Increase the threshold voltage of high voltage GaN transistors by low temperature atomic hydrogen treatment

High-electron-mobility transistors (HEMTs) based on AlGaN/GaN epitaxial heterostructures are a promising element base for the fabrication of high voltage electronic devices of the next generation. This is caused by both the high mobility of charge carriers in the transistor channel and the high electric strength of the material, which makes it possible to attain high breakdown voltages. For use in high-power switches, normally off-mode GaN transistors operating under enhancement conditions are required. To fabricate normally off GaN transistors, one most frequently uses a subgate region based on magnesium-doped p-GaN. However, optimization of the p-GaN epitaxial-layer thickness and the doping level makes it possible to attain a threshold voltage of GaN transistors close to V _th = +2 V. In this study, it is shown that the use of low temperature treatment in an atomic hydrogen flow for the p-GaN-based subgate region before the deposition of gate-metallization layers makes it possible to increase the transistor threshold voltage to V _th = +3.5 V. The effects under observation can be caused by the formation of a dipole layer on the p-GaN surface induced by the effect of atomic hydrogen. The heat treatment of hydrogen-treated GaN transistors in a nitrogen environment at a temperature of T = 250°C for 12 h reveals no degradation of the transistor’s electrical parameters, which can be caused by the formation of a thermally stable dipole layer at the metal/p-GaN interface as a result of hydrogenation.     

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.     

White electroluminescence from ZnO/GaN structures

White electroluminescence (EL) from ZnO/GaN structures fabricated by pulsed laser deposition of ZnO:In onto GaN:Mg/GaN structures MOCVD-grown on Al₂O₃ substrates has been observed. The white light is produced by superposition of the two strongest emission lines, narrow blue and broad yellow, peaked at 440 and 550 nm, respectively. The intensity ratio of different EL lines from ZnO/GaN/Al₂O₃ structures depends on the ZnO film quality and drive current. The white EL is due to the high density of structural defects at the n-ZnO/p-GaN interface. A band diagram of the n-ZnO/p-GaN/n-GaN structure is constructed and a qualitative explanation of the EL is suggested.     

Effect of hydrogen on anisotropy of the <Emphasis Type="Italic">p</Emphasis>-GaN growth rate in the case of side-wall MOCVD

The effect of the composition of the carrier gas on anisotropy of p-GaN growth rates in side-wall metal-organic chemical vapor deposition was studied. p-GaN layers with a nominal thickness of ～400 nm were grown on side-walls of GaAs mesa stripes formed preliminarily by selective-area epitaxy on Si₃N₄. It is shown that, if hydrogen is used as the carrier gas, the p-GaN growth occurs mainly in the lateral direction, so that the p-GaN layer is either absent or is thin at the top faces of mesa stripes; in contrast, if nitrogen is used as the carrier gas, growth in the normal (0001) direction is prevalent, so that a p-GaN layer is formed at all faces of the mesa stripe. The results of our study are indicative of a significant role of hydrogen in the process of epitaxial growth of GaN and can be used in the development of technology of devices with p-n junctions based on GaN and with the use of selective-area growth.     

Modified Interface Properties of Au/<Emphasis Type="Italic">n</Emphasis>-type GaN Schottky Junction with a High-<Emphasis Type="Italic">k</Emphasis> Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> Insulating Layer

The interface properties of a Au/n-GaN Schottky junction (SJ) were modified by placing a high-k barium strontium titanate (Ba_0.6Sr_0.4TiO₃) insulating layer between the Au and n-GaN semiconductor. The surface morphology, chemical composition, and electrical properties of Au/Ba_0.6Sr_0.4TiO₃ (BST)/n-GaN metal/insulator/semiconductor (MIS) junctions were explored by atomic force microscopy, energy-dispersive x-ray spectroscopy, current–voltage (I–V) and capacitance–voltage (C–V) techniques. The electrical results of the MIS junction are correlated with the SJ and discussed further. The MIS junction exhibited an exquisite rectifying nature compared to the SJ. An average barrier height (BH) and ideality factors were extracted to be 0.77 eV, 1.62 eV and 0.92 eV, 1.95 for the SJ and MIS junction, respectively. The barrier was raised by 150 meV for the MIS junction compared to the MS junction, implying that the BH was effectively altered by the BST insulating layer. The BH values extracted by I–V, Cheung’s and Norde functions were nearly equal to one another, indicating that the techniques applied here were dependable and suitable. The frequency-dependent properties of the SJ and MIS junction were explored and discussed. It was found that the interface state density of the MIS junction was smaller than the SJ. This implies that the BST layer plays an imperative role in the decreased N_SS. Poole–Frenkel emission was the prevailed current conduction mechanism in the reverse-bias of both the SJ and MIS junction.     

