Self-heating and trapping effects on the RF performance of GaN MESFETs

RF power performances of GaN MESFETs incorporating self-heating and trapping effects are reported. A physics-based large-signal model is used, which includes temperature dependences of transport and trapping parameters. Current collapse and dc-to-RF dispersion of output resistance and transconductance due to traps have been accounted for in the formulation. Calculated dc and pulsed I-V characteristics are in excellent agreement with the measured data. At 2 GHz, calculated maximum output power of a 0.3 /spl mu/m/spl times/100 /spl mu/m GaN MESFET is 22.8 dBm at the power gain of 6.1 dB and power-added efficiency of 28.5% are in excellent agreement with the corresponding measured values of 23 dBm, 5.8 dB, and 27.5%, respectively. Better thermal stability is observed for longer gate-length devices due to lower dissipation power density. At 2 GHz, gain compressions due to self-heating are 2.2, 1.9, and 0.75 dB for 0.30 /spl mu/m/spl times/100 /spl mu/m, 0.50 /spl mu/m/spl times/100 /spl mu/m, and 0.75 /spl mu/m/spl times/100 /spl mu/m GaN MESFETs, respectively. Significant increase in gain compression due to thermal effects is reported at elevated frequencies. At 2-GHz and 10-dBm output power, calculated third-order intermodulations (IM3s) of 0.30 /spl mu/m/spl times/100 /spl mu/m, 0.50 /spl mu/m/spl times/100 /spl mu/m, and 0.75 /spl mu/m/spl times/100 /spl mu/m GaN MESFETs are -61, -54, and - 45 dBc, respectively. For the same devices, the IM3 increases by 9, 6, and 3 dBc due to self-heating effects, respectively. Due to self-heating effects, the output referred third-order intercept point decreases by 4 dBm in a 0.30 /spl mu/m/spl times/100 /spl mu/m device.     

Large and small signal distortion analysis using modified Volterra series

A new approach to the Volterra analysis of analog circuits is presented. Volterra analysis is widely used for the calculation of harmonic and intermodulation distortion products. However, the analysis is limited to circuits experiencing small signal excitations and becomes inaccurate when the input signal amplitude increases, especially when MOS transistors are involved. In this paper, we analyze the cause of this drawback, which is no other than the Taylor series’ convergence properties. Moreover, we propose a solution, by calculating the nonlinearity coefficients using a different type of polynomial expansion, the Chebyshev series. This replacement improves significantly the capabilities of Volterra analysis. We also present results comparing Chebyshev series with other types of polynomial expansions. Finally, we apply the proposed method to analyze the intermodulation distortion (IMD) of a CMOS RF power amplifier, both in the small and the large signal regimes.     

Inter modulation distortion analysis of cascaded MESFET amplifiers using Volterra series representation

The third-order intermodulation distortion generated in a two-stage cascaded amplifier is derived analytically by means of the Volterra series expansion, for non-linear systems with memory. A unilateral transistor model is used, which takes into account the three major non-linearities, the gate capacitance, the trans-conductance and the output conductance, in each stage of the amplifier. The Volterra transfer functions are determined for this transistor model and closed-form expressions for the intermodulation distortion ratio are given, where the terms with a contribution less than 1% have been neglected. The equations identify the principal sources of the distortion in the amplifier circuit and the influence of the transistor parameters and load impedance is investigated. The analysis of the two-stage cascaded amplifier gives a new insight into the intermodulation distortion behaviour and is discussed in detail at the end of the paper. The results obtained here are compared with those received for single-stage amplifiers.     

