Nonalloyed ohmic contacts for high-electron-mobility transistors based on AlGaN/GaN heterostructures

A microwave field-effect transistor with nonalloyed ohmic contacts is fabricated using the technique of regrowing a heavily doped region under the contact metallization by molecular beam epitaxy through a preliminarily formed dielectric mask. The fabricated field-effect transistor with a gate length of 0.18 µm and a total width of 100 µm has a current–amplification cutoff frequency of 66 GHz and ohmic contact resistivity of 0.15-0.18 Ω mm.     

AlGaN/GaN异质结极化行为与二维电子气

AlGaN/GaN异质结及其相关器件因其优越的电学特性成为近几年的研究热点.2DEG作为其特征与材料本身的极化现象关系密切.本文主要从晶体微观结构角度介绍AlGaN/GaN异质结极化现象的产生、机理和方向性,着重讨论极化对异质结界面处诱生的二维电子气的影响.极化不仅可提高2DEG的浓度,而且还能使其迁移率得到提高.     

Parasitic source and drain resistance in high-electron-mobility transistors

The parasitic source and drain resistances of a high-electron-mobility transistor were analyzed in terms of a two layer transmission line model. The analysis showed that a highly conductive cap layer can function as an extension of the alloyed contact provided that tunneling between the cap layer and the channel is significant. The tunneling between the cap layer and the channel was analyzed in terms of a thermionic-field emission model in which a one dimensional time-dependent WKB transmission probability for the barrier was considered as well as Maxwell-Boltzman statistics for the tunneling carrier distribution. The GaAs cap, GaAlAs layer and 2-DEG channel can then be treated as a distributed resistance element with a characteristic coupling length. A reduction of the parasitic resistance can be obtained for a device structure with a short characteristic coupling length even if there exists an ideal alloyed contact to the 2-DEG channel. A multilayer cap consisting of an undoped GaAs layer inserted between the n-type GaAs and n-type GaAlAs is also proposed to reduce the barrier height for tunneling between the cap layer and the channel. The multilayer cap structure is predicted to appreciably reduce the parasitic resistance at room temperature and still be effective at 77 K.     

High-temperature transport properties of 2DEG in AlGaN/GaN heterostructures

Transport properties of the two-dimensional electron gas (2DEG) in fully strained and partially strain-relaxed Al_0.22Ga_0.78N/GaN heterostructures at temperatures from 300 to 680 K have been investigated by Hall effect measurements. The 2DEG mobility was found to decrease rapidly with increasing temperature at the initial stage and then decrease slowly as temperature is further increased. Those features indicate strongly that the 2DEG mobility is primarily limited by LO phonon scattering processes at high temperatures. Meanwhile, the calculated results show that more electrons transfer to the higher-order sub-bands with increasing temperature, and hence the effect of screening on LO phonon scattering is weakened and the alloy scattering of the AlGaN layer on the 2DEG becomes stronger. Thus variation of 2DEG occupation in different sub-bands with increasing temperature also decreases mobility of the 2DEG.     

Power switching transistors based on gallium nitride epitaxial heterostructures

The results of the development of power switching transistors based on epitaxial gallium nitride heterostructures to create an energy-efficient conversion technique are presented. The developed powerful GaN transistor operates in enrichment mode with unlocking threshold voltage V _th = +1.2 V and a maximum drain-source current I _ds = 0.15 A/mm at the drain-source voltage V _ds = +8 V. The drain-source breakdown voltage in the closed state is V _b = 300 V at the drain-source distance L _ds = 8.5 μm and drain-source voltage V _ds = 0 V.     

Efficient electro-optic semiconductor medium based on type-II heterostructures

An electro-optic medium based on the type-II semiconductor superlattice is proposed. Electrooptic modulation of the intensity of optical reflectance of the electro-optic medium integrated into a vertical Fabry-Perot cavity is studied by the optical-reflectance-spectroscopy method. The experimental data are approximated using an oscillator model of exciton absorption. The efficiency of the electro-optic medium in the case of detuning from the absorption peak by 50 meV in electric fields of 0–50 kV/cm is 10^?9 m/V at a medium filling factor of 100%.     

