Millimetre Range Nitride Devices
An increase in power output from the PA in the low-frequency L- and S- bands is mainly due to an increase in the breakdown voltage (Ubreakdown) of transistor gates in the introduction of one or more additional field-plated electrodes, which are located in the space between the gate and drain of the transistors through the insulator layers and contribute to the reduction of peak field strength between the gate and drain [2, 3]. As early as 2006 in using HEMT on SiC substrates with one field-plated electrode obtained was the saturated output power of a hybrid-type PA up to 371 W at a frequency of 2.14 GHz in a continuous mode with an efficiency of 24% at the gate breakdown voltage of about 200 V . Further PA development in the ranges in terms of enhancing their efficiency and cost reduction was due to the use of the key operational modes of transistors, which required a further increase in impulse breakdown voltage drain-gate in the closed position and transition to the silicon substrate (Ubreakdown = 1590  already received and even more than 2000  by removing the substrate Si). The achieved results have contributed to creation of key competitive transistors on nitride heterostructures for power electronics ; mass production is planned to start in the next few years . Besides, already mastered the production of high-performance key PA of the frequency range up to 2.5 GHz (RFMD ) with an efficiency of up to 70% at an output power up to 25 W. Unfortunately, this technology is applicable only in the lower frequency bands as the field-plated electrodes multiply the gate capacitance Cgs and Сgd thereby drastically reducing the frequency parameters of the transistors (in the best case, fT = 10-20 GHz and fMAX = 40-50 GHz ). For that reason, in particular, IIA PA of C-and X-band frequencies have more modest parameters, the output power of 40 W (60% PAE) and 58 W (38% PAE) respectively [11, 12] are received.
Development of the mm-range nitride device technology abroad
The general ideology of development nitride devices for mm-range abroad is shown in fig.1.
Improving nitride heterostructures to increase the operating frequency of devices mainly consisted in reducing the thickness of the upper barrier layer tB to preserve the aspect ratio of LG/tB>10÷15  to prevent short-channel effects with decreasing gate length LG. It is quite important to support the values LG/tB at the highest possible level in order to maintain high breakdown voltage, which proved  to be determined by not the heterostructure thickness but the aspect ratio. Awareness of this fact, which was established empirically based on the processing of the experimental results of numerous works, led to the development of new types of more subtle and effective wideband heterostructures from AlGaN/AlN/GaN(tB to 7 nm) to AlN/GaN (tB to 3.5 nm)  and InAlN/(AlN)/GaN (tB to 4.7 nm) having the highest potential parameters of the two-dimensional electron gas [17, 18]. The problems of creating such heterostructures and the results are provided in numerous publications partly mentioned in the reviews [13, 14]. It should be noted that to maintain a high concentration of two dimensional electron gas with decreasing tB it was necessary to increase the Al content in the barrier layer to the following values, 60% in AlGaN/AlN/GaN, 100% in AlN/GaN and 83% in InGaN/(AlN)/GaN.
A high Al content in the barrier layer initially caused serious problems with the production of ohmic contacts , for which solving there were proposed the inverted N-face heterostructures  vigorously developed by the University of California. At the latter the resistance of burnt-in ohmic contacts to 0.1 Ohm∙mm  was obtained. However, a solution was also found for the usual Ga-face heterostructures AlGaN/GaN , partial etching the AlN barrier layer in the BCl3 plasma to improve the burnt-in ohmic contacts to 0.59 Ohm∙mm, which is typical for traditional "thick" HEMT heterostructures containing 27-31% Al in the barrier layer. Further development of this idea led to the creation of non-burnt-in ohmic contacts for all types of heterostructures. This technology represents full etching the Al-containing barrier layer to the GaN channel for Ga-face heterostructures or upper undoped GaN to the AlGaN barrier for the N-face heterostructures followed by cultivation through the SiO2 mask strongly doped contact layer with n+GaN with the silicon concentration (6-8) 1020 cm-3. Then the ‘explosive’ removal of SiO2 in HF solution in an ultrasonic bath and powder metal ohmic contacts of the compositions Cr/Pt/Au  или Ti/Pt/Au  which turned out to be most stable for all types of heterostructures at the temperatures up to 400-450ºC. The results obtained for different heterostructures are summarised in the presentation of the University of Notre Dame . There were obtained the resistances of non-burnt-in resistance ohmic contacts 0.27 Ohm mm for Ga-face HEMT  and to 0.09 Ohm∙mm for N-face HEMT . It should be emphasized that the development of the non-burnt-in ohmic contact technology provided conditions for the production of HEMT according to the self-combined technology minimizing the resistance of the transistor channel. For example, in the work , which is the quintessence of all the above technological advances, the record low resistance of the transistor in the open position 0.29 Ohm mm was obtained, the resistance of ohmic contacts 0.025 Ohm∙mm, transconductance Gm = 1105 mCm/mm, the initial current Idss0 = 2.77 A/mm, the current cut-off frequency fT = 155 GHz.