Volterra series analysis of semiconductor laser diode

Theoretical models for laser nonlinearities are analyzed. Volterra transfer function G/sub n/ are calculated from the laser rate equations using an output-to-input approach. Nonlinear models for second-harmonic, third-harmonic, and two-tone third-order intermodulation distortions are calculated from G/sub n/. The transfer function-based models are simplified and a new equation for intermodulation distortion is developed. Comparisons with previous results are presented. It is suggested that this analytical technique offers a valuable tool for the performance analysis of future broadband optical communication systems.<>     

Computer simulation of negative-differential-conductivity effects, including trapping

The computer model is based on a simple approximation of GaAs, and provides for the existence of trapping. The relation between voltage, field, and diode length takes into account the internal voltage, due to doping non-uniformities. The simulation is performed without restrictive assumptions on the field or carrier concentrations at the electrodes. Simulation results focus on parastable diodes, which are stable above threshold because the doping concentrationNexceeds some critical value Npwhich depends on the electron diffusionD, velocityv, and differential mobility µ δ, in accordance with the small-signal analytical valueN_{p} = -epsilonv^{2}/4De micro delta. AsNincreases, the field EUin the uniform region of a parastable diode decreases, making parastability less sensitive to doping inhomogeneities. A positive doping gradient near the anode: 1) can make a diode unstable near threshold, and parastable at higher voltages; and 2) causes static negative differential resistance. The ac conductance of a para-stable diode become negative at sufficient modulating voltage levels in the microwave range. The diode can operate in the LSA mode, yet return to parastability when the modulating voltage is removed.     

Numerical analysis of GaAs MESFETs with p-buffer layer on semi-insulating substrate including deep traps

Numerical analysis of GaAs MESFETs with a p-buffer layer on the semi-insulating substrate is presented in which impurity compensation by traps is included. Using a p-buffer layer is shown to be effective in minimising the short-channel effects as in the case of using a substrate with high density of traps.<>     

A model for the channel noise of MESFETs including hot electron effects

Noise is an important consideration in the reliability of microelectronic circuits determining the sensitivity of the circuits and placing a lower limit on the regions of operation. Proper modeling of noise in integrated circuits is essential for reliable operation. A derivation is given for the channel noise coefficient of FET’s operating in the saturation region. Some simple approximations are made for hot electron effects which can be incorporated into the derivation and accounted for by a numerical integration technique. Experimental results of measured and calculated noise coefficients are compared for depletion mode MESFETs of different gate lengths. This model gives a much more realistic representation of the channel noise coefficients for short gate length devices rather than the simple 2/3 value currently used in circuit simulations.     

Harmonic analysis of GaN-HEMTs at different temperatures and frequencies using Volterra power series

In this study, the detailed harmonic analysis of GaN high electron mobility transistor (HEMT) at different temperatures and frequencies is presented. Volterra power series and multi-dimensional Laplace transform are used as a method. The Volterra power series is also solved up to third degree, and the small signal transfer functions of kernels (H₁, H₂ and H₃) are obtained. The relationship between drain inductance (L_d), gate–source voltage (V_gs), impedance (Z_L) and the effect of frequency (F_r) to the output gain is identified. Besides, the nonlinear gains of H₁, H₂ and H₃ kernels of the GaN-HEMT are obtained. The inverse relationship between the output gains of H₁, H₂ and H₃ kernels are derived. An unsuitable situation has also been identified for sub-carrier inter-modulation systems. In addition, an asymmetric structure is also obtained between the output gain of H₂ and side-band frequencies. The effects of other parameters are carried out for the output gain.     

Frequency-domain analysis of strongly nonlinear circuits using aconsistent large-signal model

This paper describes an analysis method that extends the applicability of the frequency-domain methods to strongly nonlinear circuits. Nonlinearities are described with Chebyshev expansions which are evaluated with a numerically stable three-term recurrence formula. The method is coupled with a novel, measurement-based consistent modeling approach which allows improved accuracy in describing the frequency-dependence of the measured small-signal parameters. The analysis method and the modeling approach are verified by comparing measurements and calculations on a MESFET mixer, driven with two and three tones     

Two-dimensional analysis of surface-state effects on turn-oncharacteristics in GaAs MESFETs

