MMIC amplifiers with built-in antennas based on nano-heterostructures
Amplifier MMIC’s with built-in antennas for the frequency ranges 5 GHz and 10-12 GHz
The high dielectric conductivity GaAs can create planar antennas with minimum dimensions. In the rectangular microstrip antennas the lowest type of resonance occurs when the length of the antenna element L≈λ/2, where λ≈λ0/√εr, εr is the dielectric conductivity of the substrate (for GaAs εr ~ 12.9), λ0 is the wave length in vacuum. The higher the conductivity of the substrate, the smaller the dimensions of the antenna. In our case, for the frequencies 5 GHz and 10 GHz, the length of the antenna element is :
Antenna elements were calculated with the help of the finite element method, while for electrodynamic modelling of amplifiers the moments method was used. The structural circuits of both LNA and PA are based on two stages with resistive feedback .
Fig.1 shows a photo of LNA for the frequency band 10-12 GHz. LNA and PA were implemented by one technology route on pHEMT-transistors on the heterostructures AlGaAs/InGaAs/GaAs. The measured amplifier gain reaches a value of more than 16 dB, the noise factor LNA is less than 2 dB (fig.2), the power of PA is more than 100 MW.
Antennas and amplifiers were calculated separately on the coordinated load of 50 ohm, which allowed integrate them into a single chip with minimal losses in the path. In the case of the receiving path, the antenna was delivered to the input of the LNA, and in case of the transmission path – to the PA output.
The size of the obtained crystals of MIC amplifiers integrated with antennas, 9.5×6.5 mm2 for the range 5 GHz and 5.4×6.5 mm2 for the range of 10-12 GHz. Fig.3 shows a photo of the MMIC antenna with PA for the range 10-12 GHz [3-7].
The photos of antennas of the frequencies 5 GHz and 10-12 GHz soldered to circuit board with the coaxial output to measure directional patterns are shown in fig.4.
Fig.5 shows the results of measurements of antenna for both bands. The observed emission peak is perpendicular to the antenna surface.
Amplifier MMIC with built-in antennas for the frequency RANGE 58-65 GHz
The 60 GHz band has the following advantages:
•it allows you to work in a wide frequency band and ensures the data transmission rate up to
5 GGbit/s or higher;
•is characterised by a high absorptivity in the atmosphere that allows the creation of isolated communication channels;
•the short wavelength enables the integration of antennas and antenna arrays on one chip.
This range makes it possible to create receive/transmit devices of the indoor broadband connection to provide high-speed reserved data transfer between the electronic devices and switch to the construction of 5G mobile broadband networks .
Amplifier MMIC’s (LNA and PA) with built-in antennas for the range of 60 GHz were designed using the same methodology as the MMIC for the 5 GHz and 10-12 GHz bands. Each antenna element was based on a monopole of the complex shape (fig.6). The diameter of the antenna element is about 0.72 mm. An amplifier for the frequency range 58-65 GHz is built on the scheme with a common source on HEMT-transistors with a total gate width of 100 µm (2×50 µm). Its dimensions are 2.26×1.15 mm. In order to standardise the elemental base, the amplifier was designed so that depending on the operating point it could function as LNA with the noise factor not more than 6.5 dB and PA with a power output up to 40 MW. Fig.6 shows a photo of the LNA with the antenna for the range of 58-65 GHz. The MMIC dimensions with the built-in antenna and amplifier were 3.4×1.15 mm [9-11].
MMIC’s for 60 GHz were made on the gallium nitride nanoheterostructures on the sapphire substrate. In that case, some constructive complexities occurred, the antenna resonance is formed in the substrate having reverse metallisation, and an essential factor for its formation is the substrate thickness (300 microns or more) that does not allow to make the sapphire plate thinner and etch holes in it. Etching holes in sapphire is also associated with a number of technological difficulties. The problems could be solved by using a coplanar technology to design amplifiers, as was done for the MMIC of 5 GHz and 10-12 GHz; however here we were faced with the inability to create any stable amplifiers with the required microwave parameters in the V frequency range.