Emission sensitization and mechanisms of electron-excitation migration in structures based on III-nitrides doped with rare-earth elements (Eu,Er, Sm)

The effect of doping with Eu, Er, and Sm rare-earth ions on the shape of the luminescence spectrum for heterostructures with GaN/In _x Ga_{1 ? x} N (0.1 < x < 0.4) quantum wells and from p-GaN〈Mg〉/n-GaN and p-AlGaN/n-GaN junctions is investigated. The results of measurements of the electroluminescence of these structures correlate with the previous data on photoluminescence and Mössbauer spectroscopy. It is shown that it is the GaN “yellow” (5000–6000 Å) band that plays the important role in the excitation of intracenter states in the structures with several GaN/InGaN quantum wells doped with Eu and Sm. In this case, Eu is most likely the sensitizer for Sm. Additional introduction of 3d metal (Fe⁵⁷) in p-GaN〈Mg〉/n-GaN:Eu results in the realization of intracenter transitions in Eu³⁺: ⁵ D ₀ → ⁷ F ₁ (6006 Å), ⁵ D ₀ → ⁷ F ₂ (6195 Å), ⁵ D ₀ → ⁷ F ₃ (6627 Å), and ⁵ D ₁ → ⁷ F ₄ (6327 Å) due to the occurrence of new, efficient channels of excitation transfer to intracenter states and in the effect of Fe on the local environment of rare-earth ions including due to the f–d hybridization enhancement.     

Electrical properties of <Emphasis Type="Italic">n-</Emphasis>GaN/<Emphasis Type="Italic">p</Emphasis>-SiC heterojunctions

Epitaxial GaN layers were grown by hydride vapor phase epitaxy (HVPE) on commercial (CREE Inc., USA) p⁺-6H-SiC substrates (Na ? Nd ≈ 7.8 × 10¹⁷ cm^s?3) and n⁺-6H-SiC Lely substrates with a predeposited p⁺-6H-SiC layer. A study of the electrical properties of the n-GaN/p-SiC heterostructures obtained confirmed their fairly good quality and demonstrated that the given combination of growth techniques is promising for fabrication of bipolar and FET transistors based on the n-GaN/p-SiC heterojunctions.     

Study of RF transistor with submicron T-shaped gate fabricated by nanoimprint lithography

The results devoted to the development of a method for creating an RF transistor, in which a T-shaped gate is formed by nanoimprint lithography, are presented. The characteristics of GaAs p-HEMT transistors have been studied. The developed transistor has a gate “foot” length of the order of 250 nm and a maximum transconductance of more than 350 mS/mm. The maximum frequency of current amplification f _t is 40 GHz at the drain-source voltage V _DS = 1.4 V and the maximum frequency of the power gain f _max is 50 GHz at V _DS = 3 V.     

Tunneling Current in Oppositely Connected Schottky Diodes Formed by Contacts between Degenerate <Emphasis Type="Italic">n</Emphasis>-GaN and a Metal

The nonlinear behavior of the I–V characteristics of symmetric contacts between a metal and degenerate n-GaN, which form oppositely connected Schottky diodes, is investigated at free-carrier densities from 1.5 × 10¹⁹ to 2.0 × 10²⁰ cm^–3 in GaN. It is demonstrated that, at an electron density of 2.0 × 10²⁰ cm^–3, the conductivity between metal (chromium) and GaN is implemented via electron tunneling and the resistivity of the Cr–GaN contact is 0.05 Ω mm. A method for determining the parameters of potential barriers from the I–V characteristics of symmetric opposite contacts is developed. The effect of pronounced nonuniformity of the current density and voltage distributions over the contact area at low contact resistivity is taken into account. The potential-barrier height for Cr–n⁺-GaN contacts is found to be 0.47 ± 0.04 eV.     

Mechanism of dislocation-governed charge transport in schottky diodes based on gallium nitride

A mechanism of charge transport in Au-TiB _x -n-GaN Schottky diodes with a space charge region considerably exceeding the de Broglie wavelength in GaN is studied. Analysis of temperature dependences of current-voltage (I–V) characteristics of forward-biased Schottky barriers showed that, in the temperature range 80–380 K, the charge transport is performed by tunneling along dislocations intersecting the space charge region. Estimation of dislocation density ρ by the I–V characteristics, in accordance with a model of tunneling along the dislocation line, gives the value ρ ≈ 1.7 × 10⁷ cm^?2, which is close in magnitude to the dislocation density measured by X-ray diffractometry.     

