High-voltage MIS-gated GaN transistors

Transistors with a high electron mobility based on AlGaN/GaN epitaxial heterostructures are promising component types for creating high-power electronic devices of the next generation. This is due both to a high charge-carrier mobility in the transistor channel and a high electric durability of the material making it possible to achieve high breakdown voltages. For use in power switching devices, normally off GaN transistors operating in the enrichment mode are required. To create normally off GaN transistors, the subgate region on the basis of p-GaN doped with magnesium is more often used. However, optimization of the p-GaN epitaxial-layer thickness and doping level makes it possible to achieve a threshold voltage close to V _th = +2 V for the on-mode of GaN transistors. In this study, it is shown that the use of a subgate MIS (metal–insulator–semiconductor) structure involved in p-GaN transistors results in an increase in the threshold voltage for the on-mode to V _th = +6.8 V, which depends on the subgate-insulator thickness in a wide range. In addition, it is established that the use of the MIS structure results in a decrease in the initial transistor current and the gate current in the on mode, which enables us to decrease the energy losses when controlling powerful GaN transistors.     

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.     

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.     

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.     

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.     

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.     

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.     

Thermal-field forward current in GaN-based surface-barrier structures

The voltage and temperature dependences of the capacitance and forward current in surface-barrier Ni-n-GaN structures are experimentally studied. The results are compared with the Padovani-Stratton thermofield emission theory. It is established that, in a temperature range of 250–410 K, the forward current of the Ni-n-GaN surface-barrier structures (the electron density in GaN is ～10¹⁷ cm^?3) is caused by a thermofield emission of electrons, whose energy is ～0.1 eV below the potential-barrier top.     

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.     

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.     

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.     

Crystalline SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Ultrathin Films Grown on AlGaN/GaN Using <Emphasis Type="Italic">In Situ</Emphasis> Metalorganic Chemical Vapor Deposition

Surface passivation by SiN _x films is indispensable for high-power operation of AlGaN/GaN heterojunction field-effect transistors (HFETs) since it can effectively suppress collapse in the drain current. So far, the plasma-enhanced chemical vapor deposition technique has been used for the SiN _x deposition; however, possible damage induced by the plasma processing may affect direct-current performance or reliability. In this paper, we present subsequent deposition of SiN _x ultrathin films on AlGaN/GaN in the same metalorganic chemical vapor deposition reactor. It is experimentally found that this in situ SiN _x passivation doubles the sheet carrier density at the AlGaN/GaN interface from that of the unpassivated sample. High-resolution cross-sectional transmission electron microscopy reveals that in situ SiN _x is crystallized on the AlGaN layer as island-like structures via the Stranski-Krastanov growth mode. The lattice constants of in situ SiN _x are estimated to be a ≈ 3.2 Å and c ≈ 2.4 Å, which are quite different from those of well-known Si₃N₄ crystal structures. First-principles calculation predicts that the crystal structure of in situ SiN _x is the defect wurtzite structure, which well explains the experimental results. The passivation technique using crystalline SiN _x films would be promising for high-power and high-frequency applications of AlGaN/GaN HFETs.     

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.     

Spin transistor based on cadmium fluoride nanostructures

A study of CdB _x F_{2 ? x} /p-CdF₂/CdB _x F_{2 ? x} planar sandwich structures fabricated on n-CdF₂ crystal surface was carried out in order to obtain the spin-transistor effect at room temperature. Features related to the band gap of CdF₂ (7.8 eV) along with those related to the spectrum for two-dimensional (2D) hole subbands in p-CdF₂ quantum well (QW) were observed in the current-voltage characteristics for ultrashallow p ⁺-n junctions. The results obtained demonstrate the important role for 2D hole subbands in the mechanism of the “proximity effect” that appears due to Andreev’s reflection in sandwich structures consisting of a narrow QW confined between superconducting barriers. Resonance behavior for the longitudinal voltage in a weak magnetic field normal to the plane of the p-CdF₂ QW gives evidence for high degree of spin polarization for 2D holes. Analysis of the dependences for the 2D-hole-gas conductance on the magnitude and direction of the magnetic field normal to the plane of the p-CdF₂ QW reveals anti-crossings for Zeeman sublevels in the singlet ground state and triplet excited state of boron dipole centers, responsible for the spin polarization of 2D holes in edge channels in the p-CdF₂ QW. The high degree of spin polarization for 2D holes in edge channels in the p-CdF₂ QW identifies the mechanism underlying spin-transistor I-V characteristics observed upon the variation of the gate voltage, which controls the magnitude of Bychkov-Rashba’s spin-orbit coupling.     

On Measurements of the Electrons and Holes Impact-Ionization Coefficients in 4<Emphasis Type="Italic">H</Emphasis>–SiC

All published results of measurements (at 300 K) of the impact ionization coefficients for electrons α_n and holes α_p in 4H–SiC are analyzed. It is shown that the most plausible approximations of dependences of α_{n, p} on electric-field strength E have the usual form α_{n, p} = a_{n, p} exp(–E_{n, p}/E) at fitting-parameter values of a_n = 38.6 × 106 cm^–1, E_n = 25.6 MV/cm, a_p = 5.31 × 106 cm^–1, and E_p = 13.1 MV/cm. These dependences α_{n, p}(E) are used to calculate the highest field strength E_b and thickness w_b of the space-charge region at the breakdown voltage U_b. A number of new formulas for calculating α_{n, p}(E) are obtained from the results of measuring the avalanche-multiplication coefficients and the excess-noise factors under the single-sided illumination of photodiodes with stepped doping.     

Polarization-Engineered Ga-Face GaN-Based Heterostructures for Normally-Off Heterostructure Field-Effect Transistors

Polarization-engineered Ga-face GaN-based heterostructures with a GaN cap layer and an AlGaN/p-GaN back barrier have been designed for normally-off field-effect transistors (FETs). The simulation results show that an unintentionally doped GaN cap and p-GaN layer in the buffer primarily deplete electrons in the channel and the Al_0.2Ga_0.8N back barrier helps to pinch off the channel. Experimentally, we have demonstrated a normally-off GaN-based field-effect transistor on the designed GaN cap/Al_0.3Ga_0.7N/GaN channel/Al_0.2Ga_0.8N/p-GaN/GaN heterostructure. A positive threshold voltage of 0.2 V and maximum transconductance of 2.6 mS/mm were achieved for 80-μm-long gate devices. The device fabrication process does not require a dry etching process for gate recessing, while highly selective etching of the GaN cap against a very thin Al_0.3GaN_0.7N top barrier has to be performed to create a two-dimensional electron gas for both the ohmic and access regions. A self-aligned, selective etch of the GaN cap in the access region is introduced, using the gate metal as an etch mask. The absence of gate recess etching is promising for uniform and repeatable threshold voltage control in normally-off AlGaN/GaN heterostructure FETs for power switching applications.