An analytic model for high-electron-mobility transistors

A new analytic model is developed for the output I–V characteristics and microwave-signal parameters of High Electron Mobility Transistors (HEMTs). In this model, the empirical formula suggested by Giblin et al. is used to approach the behavior of electron drift velocity vs electric field. The resulting I–V curves are in excellent agreement with experimental data. In order to predict the microwave performance of these devices, this model is also used to calculate the small-signal parameters, transconductance and gate capacitance, which are then used to estimate f_T, the frequency of unity current gain.     

Changes of electrical properties by Al content of AlxGa1−xAs in pHEMT structures as observed using HRXRD

Characterizations on the pseudomorphic High Electron Mobility Transistor structure under High-Resolution X-ray Diffraction (HRXRD) have been carried out at room temperature. Variation of Al contents in Al_xGa_1−xAs alloys has been found to show a shift of diffraction peaks. This variation is also found to show the change of lattice constant of crystal and also sheet carrier concentration as obtained from a Hall effect measurement. The latter phenomenon is considerably interesting to study in the early stage of the electrical properties of device based on the crystal structure.     

GaN高电子迁移率晶体管高频噪声特性的研究

基于FUKUI噪声模型,分析了GaN高电子迁移率晶体管(HEMT)器件的高频噪声特性,结果表明,由于GaN HEMT具有更高的临界电场和更大的电子饱和速度,与第2代半导体器件(GaAs HEMT等)相比具有更优越的噪声性能.对近10多年来国内外在GaN HEMT低噪声器件及其低噪声功率放大器单片集成电路(MMIC)方面的研究进行了综述,并分析了GaN HEMT在低噪声应用领域目前存在的主要问题及其发展趋势.     

Improved Long-Term Thermal Stability At 350°C Of TiB2–Based Ohmic Contacts On AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN High Electron Mobility Transistors (HEMTs) were fabricated with Ti/Al/TiB₂/Ti/Au source/drain Ohmic contacts and a variety of gate metal schemes (Pt/Au, Ni/Au, Pt/TiB₂/Au or Ni/TiB₂/Au) and subjected to long-term annealing at 350°C. By comparison with companion devices with conventional Ti/Al/Pt/Au Ohmic contacts and Pt/Au gate contacts, the HEMTs with boride-based Ohmic metal and either Pt/Au, Ni/Au or Ni/TiB₂/Au gate metal showed superior stability of both source-drain current and transconductance after 25 days aging at 350°C.     

Impact of strain relaxation of AlmGa1−mN layer on 2-DEG sheet charge density and current voltage characteristics of lattice mismatched AlmGa1−mN/GaN HEMTs

An accurate charge control model to investigate the effect of aluminum composition, strain relaxation, thickness and doping of the Al_mGa_1−mN barrier layer on the piezoelectric and spontaneous polarization induced 2-DEG sheet charge density, threshold voltage and output characteristics of partially relaxed Al_mGa_1−mN/GaN HEMTs is proposed. The strain relaxation of the barrier imposes an upper limit on the maximum 2-DEG density achievable in high Al content structures and is critical in determining the performance of lattice mismatched Al_mGa_1−mN/GaN HEMTs. The model incorporates the effects of field dependent mobility, parasitic source/drain resistance and velocity saturation to evaluate the output characteristics of Al_mGa_1−mN/GaN HEMTs. Close proximity with published results confirms the validity of the proposed model.     

Investigation of the effect of temperature during off-state degradation of AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN High Electron Mobility Transistors were found to exhibit a negative temperature dependence of the critical voltage (V_CRI) for irreversible device degradation to occur during bias-stressing. At elevated temperatures, devices exhibited similar gate leakage currents before and after biasing to V_CRI, independent of both stress temperature and critical voltage. Though no crack formation was observed after stress, cross-sectional TEM indicates a breakdown in the oxide interfacial layer due to high reverse gate bias.     

Correlating gate sinking and electrical performance of pseudomorphic high electron mobility transistors

Ti interdiffusion from the Ti/Pt/Au gate into the AlGaAs Schottky barrier layer (SBL) of 0.25-μm GaAs Pseudomorphic High Electron Mobility Transistors (PHEMTs) has been studied using the accelerated life testing technique. Based on measurements and modeling, analytical expressions for quantitative correlation between the positive pinch-off voltage (V_P) shift as well as the saturation drain current (I_Dsat) decrease and the physical damage occurring during gate sinking has been developed. It is suggested that the main cause for device failure is the growth of the TiAs phase leading to the decrease in the SBL thickness. Additionally, it is suggested that V_P may be used as a better indicator for device degradation than I_Dsat since it is linearly proportional to the degrading physical characteristic – the Schottky barrier layer thickness.     

