Keywords

1 Introduction

It is already well known fact that gallium nitride (GaN) is a potential material for realizing micro- and nano-scale devices which are capable of radiating high power terahertz (THz) waves [1,2,3,4,5,6,7,8,9,10]. Out of various solid-state THz radiators like resonant tunneling diodes, heterojunction bipolar transistors (HBTs), high electron mobility field effect transistors (HEMTs), quantum cascade lasers (QCLs), etc. [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37] avalanche transit time (ATT) devices, more specifically impact avalanche transit time sources (IMPATT) diodes have more capabilities of generating high-power, high-efficient THz waves [38, 39]. Theoretical studies predict that the GaN based IMPATT oscillators are capable of THz power of the order of milli-watt (mW) up to 5 THz. In this chapter, the authors have presented an elaborated discussion on the possibilities of realizing a novel integrated power module by integrating the passive radiating element (i.e. the antenna) with the active source (i.e. the IMPATT structure).

Initially, the chapter can be organized into six sections which provided brief discussions on the primary developmental steps of the proposed integrated power module. Those are given by.

  1. (i)

    Design and Fabrication of the Seed IMPATT diode Structure,

  2. (ii)

    Bonding and Packaging,

  3. (iii)

    Resonant-Cap Cavity for THz IMPATT Source,

  4. (iv)

    Broadband Oscillator Realization,

  5. (v)

    Source-Antenna Integration, and

  6. (vi)

    Power Combining.

Finally, in the final section device structure, material properties, simulation technique and simulation results are presented.

2 Design and Fabrication of the Seed IMPATT Diode Structure

Design and simulation of GaN based DDR IMPATT seed structure shown in Fig. 1 for 1.0 THz frequency generation has already been carried out [39]. Doping and thickness of different layers of the DDR structure are already chosen subject to obtain maximum DC to RF conversion efficiency [39]. The metal contacts for both anode and cathode have been confirmed by acquiring knowledge from the published literature. All details regarding the proposed DDR structure and its large-signal performance have been already published elsewhere [39]. However, after several close investigations, some issues have been raised regarding the proposed structure; those are point-wise briefed below.

Fig. 1
A schematic diagram has eight main layers and four sub layers in the cathode after the fifth main layer. The prominent layers are anode, cathode, buffer, and sapphire.

Schematic diagram showing the vertical section of the 1.0-THz GaN DDR IMPATT structure grown on sapphire substrate [39]

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  1. 1.

    Instead of using Sapphire [c(1000)-Al2O3] as the substrate for growing the entire DDR structure, it can be grown on GaN substrate. Primary advantage of homo-epitaxial growth over hetero-epitaxial growth is the reduction of dislocation at the interface of the substrate and grown layer. Moreover, better thermal conductivity of GaN than Sapphire enables the GaN substrate to act as an internal heat sink.

  2. 2.

    Since the efficiency of the THz diode is expected to be smaller than 10%; therefore a large amount of heat energy is supposed to be dissipated within the diode during its continuous wave steady-state operation. This will lead to a 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 to be attached below the substrate layer (Fig. 2a) of the diode chip by using an appropriate adhesive substance (having high thermal conductivity) [40,41,42]. The temperature distribution inside the type-IIa diamond heat sink is shown in Fig. 2b.

    Fig. 2
    An illustration on the left has a diode chip on top and a circular based type II a diamond heat sink placed below. The temperature distribution graph at the right has a raised middle portion.

    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 [40]

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  3. 3.

    Proposed structure may face an electric field crowding effect which may lead to the edge (local) a breakdown or premature breakdown. Necessary structural modification has to be incorporated into the device structure in order to avoid such premature breakdown.

After finding the appropriate solutions for the abovementioned three issues, the bonding and packaging issues will have to be taken into consideration.

3 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 the thermal sonic compressor and both ends of the gold wire can be connected with gold coated ring-cap (package anode) of the S4 package by using silver epoxy baked at 150 °C for half an hour. The diode chip attached with the external type-IIa diamond heat sink has to be die-bonded to gold coated copper cylinder at the lower surface of the S4 package; it will act as an integral heat sink along with the diamond heat sink. However, the bonding of the cathode with the gold coated copper cylinder (package cathode) is a tricky job. Figure 3 shows the bonding and packaging of the diode chip in an S4 package. Both anode and cathode of the diode chip have to be bonded with gold coated cap and gold plated copper cylinder respectively by using multiple numbers of gold wires in order to reduce effective parasitic series resistance; however, only single wire bonding is shown in Fig. 3. After packaging, the overall equivalent circuit of the packaged diode is shown in Fig. 4. Here, Lp and Cp are the package inductance and capacitance, -RD, CD and RS are the diodes negative resistance, capacitance and parasitic series resistance (all are functions of frequency). At THz regime, stud-type package may be the better option as compared to the S4 package [43].