Hopping conductivity and dielectric relaxation in Schottky barriers on GaN

A study of the current and capacitance dependences on the forward voltage in Au/n-GaN Schottky diodes, the sub-band optical absorption spectra, and the defect photoluminescence in n-GaN bulk crystals and thin layers is reported. It is shown that defect-assisted tunneling is the dominant transport mechanism for forward-biased Schottky contacts on n-GaN. The dependences of the current and capacitance on forward bias reflect the energy spectrum of defects in the band gap of n-GaN: the rise in the density of deep states responsible for yellow photoluminescence in GaN with increasing energy and the steep exponential tail of states with an Urbach energy of E _U = 50 meV near the conduction-band edge. A decrease in the frequency of electron hops near the Au/n-GaN interface results in a wide distribution of local dielectric relaxation times and in a dramatic transformation of the electric-field distribution in the space-charge region under forward biases.     

Effect of Temperature Variation on the Characteristics of Microwave Power AlGaN/GaN MODFET

The prime motivation for developing the proposed model of AlGaN/GaN microwave power device is to demonstrate its inherent ability to operate at much higher temperature. An investigation of temperature model of a 1 μm gate AlGaN/GaN enhancement mode n-type modulation-doped field effect transistor (MODFET) is presented. An analytical temperature model based on modified charge control equations is developed. The proposed model handles higher voltages and show stable operation at higher temperatures. The investigated temperature range is from 100 °K–600 °K. The critical parameters of the proposed device are the maximum drain current (I_Dmax), the threshold voltage (V_th), the peak dc trans-conductance (g_m), and unity current gain cut-off frequency (f_T). The calculated values of f_T (10–70 GHz) at elevated temperature suggest that the operation of the proposed device has sufficiently high current handling capacity. The temperature effect on saturation current, cutoff frequency, and trans-conductance behavior predict the device behavior at elevated temperatures. The analysis and simulation results on the transport characteristics of the MODFET structure is compared with the previously measured experimental data at room temperature. The calculated critical parameters suggest that the proposed device could survive in extreme environments.     

Determination of the absolute value of the semiconductor surface potential by the quasi-static capacitance-voltage characteristics of an MIS structure

A simultaneous analysis of the derivatives C_V′V_g of the experimental and ideal quasi-static capacitance-voltage characteristics (plotted as a function of the normalized differential capacitance of a metal-insulator-semiconductor (MIS) structure) allows identification of regions within the semiconductor band gap E_g, in which interface states are virtually aTSent and the relation between the surface potential ψ_S of the real semiconductor and the voltage V_g applied to the MIS structure may be readily ascertained. This allows an accurate enough determination of the additive constants ψ_S0(V_g0) necessary to calculate the dependence ψ_S(V_g) in the entire range of V_g by numerical integration of the experimental quasi-static C-V characteristic. The comparison of this dependence with the ideal one characterizes in detail the integral electronic properties of the semiconductor-insulator heterojunction: the E_g-averaged density of interface states, the qualitative pattern of their distribution over the band gap, and the flat-band voltage V_FB and its components caused by a charge fixed in the undergate insulator and a charge localized at boundary states. A high accuracy of the V_FB measurements allows detection of even a weak physical response of MIS structures to external factors or to variations in the heterojunction technology. Results of such an analysis for a typical SiO₂/Si interface of an n-Si-MOS (metal-oxide-semiconductor) structure are considered. The application of C_V′-C_V diagrams for analyzing the high-frequency C-V characteristics is considered.     

Electron transport and detection of terahertz radiation in a GaN/AlGaN submicrometer field-effect transistor

Electron transport and photoresponse in the terahertz range in a GaN/AlGaN field-effect transistor with the submicrometer gate (0.25 μm) and two-dimensional electron gas in the channel (the electron concentration n _s = 5 × 10¹² cm^?2) were studied at 4.2 K. The charge-carrier mobility in the transistor’s channel μ = 3500 cm²/(V s) was determined from the dependence of the conductance on magnetic field. It is found that the dependence of photovoltage at the radiation frequency f = 574 GHz on the gate voltage (i.e., on the concentration of two-dimensional electrons) features a characteristic maximum, which is related to a resonance response of the subgate plasma in the transistor channel.     

Tunnel-recombination currents and electroluminescence efficiency in InGaN/GaN LEDs

The mechanism of injection loss in p-GaN/InGaN/n-GaN quantum-well LEDs is analyzed by studying the temperature and current dependences of external quantum efficiency in the temperature range 77–300 K and by measuring transient currents. The data obtained are interpreted in terms of a tunnel-recombination model of excess current, which involves electron tunneling through the potential barrier in n-GaN and the over-barrier thermal activation of holes in p-GaN. At a low forward bias, the dominant process is electron capture on the InGaN/p-GaN interface states. At a higher bias, the excess current sharply increases due to an increase in the density of holes on the InGaN/p-GaN interface and their recombination with the trapped electrons. The injection of carriers into the quantum well is limited by the tunnel-recombination current, which results in a decrease in efficiency at high current densities and low temperatures. The pinning of the Fermi level is attributed to the decoration of heterointerfaces, grain boundaries, and dislocations by impurity complexes.     