Composition dependence of the band offsets in wurtzite nitride-based heterojunctions

We theoretically calculate the composition dependence of the valence- and conduction- band discontinuities at the interfaces between selected III-nitride ternary materials with wurtzite structure, e.g. Al_xGa_1−xN/Al_xGa_1−xN, In_xGa_1−xN/In_xGa_1−xN and In_xA_1−xN/In_xA_1−xN. Calculations are performed using a theoretical model, initially proposed by Chuang et al. 1997 [1]. Depending on a particular set of input parameters, simulation results show that band offsets change more or less with strain. The valence band offsets, together with the resulting conduction band offsets, indicate that a type-I, type-II band line-up forms at In_xGa_1−xN/GaN, Al_xGa_1−xN/GaN heterojunctions with varying In, Al contents respectively. Also, based upon the same model, we propose a type I Indium-dependent band alignment in In_xAl_1−xN/AlN interfaces. The failure of the transitivity rule, which is often used to determine the band offsets in heterojunctions, was demonstrated and its cause was explained. The obtained results are well compared with experiment and theory in various reliable test cases and therefore provide a basis for optimization and design of novel interface structures.     

Hemt’s capability for millimeter wave applications

This paper presents a tutorial review on the High Electron Mobility Transistors (HEMTs)for low-noise and power applications at millimeter wave range frequencies. The major parameters to achieve high performance with such devices are discussed. Present status on power and noise performances is given for InAlAs/InGaAs HEMTs on InP and GaAs substrates.     

Crystal structure and band gap of AlGaAsN

Quantum dielectric theory is applied to the quaternary alloy Al_xGa_1−xAs_1−yN_y to predict its electronic properties as a function of Al and N mole fractions. Results are presented for the expected crystal structure, minimum electron energy band gap, and direction in k-space of the band gap minimum for all x and y values. The results suggest that, for a proper choice of x and y, Al_xGa_1−xAs_1−yN_y could exhibit certain advantages over Al_xGa_1−xAs when utilized in field-effect transistor structures.     

Deep level investigation on n-In0.35Ga0.65As/GaAs structures

We have investigated the deep electronic levels in n-In_0.35Ga_0.65As epitaxial layers grown by molecular beam epitaxy (MBE) on graded In_xGa_1−xAs buffer/GaAs structures. In_xGa_1−xAs buffer layers with linear, parabolic and power composition grading law, respectively have been considered. The dependence of the deep levels distribution on the buffer grading law as well as on growth parameters such as the growth temperature and use of As₂ or As₄ beams is reported.     

Interaction of electrons with optical phonons localized in a quantum well

The scattering rate of electrons in a quantum well by localized polar optical and interface phonons is considered. The dependence of the force of the electron-phonon interaction on the frequency of optical phonons in materials of the heterostructure forming the electron and phonon quantum wells is determined. It is shown that, by varying the composition of semiconductors forming the quantum well and its barriers, it is possible to vary the scattering rates of electrons by a factor of several times. The scattering rates of electrons by polar optical phonons are calculated depending on the fractions In _x and In _y in the composition of semiconductors forming the In _x Al_{1 ? x} As/In _y Ga_{1 ? y} As quantum wells. Dependences of the mobility and saturated drift velocity of electrons in high electric fields and quantum wells In _y Ga_{1 ? y} As on the composition of the In _x Al_{1 ? x} As barriers introduced into quantum wells are determined experimentally. The electron mobility increases, while the saturated drift velocity decreases as the fraction of In _x in the composition of barriers is increased.     

Band gap behavior of scandium aluminum phosphide and scandium gallium phosphide ternary alloys and superlattices

Full-potential linear muffin-tin (FP-LMTO) has been implanted within density functional theory (DFT) and within local density approximation. The structural and electronic properties of GaP, AlP and ScP binary compounds and their ternary alloys Sc_xAl_1−xP and Sc_xGa_1−xP over a whole range of compositions (0≤x≤1) and the superlattices Sc_0.25Ga_0.75P/Sc_0.5Ga_0.5P and Sc_0.25Al_0.75P/Sc_0.5Ga_0.5P in the zinc-blende structure are investigated. The calculated resulted gave reasonable agreements with other published data.     

Structural characterization of AlxGa1−xSb grown by LPE

Al_xGa_1−xSb ternary solid solutions lattice-matched to the GaSb (001) substrate with composition in the range 0.05≤x≤0.2 were grown by liquid phase epitaxy. High resolution X-ray diffraction and Raman scattering techniques were applied to characterize Al_xGa_1−xSb alloys. The out of plane lattice parameter was estimated directly from the asymmetrical diffractions of planes (115) and (−1−15). The out of plane lattice parameter as a function of Aluminium content is higher than the corresponding bulk lattice parameter of Al_xGa_1−xSb layers obtained with Vegard's law. These results show that some of the layers are more strained than others. Two peaks are observed in their Raman spectra over this composition range. The assignment of the observed modes to GaSb-like modes is discussed. The studies of the chemical composition of the layers by SIMS exhibit the presence of tellurium, carbon and oxygen like the main residual impurities.     