To solve this problem an engineering and design solution was found to create a "ground plane" on the front surface of the wafer with some pre-fabricated active and passive microwave components on top of a layer of polymeric dielectric with a thickness of 10-15 microns (photoresistive coating developed by the Institute of Macromolecular Compounds of Russian Academy of Sciences). Fig.6 shows the appearance of the PA MMIC with an antenna before and after applying the photoresistive coating. Grounding the respective elements is effected through holes in the photoresistive coating (fig.7) providing a protective passivation at the same time. Photoresistive coating is removed from pads and antenna radiation field.
According to the technological process developed on nitride heterostructures for the first time in Russia on sapphire substrates amplifier MMIC’s were made with integrated antennas for use in the receive/transmit modules. Fig.8 shows the measured characteristics of an amplifier: amplifier gain in the context of continuous power supply is 12-13 dB in the range 58-65 GHz (fig.8a); PA output power at pulsed supply (duration of 1 ms, the pulse/pause ratio is 99) is 30-50 MW in a saturated mode (8b). Fig.9 shows the measured dynamic characteristics of amplifiers under the pulsed power, i.e. 15-17 dBm saturated output (30-50 mW); 1 dB compression point is from -3 to 0 dBm.
To determine the MMIC radiation pattern with built-in antennas on a wafer, a special test stand was developed (fig.10) . The radiation pattern is measured with the lab antenna located at a distance R from the measured antenna. A lab antenna is mounted on a rotary mechanism in a way to allow its movement along the circumference, the centre of which is studied antenna, in increments of 5 degrees, and the distance R between the antennas remains unchanged.
The radiation pattern measurements were taken in two planes, ZX and ZY. In the ZY plane the rotation range of the horn antenna was 180°, whereas in the ZX plane 130°. The angle of rotation of the horn antenna was limited by the dimensions and location of the probe holder. Fig.11 shows the measured radiation pattern of the antenna at a frequency of 58 GHz.
Since the measurements of samples were carried out directly on the wafer, the metallisation of adjacent samples of MMIC and measuring equipment (microwave probe, probe holder and probe table) distorted the radiation pattern of an antenna thus explaining irregularities in the resulting characteristics. It did not seem to be possible to measure the radiation pattern of the antenna amplifier MMIC with the probe method as in this case additional feeding probes were needed on both longitudinal sides of MMIC. Thus, the chip is surrounded on three sides by the probes that was greatly distorting the directional pattern and limiting the angle of rotation of the horn antenna up to ±10 degrees. However, measurements in a narrow range of rotation of the horn antenna (±10 degrees) show that the power radiated from the amplifier MMIC with an antenna is 15-20 dB higher in comparison with the power radiated by a single antenna. A review of the foreign literature on creating built-in antennas with amplifiers for 58-65 GHz frequency proved that in this range antenna MMIC’s with amplifiers manufactured following the CMOS or SiGe technologies on silicon substrates are used. MMIC’s with integrated antennas on the nanoheterostructures AlGaN/GaN/Al2O3 are not discovered by us, only some individual elements (amplifiers, mixers).
The recent developments in the frequency range 59-65 GHz are MMIC antennas with amplifiers following the 65-nm CMOS technologies which allows to create the device 1.96 × 1.96 mm with an output of 17.78 MW. . With the use of the silicon-germanium technology, an LNA SSI circuit with the integrated antenna with a size of 3×2.8 mm and a gain above 12 dB  was made. In terms of the output power, these MMIC are lagging behind by 2-3 times the amplifier MMIC’s designed with integrated antennas on gallium nitride (30-50 MW) with a minimum topological size of 120 nm.
The work was financially supported by the Ministry of education and science of the Russian Federation (grant No. 14.607.21.0087, the unique