Fig. 3
A schematic illustration of the diode chip has a gold plated cap, wire, and cylinder, a diode chip and a diamond heat sink in the middle.

Schematic illustrating the bonding and S4 packaging of the diode chip

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Fig. 4
A circuit diagram has an anode at the top, leading via a gold wire package to the diode and leading below to covers and ends in a cathode.

Equivalent circuit of the S4 packaged IMPATT chip

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4 Resonant-Cap Cavity for THz IMPATT Source

The packaged diode has to be embedded in an appropriately designed rectangular waveguide cavity resonator as shown in Fig. 5. The diode has to be reverse-biased and may be embedded inside the cavity via a bias post as shown in Fig. 5. The packaged diode mounted inside the suitable cavity resonator (circuit) leads to device-circuit interaction which results in oscillation. 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 [39]. On the other hand, the real part 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 this point and very inefficient power transfer can occur from the diode to the resonator. The impedance matching between the device and circuit can be achieved by using two possible methods. Those are.

Fig. 5
A schematic diagram has a sliding short at the left, a rectangular waveguide at the bottom leading via a post, a packaged diode having a bias. It moves further right through the E H tuner to the load.

Schematic of the packaged diode embedded inside a rectangular waveguide cavity resonator via a bias post

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  1. (i)

    By using reduced height waveguide cavity, and.

  2. (ii)

    By using resonant-cap cavity.

First method may be suitable for microwave/millimetre frequency range up to even 94 GHz. However, to match impedance from the reduced height waveguide in which the diode is mounted needs to be matched with the full-height waveguide system either with stub matching or stepped impedance/exponential taper transformer. But for higher millimetre-wave frequencies and THz (0.3–10 THz) range it is impractical to use the post-mounting technique. Thus at the THz frequency range, resonant-cap cavity using a disk-cap resonator circuit is the best choice for impedance matching at THz regime [44, 45].

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 Fig. 6 [44]. The disk of the resonant-cap and the 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 [46]. 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, …… [46]. By choosing the suitable value of m, an appropriate cap-cavity structure can be designed.

Fig. 6
A schematic diagram has a rectangular waveguide as the base. It has a diode at the right and a disc of the resonant cap is marked out on the top, at the left.

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

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5 Broadband Oscillation

The disk can be made slotted (Fig. 7) in order to increase the bandwidth of the cap-cavity oscillator [47,48,49,50].

Fig. 7
The side view provides a plus shaped perspective and the top view provides a circular structure with two radii. A line graph at the right provides the frequency response in which the slotted disk has two peaks and the un-slotted disk has a single, central peak.

a Side and top views of slotted disk structure, and b frequency response of slotted and un-slotted disk structure together with the bottom broad-wall of the waveguide

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6 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 Fig. 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 the best radiation efficiency.

Fig. 8
A radial transmission diagram has a major lobe of radiation in the middle, leading below to a disk cap, packaged diode, and fringing lines at the sides on top of a bottom broad wall.

Radial transmission line structure for source-antenna integration

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7 Power Combining

THz power output from a single source may be very small (practically < 10 mW). Therefore, suitable power combining multiple sources must be implemented in order to enhance the radiated THz power. Twin-cap IMPATT power combining technique [50], vide Fig. 9, may be used that may be optimized for phase coherence using some form of Meta-surface too.

Fig. 9
A schematic diagram of the twin cap I M P A T T power combiner, in which its twelve parts are labelled.

Twin-cap IMPATT power combiner [50]

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8 Proposed Device Structures and Simulation Results

Design and simulation of the GaN ATT diode have been carried out by using an indigenously developed large-signal simulation tool [39]; diode structure and its detailed simulation results are already reported elsewhere [39]. The important material parameters used in the simulation are tabulated in Table 1 [51,52,53,54]. In this work, two integrated power module structures consisting of (i) a disk-cap circular microstrip patch antenna for narrowband operation and (ii) a slotted-disk circular microstrip antenna for the broadband operation to be fabricated on the diode-head are proposed and analyzed. High Frequency Structure Simulator (HFSS) is used to simulate the frequency response of the proposed structures. Figures 10 and 11 show the device structures and Figs. 12 and 13 show corresponding HFSS layouts. The two-dimensional (2D) electric field and carrier concentrations plot for the centre voltage 30 V are shown in Figs. 16 and 17 respectively. Maximum electric field at the metallurgical junction is found to be ξmax = 22.5476 × 107 V m−1. The electric field crowding effect was not considered for our simulation; however, it is better to study its impact while simulation its actual structure. The real and imaginary parts of the device (Zd(f) = Rd(f) + jXd(f)) and antenna (Zc(f) = Rc(f) + jXc(f)) impedances are plotted against frequency in Fig. 18. Antenna part is designed such a way that perfect impedance match between the device and circuit is achieved (i.e. |Rd(f)|f = 1 THz = |Rc(f)|f = 1 THz and |Xd(f)|f = 1 THz = |Xc(f)|f = 1 THz) at f = 1 THz (Figs. 14 and 15).