Work Function Tuning and Doping Optimization of 22-nm HKMG Raised SiGe/SiC Source–Drain FinFETs

The basic requirements on process design of extremely scaled devices involve appropriate work function and tight doping control due to their significant effect on the threshold voltage as well as other critical electrical parameters such as drive current and leakage. This paper presents a simulation study of 22-nm fin field-effect transistor (FinFET) performance based on various process design considerations including metal gate work function (WF), halo doping (N _halo), source/drain doping (N _sd), and substrate doping (N _sub). The simulations suggest that the n-type FinFET (nFinFET) operates effectively with lower metal gate WF while the p-type FinFET (pFinFET) operates effectively with high metal gate WF in 22-nm strained technology. Further investigation shows that the leakage reduces with increasing N _halo, decreasing N _sd, and increasing N _sub. Taguchi and Pareto analysis-of-variance approaches are applied using an L₂₇ orthogonal array combined with signal-to-noise ratio analysis to determine the best doping concentration combination for 22-nm FinFETs in terms of threshold voltage (V _t), saturation current (I _on), and off-state current (I _off). Since there is a tradeoff between I _on and I _off, the design with the nominal-is-best V _t characteristic is proposed, achieving nominal V _t of 0.259 V for the nFinFET and ?0.528 V for the pFinFET. Pareto analysis revealed N _halo and N _sub to be the dominant factor for nFinFET and pFinFET performance, respectively.     

Radiation-stimulated processes in transistor temperature sensors

The features of the radiation-stimulated changes in the I–V and C–V characteristics of the emitter–base junction in KT3117 transistors are considered. It is shown that an increase in the current through the emitter junction is observed at the initial stage of irradiation (at doses of D < 4000 Gy for the “passive” irradiation mode and D < 5200 Gy for the “active” mode), which is caused by the effect of radiation-stimulated ordering of the defect-containing structure of the p–n junction. It is also shown that the X-ray irradiation (D < 14000 Gy), the subsequent relaxation (96 h), and thermal annealing (2 h at 400 K) of the transistor temperature sensors under investigation result in an increase in their radiation resistance.     

Investigation of the ZnS-CdHgTe interface

The ZnS-Cd_xHg_1?xTe interface was investigated using the capacitance-voltage characteristics of MIS structures in experimental samples. During fabrication of the n⁺-p junctions based on p-Cd_xHg_1?xTe, the density of states within the range N _ss =(1–6)×10¹¹ cm^?2 eV^?1 at T=78 K was obtained. The experiments showed that the conditions in which n⁺-p junctions are fabricated only slightly affect the state of the ZnS-CdHgTe interface. The negative voltages of the at bands V _FB , even if immediately after deposition of the ZnS films V _FB >0, point to the enrichment of the ZnS-p-CdHgTe near-surface layer with majority carriers, specifically, holes. This led to a decrease in the leakage current over the surface. During long-term storage (as long as ～15 years) in air at room temperature, no degradation of differential resistance R _d , current sensitivity S _i , and detectivity D* of such n⁺-p junctions with a ZnS protection film was observed.     

Analysis and Simulation of Vertical Complementary Silicon Bipolar Transistors

The features of modern complementary bipolar technologies (CB-technologies) for analog applications have been analyzed and the main trends in their development are considered. Different industrial CBtechnologies are compared on the basis of two parameters of complementarity, namely the quality factor (βV _A ) and Johnson’s parameter (BV _CEO f _T ). The p-epitaxial-planar CB-technology has been studied by the method of two-dimensional numerical simulation using the TCAD Sentaurus software for vertical n–p–n and p–n–p transistors. A careful calibration of the parameters of technological and electrophysical 2D models on the basis of test structures has shown sufficient accuracy of the used methodology for practice.     

Electron transport in unipolar heterostructure transistors with quantum dots in strong electric fields

A model for the explaining specific features of the electron transport in strong electric fields in the quantum-dot unipolar heterostructure transistor (AlGaAs/GaAs/InAs/GaAs/InAs) is presented. It is shown that the two-step shape of the output current-voltage characteristic I _D (V _D ) and the anomalous dependence of the drain current I _D on the gate voltage V _G are caused by the ionization of quantum dots in the strong electric field at the drain gate edge. The ionization of quantum dots sets in at the drain voltage V _D that exceeds the V_D1 value, at which the I _D (V _D ) dependence is saturated (the first step of the I-V characteristic). With the subsequent increase in V _D , i.e., for V _D >V_D1, the I _D (V _D ) dependence has a second abrupt rise due to the ionization of quantum dots, and then, for V _D =V_D2>V_D1, the current I _D is saturated for the second time (the second step in the current-voltage characteristic). It is suggested to use this phenomenon for the determining the population of quantum dots with electrons. The model presented also describes the twice-repeated variation in the sign of transconductance g _m =dI _D /dV _G as a function of V _G .