Multi-wavelength integrated optical-laser emission modulator based on semiconductor heterostructures

Small-signal-controlled optical modulator structure capable of operation at several wavelengths is suggested. It is shown that the application of spectrally selective mirrors makes it possible to develop optically integrated cavities that provide lasing at prescribed discrete wavelengths and a control section that can vary the internal optical loss. The main relationships between the lasing-mode switching conditions and optical-modulator structure parameters are determined. Optical modulators for the 1060–1080 nm spectral range, with a distributed Bragg reflector as one of the mirrors, are developed and experimentally investigated. It is shown that small-signal control by a forward current can switch the lasing between optically integrated Fabry-Perot cavities at a 5–20 ns rate.     

Photosensitivity of heterostructures based on finely ground semiconductor phases

A new type of heterostructure is suggested and developed. The heterostructures are based on the direct contact of a bulk semiconductor with a dielectric layer in which a finely ground semiconductor phase is dispersed. In Si-and GaAs-based heterostructures of this type, rectification and photovoltaic effects are observed. It is shown that illumination of such structures so that the side of the dielectric layer with the built-in finely ground semiconductor phase is exposed to light induces a broadband photovoltaic effect deep within the fundamental absorption band of the bulk semiconductor.     

Spin-dependent transmission of holes in III-V diluted magnetic semiconductor based heterostructure

Based on the mean field approximation, we investigated analytically the spin-transport of holes through heterostructure formed by magnetic layers Ga_(1−x)Mn_xAs separated by a non-magnetic spacer GaAs. Injected holes of up (down) spin have different transmission coefficient which oscillates for spin-down and increases fast for spin-up. The significant quantum size and (RKKY) interaction are considered simultaneously. The results indicate also that as the strength of the magnetic and non-magnetic layers increases, the spin-transmission changes. The results can be used to create efficient spin-filters.     

High-power high-voltage bipolar transistors based on complex semiconductor structures

Physical processes in transistors in which a new structural element (an array of bulk inclusions in the collector region) is introduced were studied. This array gives rise to a decrease in the electric field at the metallurgical boundary of the collector p-n junction and inhibits the development of the secondary breakdown.     

2DEG base hot electron transistors fabricated using MBE and in situion beam lithography

A vertical hot electron transistor incorporating a two-dimensional electron gas (2DEG) base has been fabricated in the GaAs-AlGaAs materials system. The difficulties caused by the need to form selective ohmic contacts to the different conducting layers have been overcome using a combination of in situ focused ion beam (FIB) isolation and molecular beam epitaxial (MBE) regrowth. This has allowed a high yield of working devices to be achieved with a typical common emitter current gain of h_FE=6 at low temperatures     

III-V semiconductor and LDS research activities in Slovakia

Research in the field of III-V semiconductors and Low Dimensional Structures (LDS) in the Slovak Republic (eastern part of the former Czechoslovakia) had to develop under rather unfavourable conditions of economic and political barriers. However, the research history in this field is more than 20 years old and this is an excellent background to the research activities that are presently expanding rapidly. They are concentrated at the Department of Microelectronics of Slovak Technical University (STU) and at Institutes of the Slovak Academy of Sciences.     

Ultra-wide electroluminescence spectrum of LED heterostructures based on GaPAsN semiconductor alloys

In this paper, we present the results of the white light-emitting diodes (LEDs) construction based on GaPAsN semiconductor alloys on a GaP substrate. Heterostructure electroluminescence with a continuous emission spectrum in the range from 350 nm to 1050 nm is observed. The output of light through the side walls and the face side of the sample enabled us to achieve white light emission by means of ultra-wide electroluminescence spectrum covering all visible spectrum and part of the near IR spectral range. While extracting emission through substrate, the short-wavelength part of the visible spectra is absorbed at GaP layer.     

Selection of modes in transverse-mode waveguides for semiconductor lasers based on asymmetric heterostructures

Semiconductors - Asymmetric Al0.3Ga0.7As/GaAs/InGaAs heterostructures with a broadened waveguide produced by the method of MOCVD epitaxy are studied. It is established that the precision shift of...     

Electrical characteristics of single-gate interference transistors based on various semiconductor materials

The current-voltage and high-frequency characteristics of single-gate interference T transistors based on quantum wires made of various semiconductors, specifically, Si, Ge, GaAs, InAs, GaSb, InSb, GaP, and InP, were studied theoretically. Two scattering mechanisms were taken into account in the T-transistor model in order to assess their effect on the electrical characteristics of devices. The adequacy of the suggested model was verified by comparing the results of simulation with experimental data. The calculations were performed using the QW-NANODEV subsystem for simulating the devices based on quantum wires.     