Table 1 Important material parameters of GaN at room temperature
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Fig. 10
Two schematic diagrams of the front and top views call out the disk-cup, feed line, and edges of the plane and diode mesa.

Schematic diagrams of the a front-view and b top-view of the GaN integrated power module

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Fig. 11
Two schematic diagrams of the front and top view call out the disk-caps, edges, cathode ground plane, and pad window.

Schematic diagrams of the a front-view and b top-view of the slotted-disk GaN integrated power module

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Fig. 12
A 3-D H F S S layout marks the three axes, x, y, and z for a circular shape atop a rectangular structure.

HFSS layout of the GaN integrated power module

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Fig. 13
A 3-D H F S S layout marks the three axes, x, y, and z for a wheel shape atop a rectangular structure.

HFSS layout of the slotted-disk GaN integrated power module

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Fig. 14
A 2 D plotted profile is in the shape of a triangle with the peak in the middle.

2-D Electric field profile

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Fig. 15
Two 3-D views of a density profile. The left one is right-top inclined and the right one moves from left-top towards bottom right.

2-D carrier density profiles

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Figures 17 and 18 show the variations of S11 parameter and VSWR of the disk-cap and slotted-disk GaN integrated power modules respectively. Narrowband operation of the disk-cap structure and broadband operation of the slotted-disk structure can be confirmed from Figs. 17 and 18. Two-dimensional field plots, 3D Gain and directivity plots of those structures shown in Figs. 19 and 20 depict the antenna performance at 1.0 THz (Table 2).

Fig. 16
Two line graphs of real and imaginary parts of the large signal diode impedance are depicted. The graphs are top-right inclined.

Variations of real (Rd) and imaginary (Xd) parts of the large-signal diode impedance as well as real (Rc) and imaginary (Xc) parts of the cap-circuit impedance at the diode plane (i.e. at r = Dj/2)

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Fig. 17.Variation of S11 parameter of the disk-cap and slotted-disk GaN integrated power modules with frequency

Fig. 17
Two line graphs indicate the V S W R frequencies of the disk cap and slotted disk power modules. The disk cap module is centered at the top and the slotted disk module has a big low at the center.

Variation of S11 parameter of the disk-cap and slotted-disk GaN integrated power modules with frequency

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Fig. 18
Two line graphs mark the V S W R frequencies of the disk cap and slotted disk power modules. The disk cap module is centered and the slotted disk module has highs and lows all over.

Variation of VSWR of the disk-cap and slotted-disk GaN integrated power modules with frequency

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Fig. 19
Two top views of a disk cap and a slotted disk power module. The slotted disk module's graphs are a bit wider.

2-D field patterns of a disk-cap and b slotted-disk GaN integrated power module for ϕ values of 00, 900 and 1800 at 1.0 THz

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Fig. 20
Two sets of two 3-D views of gain and directivity plots for disk-cap and slotted-disk modules. The first two are a bit more bulging outwards than the second set.

3-D a gain and b directivity plots (3-D radiation patterns) of disk-cap GaN integrated power module at 1.0 THz; 3-D c gain and d directivity plots (3-D radiation patterns) of slotted-disk GaN integrated power module at 1.0 THz

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Table 2 Design parameters of the 1.0 THz GaN integrated power module considering the effect of fringing field
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9 Summary

In this chapter, possibilities of realizing two GaN ATT diode based integrated THz power module structures have been discussed. Design and simulation of the GaN ATT diode have been carried out by using an indigenously developed large-signal simulation tool; diode structure and its detailed simulation results are already reported elsewhere. In this work, two integrated power module structures consisting of a disk-cap circular microstrip patch antenna for narrowband operation and a slotted-disk circular microstrip antenna for the broadband operation to be fabricated on the diode-head are proposed and analyzed. High Frequency Structure Simulator (HFSS) is used to simulate the frequency response of the proposed structures. The proposed integrated power module structures have immense potentialities to be used as powerful and efficient THz source in various THz biomedical applications.