators through appropriate modification of the resonant-cap cavity,”
International Journal of Electronics, vol. 75, pp. 941-950, 1993.
12. Subal Kar, “Experimental studies on a modified resonant-cap microwave IMPATT amplifier withimproved performance,” International Journal of Electronics, vol. 82, pp. 555-566, 1997.
13. Subal Kar, “A twin-cap IMPATT power combiner”, Indian Patent No.: 18414,[Issued on 04.07.1996].