Optoelectronics based on 2D TMDs and heterostructures

2D materials including graphene and TMDs have proven interesting physical properties and promising optoelectronic applications. We reviewed the growth, characterization and optoelectronics based on 2D TMDs and their heterostructures, and demonstrated their unique and high quality of performances. For example, we observed the large mobility, fast response and high photo-responsivity in MoS₂, WS₂ and WSe₂ phototransistors, as well as the novel performances in vdW heterostructures such as the strong interlayer coupling, am-bipolar and rectifying behaviour, and the obvious photovoltaic effect. It is being possible that 2D family materials could play an increasingly important role in the future nano- and opto-electronics, more even than traditional semiconductors such as silicon.     

Nonlinear-optical effects in semiconductor lasers based on InGaAs/GaAs/AlGaAs quantum-confinement heterostructures

Generation of a difference-frequency wave by two electromagnetic waves propagating in a heterolaser is analyzed theoretically. Calculations are carried out for InGaAs/GaAs/AlGaAs heterostructures of design optimized to attain maximum lasing power. It is shown that phase matching between the primary waves and the difference-frequency wave may persist over a distance of ～1 mm, comparable to the cavity length (2–3 mm), and the conversion coefficient can be as large as several percent.     

Prospects for the development of high-power field-effect transistors based on heterostructures with donor-acceptor doping

We report the first results on the development of high-power field-effect transistors on gallium-arsenide heterosrtuctures with a quantum well and additional potential barriers, formed from layers with different doping types, optimized to reduce transverse spatial electron transport and enhance quantum confinement. The transistors yield a doubled output power at a trapezoidal gate length of 0.4–0.5 μm and a total gate width of 0.8 mm at a frequency of 10 GHz in the continuous mode of operation. The gain exceeds 9.5 dB at a specific output power above 1.6 W/mm and a power-added efficiency of up to 50%. Prospects for the development of such devices are presented.     

A high-yielding evaporation-based process for organic transistors based on the semiconductor DNTT

We report on the performance of organic thin film transistors manufactured in an all-evaporated vacuum roll-to-roll process. We show that dinaphtho [2,3-b:2′,3′-f] thieno[3,2-b]thiophene (DNTT) is a suitable semiconductor material for deposition onto a flash evaporated polymer insulator layer to make bottom-gate top-contact transistors. Significantly, in batches of 90 transistors, the process approached a 100% yield of high mobility transistors with high on/off ratios and low gate-leakage. By contrast, a solution-deposited insulator layer led to significant gate leakage in a high proportion of transistors leading to poor yield. The performance of DNTT devices is shown to be superior to that of previously reported pentacene devices. Transistor performance is further enhanced by inclusion of a low-polarity surface modification, such as polystyrene, to the acrylate. The devices show good environmental stability but we demonstrate also that they can be in-line encapsulated with an acrylate and a SiOx overlayer without damaging the underlying transistor. Finally, a first demonstration is made of organic vapour jet printing of the DNTT to manufacture transistors with a high semiconductor deposition rate.     

Defects and defect reduction processing in semiconductor heterostructures

Defects such as dislocations and interfaces play a crucial role in the performance of heterostructure devices. The full potential of GaAs on Si heterostructures can only be realized by controlling the defect density. The nature of misfit dislocations at the heterointerface has been studied and a mechanism for the formation of 60° and 90° misfit dislocations has been proposed. Threading dislocations in the epilayer are the most prominent defects and their density has to be controlled to fully utilize the properties of semiconductor heterostructures. Various processes to reduce defect densities in the epilayers have been discussed and in particular, the use of strained layer superlattices to reduce the threading dislocation density has been presented in this paper. Several superlattice structures have been used to reduce the density of threading dislocations in the GaAs epilayer. In this study, we have optimized the use of strained layer superlattices with respect to the position, period and number to reduce and control the dislocation density. The use of strained layer superlattices in conjunction with rapid thermal annealing was found to be the most effective method in reducing threading dislocation density. Transmission electron microscopy has been used to study the dislocation density reduction and the interaction of threading dislocations with the strained layers.