Surface-state effects on gate-lag or slow current transient in GaAs MESFETs are studied by two-dimensional (2-D) simulation. It is shown that the gate-lag becomes remarkable when the deep-acceptor surface state acts as a hole trap. To suppress it, the deep acceptor should be made electron-trap-like, which can be realized by reducing the surface-state density. Device structures expected to have less gate-lag, such as a self-aligned structure with n⁺ source and drain regions and a recessed-gate structure are also analyzed. An analysis of the possible complete elimination of gate-lag in these structures is given     

Two-dimensional analysis of substrate-trap effects on turn-oncharacteristics in GaAs MESFETs

Effects of substrate traps on turn-on characteristics of GaAs MESFETs are studied by two dimensional (2-D) simulation. When the off-state gate voltage is much more negative than the threshold (pinch off) voltage and the surface-state effects are small, abnormal current overshoot and subsequent slow transients are observed for the case with undoped semi-insulating substrate including an electron trap: EL2. Even if the surface-state effects are pronounced to show the large gate-lag, the drain current may show the overshoot-like behavior at relatively early periods. The case of Cr-doped substrate with a hole trap: Cr is also discussed     

Numerical analysis of surface-state effects on kink phenomena of GaAs MESFETs

Effects of surface states on the "kink" (or an abnormal increase in output conductance with the drain voltage) in GaAs MESFETs are studied by two-dimensional (2-D) simulations. It is shown that the kink could arise due to a space-charge effect originated from impact ionization of holes and the following hole trapping by the surface states. The onset voltage for current rise depends on the nature of surface states, which strongly affects the potential profiles. Transient or dynamic simulation indicates that the trap-related kink phenomenon should be a rather slow process. Substrate-related kink dynamics are also analyzed.     

Comparison of zincblende-phase GaN, cubic-phase SiC, and GaAs MESFETs using a full-band Monte Carlo Simulator

We present a theoretical study of metal-semiconductor field-effect transistor (MESFET) devices for three different materials: zincblende-phase gallium nitride (ZB-GaN), cubic-phase silicon carbide (3C-SiC) and gallium arsenide (GaAs). The dc breakdown voltage of comparable MESFETs made with the two wide bandgap materials, ZB-GaN and 3C-SiC are compared to that made with the well studied material, GaAs. In this way, the GaAs calculations serve as a control, enabling an accurate comparison of the device behaviors. The simulations are performed with a new, generalized, self-consistent, full-band Monte Carlo simulator. The new simulator includes fully numerical scattering rates and a fully numerical, overlap-based final-state selection process. A 0.1 /spl mu/m gate-length MESFET is used for all of the simulations, and rectangular wells of lightly doped material are used to model interface states. The calculated dc breakdown voltages of the ZB-GaN, 3C-SiC, and GaAs MESFETs are 18, 16, and 5 V respectively. The previously estimated factor-of-four difference between the breakdown voltage of ZB-GaN and GaAs devices is verified.     

Two-dimensional analysis of the surface state effects in 4H-SiC MESFETs

Two-dimensional small-signal ac and transient analysis of surface trap effects in 4H-SiC MESFETs have been performed in this paper. The mechanism by which acceptor-type traps effect the transconductance and drain current changes has been discussed. The simulation results show that transconductance exhibits negative frequency dispersion behavior, which is caused by the charge exchange via the surface states existing between the gate-source and gate-drain terminals. The current degradation behavior is also observed due to acceptor-type traps, acting as electron traps, in MESFET devices. A detailed study involving the density, ionization and energy level of traps reveals conclusive results in the devices analyzed.     