Analytical modeling of InSb/AlInSb heterostructure dual gate high electron mobility transistors

This paper presents an approximate solution of a 2D Poisson’s equation within the channel region for Double-Gate AlInSb/InSb High Electron Mobility Transistors (DGHEMTs), using variable separation technique. The proposed model is used to obtain the surface potential, electric field, threshold voltage and drain current of both tied and separated gate bias conditions for Double-Gate AlInSb/InSb HEMTs. The surface potential and electric field are derived for both effective conduction paths of front and back heterointerface by a simple analytical expression and an analytical solution is verified with sentarus TCAD device simulator.     

Compound semiconductor MOSFETs

Enhancement mode, high electron mobility MOSFET devices have been fabricated using an oxide high-κ gate dielectric stack developed using molecular beam epitaxy. A template layer of Ga₂O₃, initially deposited on the surface of the III-V device unpins the GaAs Fermi level while a (Gd_xGa_1−x)₂O₃ bulk ternary layer forms the highly resistive layer to reduce leakage current through the dielectric stack. A midgap interface state density of ∼2 × 10¹¹ cm⁻² eV⁻¹ and a dielectric constant of 20 are determined using electrical measurements.. N-channel MOSFETs with a gate length of 1 μm and a source-drain spacing of 3 μm show a threshold voltage, saturation current and transconductance of 0.11 V, 380 mA/mm and 250 mS/mm, respectively.     

Impact of strain on the band offsets of important III–V quantum wells: InxGa1−xN/GaN,GaAs/InxGa1−xP and InxGa1−xAs/AlGaAs

Lattice strain has immense effect on the optoelectronic properties of III–V semiconductor quantum wells (QWs), since it introduces a pronounced change on the band properties of QWs and it is often purposefully introduced to improve device performance. In this paper we report the results of our experimental and theoretical studies on, how the important parameter, the band offset, changes with strain for In_xGa_1−xN/GaN, GaAs/In_xGa_1−xP and In_xGa_1−xAs/AlGaAs QWs. Experimentally the band offsets have been studied through capacitance transient measurements in the form of deep level transient spectroscopy (DLTS) on suitable QWs within a Schottky diode. The energy levels in a QW are considered to be analogous to a deep trap in the forbidden energy gap. From detailed balance between the emission and capture, Arrhenius type expressions were derived to analyze transient emission data, from which the band offsets were computed. Theoretically the band positions at the heterointerfaces have been calculated from the equations developed, which directly correlate the position of the bands with the strain at the interface. The strain is calculated from the In mole fractions and lattice constants. The parameters implicitly involved are the elastic stiffness constants (C₁₁ and C₁₂), the hydrostatic deformation potential of the conduction band (a′), the hydrostatic deformation potential (a) and the shear deformation potential (b) for the valance band. The results should be useful to research workers in the field of optoelectronics.     

Optoelectronic properties of cubic BxInyGa1−x−yN alloys matched to GaN for designing quantum well Lasers: First-principles study within mBJ exchange potential

A Full-Potential Linearized Augmented Plane Wave calculation within density functional theory is performed to investigate the electronic and optical properties of cubic B_xIn_yGa_1−x−yN alloys matched to GaN with low-Boron content (x≤0.187). The exchange-correlation potential is treated by the local density approximation (LDA) to calculate the structural properties. The band structure and density of states of these compounds are well predicted by modified Becke–Johnson (mBJ) exchange potential compared to LDA and generalized gradient approximation (GGA). Also, the optical properties are calculated by the mBJ exchange potential. The computed structural parameters are found to be in good agreement with experimental and theoretical data. The B_xIn_yGa_1−x−yN alloy is expected to be lattice matched to GaN substrate for (x=0.125, y=0.187). The incorporation of B and In into GaN substrate allows the reduction of the band gap energy. The real and imaginary parts of the dielectric function, refractive index, reflectivity and absorption coefficient are discussed on the basis on the energy band structure and the calculated density of states. The optical properties of B_xIn_yGa_1−x−yN depend on the incorporated Boron content (with y=0.187). This means that B_xIn_yGa_1−x−yN could constitute an active layer in single quantum well for the design of high-efficiency solar cells and optoelectronic devices as Laser Diodes operating in the UV spectral region.     

Bulk moduli of PbSxSe1−x,PbSxTe1−x and PbSexTe1−x from a thermodynamical model compared to generalized gradient approximation approach

Very recently, first-principle technique of full-potential linearized augmented plane-wave method, by using for exchange-correlation potential the generalized gradient approximation (GGA), was employed for the study of the lead chalcogenide semiconductors’ alloys PbS_xSe_1−x, PbS_xTe_1−x and PbSe_xTe_1−x. These density functional calculations led to the determination of structural, electronic and optical properties, including the values of lattice constants and bulk moduli as a function of composition. Here, we investigate the latter properties, but by employing a thermodynamical model which has been suggested for the formation and migration of defects in solids including several recent applications in semiconductors. The following crucial difference emerges when comparing the present results with those deduced by density functional calculations: Among the alloys studied, GGA calculations identify that PbS_xTe_1−x exhibits the most evident non-linear variation of the bulk modulus versus the composition, while according to the thermodynamical model such an evident non-linear behavior – and maybe somewhat stronger – is also expected for PbSe_xTe_1−x. A tentative origin of this diversity is discussed.