A. F. M. Anwar

Transmission line analogy of resonance tunneling phenomena: The generalized impedance concept

This paper presents a simple yet accurate method for solving the Schrodinger equation to calculate the quantum mechanical transmission probability across arbitrary potential barriers. It is based on the concept of wave impedance analogous to transmission line theory. The quantum mechanical transmission probability in a resonant tunneling device can be easily calculated using this method.

Calculation of the traversal time in resonant tunneling devices

Transmission line techniques are used to calculate the traversal time of electrons in resonant tunneling structures. It is suggested that the real part of the quantum‐mechanical wave impedance Re[(2ℏ/m*)ψ’/ψ], at resonance, can be used to calculate the electron traversal time. Furthermore, it is shown that for symmetric structures the lifetime and evolution time of electrons are the same. The sum of the lifetime and the evolution time equals the electron traversal time. Traversal time variations for the asymmetric well are discussed.

Impact ionization in InAlAs/InGaAs/InAlAs HEMT's

The kink effect and excess gate current in InAlAs/InGaAs/InAlAs HEMTs have been linked to impact ionization in the high field region of the channel. In this letter, a relationship is established between experimentally measured excess gate current and the tunneling of holes from the quantum well formed in the channel. The channel hole current is then obtained as the quotient of the excess gate current to the gate-voltage-dependent transmission probability. This channel hole current follows the exponential dependence of the ionization constant on the inverse electric field.

Temperature dependent transport properties in GaN, Al/sub x/Ga/sub 1-x/N, and In/sub x/Ga/sub 1-x/N semiconductors

Ensemble Monte Carlo simulation is used to determine the electron saturation velocity and low-field mobility for Al/sub x/Ga/sub 1-x/N and In/sub x/Ga/sub 1-x/N. Acoustic phonon, optical phonon, intervalley, ionized impurity, alloy, and piezoelectric scattering are included in the simulation. Doping concentration ranging from 10/sup 17/ cm/sup -3/ to 10/sup 19/ cm/sup -3/ is considered in the temperature range from 50 K to 500 K, Theoretical calculation shows excellent agreement with low-field mobility experimental data. Empirical expressions for low field mobility and saturation velocity are provided as functions of temperature, doping concentration and mole fraction.

Bias induced strain in AlGaN∕GaN heterojunction field effect transistors and its implications

We report gate bias dependence of the charge due to piezoelectric polarization obtained by using a fully coupled formulation based upon the piezoelectric constitutive equations for stress and electric displacement. This formulation is significant because it fully accounts for electromechanical coupling under the constraint of global charge control. The coupled formulation results in lower charge due to piezoelectric polarization as compared to the uncoupled formulation for a given Al mole fraction. With increasing two dimensional electron gas concentration, that is, for gate biases greater than threshold, the compressive strain along the c axis in the barrier AlGaN layer increases with a concomitant increase of in-plane stress. Current collapse is correlated to the increase in source and drain resistances through their dependence upon surface charge. An alternate explanation of current collapse using local charge neutrality is also presented.

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.

Carrier trapping and current collapse mechanism in GaN metal-semiconductor field- effect transistors

A mechanism for current collapse in GaN metal–semiconductor field-effect transistors is proposed, which assumes the existence of acceptor traps with multiple states in the band gap. Current collapse has been experimentally observed in the current–voltage characteristic after the drain voltage sweep had exceeded the threshold for impact ionization in a previous measurement. In the proposed model, electrons generated by impact ionization are captured by neutral acceptor trap states in the substrate located above the valence band. The charged trap states move to an energy level located near midgap, creating a positively charged depletion region in the channel, and causing current collapse. With increasing drain bias, the quasi-Fermi level approaches the charged trap states at the drain end of the gate, initiating detrapping of the electrons and restoring the current. The calculated results show good agreement with published experimental data.

A physics-based frequency dispersion model of GaN MESFETs

A physics-based model for GaN MESFETs is developed to determine the frequency dispersion of output resistance and transconductance due to traps. The equivalent circuit parameters are obtained by considering the physical mechanisms for current collapse and the associated trap dynamics. Detrapping time extracted from drain-lag measurements are 1.55 and 58.42 s indicating trap levels at 0.69 and 0.79 eV, respectively. The dispersion frequency is in the range of megahertz at elevated temperature, where a typical GaN power device may operate, although at room temperature it may be few hertz. For a 1.5 /spl times/ 150 /spl mu/m GaN MESFET with drain and gate biases of 10 V and -1 V, respectively, 5% decrease in transconductance and 62% decrease in output resistance at radio frequencies (RFs) from their DC values are observed. The dispersion characteristics are found to be bias dependent. A significant decrease in transconductance is observed when the device operates in the region where detrapping is significant. As gate bias approaches toward cutoff, the difference between output resistance at dc and that at RF increases. For drain and gate biases of 10 and -5 V, output resistance decreases from 60.2 k/spl Omega/ at dc to 7.5 k/spl Omega/ at RF for a 1.5 /spl mu/m /spl times/ 150 GaN MESFET.

A temperature-dependent nonlinear analysis of GaN/AlGaN HEMTs using Volterra 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 /spl mu/m/spl times/500 /spl mu/m Al/sub 0.15/Ga/sub 0.85/N/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 (OIP/sub 3/) 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 IM/sub 3/ for 10-dBm output power is -72 dB at 300 K and -56 dB at 600 K. At 300 K, IM/sub 3/ is -66 dB at 5 GHz and -51 dB at 10 GHz. For the same frequencies, IM/sub 3/ increases to -49.3 and -40 dB, respectively, at 600 K.

Electron escape time from single quantum wells

A theoretical model for electrons escaping a quantum well under the influence of an applied electric field is developed. Both the thermionic emission and tunneling components of the currents are calculated, taking into account the proper partitioning between the two currents. The group velocity for a nonuniform electron distribution within the quantum well, which is a function of position and energy, and the continuous energy dependence of the quantum well density of states is considered. A comparison between this model and previously reported experimental results are made which demonstrates excellent agreement.