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The GaN based devices with respect to most parameters exceed the devices based on traditional semiconductor materials. The AlGaN-transistors are the devices, operating in the depletion mode. For most applications it is necessary to implement the operating mode E , when the current in the channel is closed at zero gate voltage. In this paper the novel method namely using the p -GaN layer under the gate has been considered. The plasma-chemical removal of p -GaN layer in the non-gated active region has been chosen as a formation method of this layer. In this case the challenges, namely the non-uniformity in the depth of etching and poor control of the etching rate, arise. To exclude these problems, the heterostructures with additional AlN barrier layer has been developed. The research results of the heterostructure parameters affecting the carrier concentration in the channel, and, respectively, the transistor output characteristics have been presented and the developed design process has been shown. According to it the normally-off transistors have been formed. The maximum drain current in the open state is 350 mA/mm at 4 V gate voltage and the breakdown voltage is about 550 V in closed state at 0 V gate voltage.
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The production of ohmic contacts to heterobipolar nanostructures has a number of peculiarities. In addition to the basic requirement of minimizing the contact resistance, the contacts to this type of structures have a transient layer, the penetration depth of which should not exceed the emitter layer thickness, otherwise the short-circuit of the emitter-base p - n -junction is possible. We have examined the influence of the main technological parameters of annealing on contact characteristics. As a result, optimization of the low-resistance ohmic contact to heterobipolar transistor layers obtaining process has been carried out. The ohmic contacts to the n -layers of heterobipolar nanoheterostructures based on gallium arsenide, obtained by the layer-by-layer electron-beam deposition of Ge/Au/Ni/Au, have been investigated. The diffusion profiles of the Ge impurity doping distribution as a function of time and the temperature of RTA have been calculated and SEM investigated. It has been determined that RTA for 60 sec at temperature of 398 °C makes it possible to obtain the ohmic contacts with low resistance, smooth morphology of the surface and the minimum size of the transition layer.
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For most commonly used semiconductor materials the contact metal systems, giving low-resistance ohmic contacts and their formation methods are known and investigated. For the materials, relatively recently applied for serial production of semiconductor devices, particularly for silicon carbide, the task of producing low-resistance ohmic devices, in spite of a significant quantity of experimental data remains an urgent one. For this purpose the works directed at production of the ohmic contacts to silicon carbide with the improved operation characteristics were carried out and are conducted. In the work the influence of the metallization composition (Ni and TiAl) the influence of the metallization composition (Ni and TiAl), the annealing process parameters, the doping of the silicon carbide contact layer by nitrogen N ions, as well as the crystallographic face (Si or C) on the resistance of ohmic contacts to 6H-SiC has been considered. It has been found that the most important influence on the process of forming the ohmic contacts to n-6H-SiC has the annealing process, as a result of which the contact resistance becomes approximately 6 times reduced. The process of contact layer doping is the second in importance. This process can reduce the contact resistance on the average by 4 times. As a result of the study it has been established that the low-resistance contacts can be obtained on both surfaces with approximately equal low resistances. The TiAl metallization is the most optimal for the C surface and Ni for the Si surface. This makes it possible to obtain the ohmic contacts on both polar surfaces with approximately the same resistances, equal to 2,5·10 Ohm·cm.
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The method for fabrication of HEMTs, in which T-gate is formed using the nanoimprint lithography technology, has been presented. The characteristics of the created GaAs pHEMT transistors have been investigated. The developed transistor has the gate base length of 250 nm order and maximum transconductance over 350 mS/mm, the current-gain cutoff frequency ( f ) and maximum oscillation frequency ( f ) of 40 and 50 GHz, respectively.
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The study results for the THz detectors with a spatial serial and parallel connected transistor structure based on GaAs heterostructure with an asymmetric T-shaped gate in each transistor have been presented. It has been shown that the photovoltaic/photoconductive detectors demonstrate terahertz responses without using additional antenna elements. The sensitivity of up to 1000 V/W and 50 mA/W for the serial and parallel connection of transistors has been measured. An equivalent noise power below 10 W/Hz has been obtained.
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