In recent years, the nitride device passivation technology has developed in order to eliminate traps at heterointerfaces, in particular, in-situ passivation in the growth chamber has become an industrial technology in the production of heterostructures.
As a result of technological breakthroughs in recent years by foreign researchers achieved were the frequency parameters of the nitride HEMT close to the record parameters of the arsenide mNEMT and rNEMT on the GaAs and InP substrates. For example, in 2008 the value fT = 190 GHz  was obtained, and then in 2010 with a gate length of LG= 40 nm the values fT = 220 GHz and fMAX = 400 GHz  were derived, which were blocked by the value of fT = 343 GHz  as early as 2011.
The nitride nanoheterostructures provided the basis for the development and creation of highly radiation-resistant MIC PA Ka-band of 10-15 times higher than the MIC based on GaAs rNEMT in terms of weight and size parameters (UMS, 2012). The transceiver AESA modules for the radar range of 94 GHz (QuinStar Technology together with HRL) with output power up to 5 W and a specific power output of more than 2 W/mm are also being developed.
The leading overseas manufacturers (Northrop Grumman, Cree, TriQuint, Fujitsu etc.) are rapidly improving the technology and developing a wide range of MIC on nitride heterostructures with the operating frequencies up to 100 GHz and above, and not only PA. For example, a MIC is designed for a low-noise amplifier with the range of 75-82 GHz with NF = 3.8 dB at 80 GHz with amplification coefficient >20 dB , which exceeds the parameters of the best MIC on GaAs and InP.
The established industrial production of nitride heterostructures on silicon substrates with a diameter of up to 8" (NITRONEX Corp, USA ) provided the conditions for the mass production of cheap MIC, which can completely replace HF devices and microwave devices on the traditional arsenide and silicon heterostructures.
Development of millimetre range nitride devices in IUHFSE RAS
Based on the review of the existing international developments in the field of wide nanoheterostructures AlGaN/AlN/GaN in the microwave bands and EHF bands and work experience of IUHFSE RAS with the AlGaN/GaN heterostructures obtained during the execution of R&D, it was concluded that it was possible and necessary to put greater emphasis on developing a technology to ensure the design and manufacture of a wide range of radiation-proof MIC for millimetre range transceiver modules based on wide-range HEMT heterostructures of domestic producers (Elma-Malachit Co, Svetlana-Rost Co and Kurchatov Institute). The current R&D activities of IUHFSE RAS are generally shown in fig.2. In this article we will not talk about them in detail, we will consider only the basic problems, solutions and outputs.
For 3-4 years we have studied a large number of nitride heterostructures AlGaN/GaN with the AlGaN barrier thicknesses from 28 to 33 nm (Type 1) as well as the specially grown heterostructures AlGaN/AlN/GaN with a thickness of the barrier layer from 28 to 7 nm (Type 2) on the sapphire and SiC substrates (Table 1). The research resulted in the determined criteria for the selection of optimal heterostructure parameters for different frequency ranges.
In particular, it was found that for the Ka- frequency band optimal were the heterostructures of Type 2 with tB = 15 nm, of which by now V-1400 has shown the best parameters (Elma-Malachit) on the SiC substrate providing the creation of transistors with the initial current up to 1.1 A/mm at a maximum slope of up to 380 mA/mm and cutoff voltage 4 V. The field transistors with LG= 180 nm (LG/tB=12) have fT/fMAX=62/130 GHz with the absence of short-channel effects being optimal for the Ka-band PA. At the same time, transistors with LG=100 nm (LG/tB=8) on the same heterostructure have higher frequencies fT/fMAX=77/161 GHz meaning that they can be used in a higher V- and E-band but are not optimal for those frequencies due to the short-channel effects.
The heterostructures with potentially higher frequencies with lower thicknesses tb = 13 nm and 11 nm made by Svetlana-Rost Co yet have much lower initial currents of transistors (500 mA/mm and 300 mA/mm respectively). More successful was the work on the creation of thin heterostructures AlGaN/AlN/GaN (11 nm) and AlN/GaN (3.5 nm) on sapphire substrates performed jointly with Kurchatov Institute. For the first time in Russia obtained transistors with initial currents exceeding 1 A/mm on the AlN/GaN/Sapphire heterostructures to ensure prospective development of the W-band frequencies.
The development of microwave devices based on the nitride heterostructures on silicon substrates (Elma-Malachit Co) has started to ensure cheaper output and mass production.
Technology advances and developments
The key output is to create a reproducible technology to develop and manufacture transistors and MIS on nitride heterostructures with set parameters in the range from DC to 40 GHz. The frequency bands 56-64 GHz and 92-96 GHz are being developed, and an increase in the power output of PA in the Ka-band is going to be achieved. It required to meet the technological challenges as follows:
•development of the heterostructure passivation technology directly in the growth chamber ‘in-situ’ to eliminate the lagg effects and increase concentration of the two-dimensional electron gas;
•improving the ohmic contact production technology (within the limit, the creation of non-burnt-in contacts) including the operation of plasma chemical etching of the barrier layer AlGaN or AlN followed by epitaxial completing the contact layer n+GaN through the mask SiO2.