A large-signal analysis of broad-band klystrons with design applications

For the design of a high-performance broad-band klystron, an accurate large-signal analysis is made. By a one-dimensional analysis program using a disk model of the electron stream, the interaction between a bunched electron beam and the electric field in each cavity gap is studied, taking into consideration space-charge forces, relativistic effects, and electric field distributions in ungridded gaps; beam loading is included by an iterative method at each intermediate gap, and the energy exchange in the output gap is obtained from the change in kinetic energy. On the five-cavity UHF-TV klystron designed with this program and having a conversion efficiency of over 60 percent, the debunching effect of the electric field in intermediate cavity gaps and the behavior of vibrating electrons in the output gap are clarified. Calculated conversion efficiency is shown to agree well with measured efficiency over the pass-band. Additionally, by a two-dimensional analysis program, the effect of the cathode magnetic coefficient on the electron bunching is studied.     

Comparison of FWM- and XPM-induced crosstalk using the Volterra series transfer function method

New analytical tools to calculate the variance due to cross-phase modulation (XPM) and four-wave mixing (FWM) induced intensity distortion are derived based on the Volterra series transfer function method. The analysis for both the XPM and FWM effects is based on the same system configuration with a continuous-wave (CW) probe channel plus modulated pump channels, which makes possible a fair comparison between the two nonlinear effects. Effective ways to reduce the XPM- and FWM-induced intensity distortion are given. The new results on the variance of the nonlinearity-induced intensity fluctuation also make it possible to study both synchronous wavelength-division multiplexing (WDM) systems with fixed channel delays and asynchronous WDM systems with random channel delays. The new analytical results provide accurate and efficient ways for system parameter optimization to reduce these two nonlinear effects.     

Time-domain analysis of nonlinear circuits performed on the basis of functional Volterra series

A method is proposed for determination of Volterra kernels from known images. It is shown that frequency-domain kernels should be considered at the points where their analyticity is violated. The timedomain structure of Volterra kernels is determined. A nonlinear microwave amplifier is simulated in the presence of a wideband input signal.     

基于灵敏度分析及Volterra级数的非线性模拟电路故障诊断

为了有效解决非线性模拟电路故障诊断问题,提出了一种基于灵敏度分析及Volterra级数理论的故障诊断方法。首先采用灵敏度分析估计激励信号的有效频带范围,根据Volterra级数确定激励信号的频率成分,以使故障可测度最大化;其次基于多音激励下Volterra核的频域输出特性进行故障特征提取,并采用主元分析(principal componentanalysis,PCA)对故障特征进行维数压缩;最后构造多值分类支持向量机(support vector machine,SVM)网络进行故障模式判别。理论分析及仿真结果表明,与以往方法比较,该方法可以显著提高非线性模拟电路故障识别率,验证了该方法的有效性。     

Optimization of technology of growing GaN heterostructures with using diffractometry analysis

The procedure of optimization of growing the layers of III-N nitrides by the MOCVD technique using X-ray-diffraction analysis is considered. It is established that the structural perfection of the layers estimated from the measurement of rocking curves correlates with radiation characteristics of structures of light-emitting diodes (LED).     

A temperature-dependent nonlinear analysis of GaN/AlGaN HEMTs usingVolterra series

Gain and intermodulation distortion of an AlGaN/GaN device operating at RF have been analyzed using a general Volterra series representation. The circuit model to represent the GaN FET is obtained from a physics-based analysis. Theoretical current-voltage characteristics are in excellent agreement with the experimental data. For a 1 μm×500 μm Al_0.15Ga_0.85N/GaN FET, the calculated output power, power-added efficiency, and gain are 25 dBm, 13%, and 10.1 dB, respectively, at 15-dBm input power, and are in excellent agreement with experimental data. The output referred third-order intercept point (OIP₃) is 39.9 dBm at 350 K and 33 dBm at 650 K. These are in agreement with the simulated results from Cadence, which are 39.34 and 35.7 dBm, respectively. At 3 GHz, third-order intermodulation distortion IM₃ for 10-dBm output power is -72 dB at 300 K and -56 dB at 600 K. At 300 K, IM₃ is -66 dB at 5 GHz and -51 dB at 10 GHz. For the same frequencies, IM ₃ increases to -49.3 and -40 dB, respectively, at 600 K