It is especially important to solve the problems for thin heterostructures with a higher Al mole fraction in the barrier layer for the W-band of frequencies and above. Work is being done in cooperation with Kurchatov Institute. First obtained in Russia the resistance of non-burnt-in ohmic contacts to 0.11 Ohm/mm on the heterostructures shown in Table 1, which corresponds to the best world’s standards. The results were presented at the 9th Russian Conference "Gallium Nitrides, Indium and Aluminum – Structures and Devices" in 2013.
Another set of technological challenges is associated with the transition from the coplanar topology to the microstrip topology implying the presence of ‘ground plane’, to which there should be brought out the sources of the transistors and ‘ground’ of the MIC elements through plated-through holes. It should be noted that this is the key to creating quadrature mixers and voltage-controlled generators for transmit-receive modules of the millimetre range on nitride heterostructures and increase in the output power of PA MIC. By analogy with the arsenide technology, this problem was solved abroad by ‘drilling’ through-holes on the back side of the thinned substrate (fig.3a), which demonstrates the following serious drawbacks:
•it is impossible to drill holes in the sapphire substrates, the lowest rate of plasma-chemical etching of a SiC substrate (less than 1 micron/min), the rate of etching the buffer layer AlGaN/GaN is even lower;
•there is a need for precise homogenous thinning of substrates to provide the required parameters of microwave microstrips and uniform hole etching, which has impact on the performance of technological processes and suitable MIC going from the plate.
Solving the problems can be somewhat facilitated by switching to heterostructures on silicon substrates thus creating an additional incentive to the development of that kind of production. However, we believe that a more promising solution would be to create a ground plane on the face of the plate with the ready-made active and passive microwave components, e.g. by applying polyimide coating (fig.3b). This solution is very well in line with the crystal surface mounting technology in the assembly of microwave modules of the Raduga Co research and production company (fig.3c, d), and it will make it easier and cheaper to arrange for the industrial production of small-sized multifunctional microwave modules for the millimetre range transceiver systems on nitride heterostructures.
Development of MIC sets
The developed technology provided the basis for the creation of MIC sets of on nitride heterostructures for receipt and transmission microwave devices instead of arsenide heterostructures traditionally used by MIC. The status of activities in IUHFSE RAS in this area is illustrated in fig.2, which shows MIC already completed (solid lines) or MIC at various stages of development (dotted lines). A detailed description of the parameters is beyond the scope of this article. However, some conclusions can be drawn from fig.4, which shows the noise and amplifying parameters of some MIC on the arsenide and nitride heterostructures and in the frequency range up to 40 GHz developed by IUHFSE RAS as well as abroad. As it can be seen, the monolithic integrated circuit of the low-noise amplifier (MIC LNA) on nitride heterostructures can be quite competitive with the MIC based on pNEMT on GaAs. Some disadvantages in terms of noise parameters we believe represent a consequence of still existing imperfections related to heterostructures and the nitride MIC manufacturing technology. We would like to note that presented here are the parameters of nitride MIC produced by the coplanar technology on heterostructures with the barrier thickness higher than 18 nm. Calculations show that a further reduction of NF of transistors and MIC on the nitride heterostructures in the mm-range is possible by using thinner barrier layers together with the technology of non-burnt-in ohmic contacts and in-situ passivation. Devices, which use new technological solutions, are currently at the stage of production.
As an illustration of the level of technological advances of IUHFSE RAS in the millimetre band development on nitride heterostructures can provide microwave parameters developed for the first time in Russia MIC PA frequency range 85-95 GHz. Measurements were made on the equipment of the research and production company Istok. Focus should be put on good match in a given frequency range thus proving the appropriate level of MIC design. Currently, the work in this direction on thinner heterostructures by Kurchatov Institute  is in progress.
The outcomes of research on the creation of a technology to design and manufacture of the millimetre range MIC on nitride heterostructures in IUHFSE RAS indicate the possibility of creating MIC sets in Russia for transceiver systems that exceed by parameters the corresponding devices on arsenide heterostructures, which corresponds to the world level of development in this direction. For successful industrial development and mass production of MIC for transceiver modules of different frequency bands, it is vital to coherently address this issue, heterostructures – MIC manufacturing technology – microwave module assembly technology. It can be expected that only this approach will help quickly establish the inexpensive and mass production of radiation-proof transceiver modules for different purposes which are so necessary to our country.
The research was carried out under the Public Contract No 14.427.12.0001 dated 30 September 2013 by order of the Ministry of Education and Science of the Russian Federation.