Wudyalew Wondmagegn

Observations on frequency sensitivity of memristors

Comparison is made between established one dimensional linear and nonlinear memristor drift models with Hewlett Packard (HP) experimental data to test level of agreement. Models used in first phase of work are based on a sinusoidal applied voltage. Linear and nonlinear model data as well as HP experimental data were employed to make instantaneous power predictions during one full time cycle of the input waveform. Instantaneous power profile comparisons between both linear and nonlinear models for higher and lower frequencies with experimental data indicate that the power profiles are less sensitive to change in frequency though memristor hysteresis is highly sensitive. Model based simulation tests confirm that the frequency response scales inversely with an identified time constant based on physical properties and memristor dimensions.

Quasi-static modeling of an organic Schottky diode with trapped charge

The fundamental aspects of trap analysis in charge transport of organic semiconductors are reviewed by focusing on the role of traps in I-V characteristics of an organic Schottky diode. A p-type organic semiconductor based device is considered and the trap distribution is assumed to vary exponentially with energy. A numerical model is developed to determine the trapped charge concentration in the bulk region of organic semiconductor during the steady state trap free space charge limited current conduction. This model was used to deduce an analytical expression that relates the charge ratio θ with the Fermi level energy EF. This expression was utilized to determine the value of Fermi level energy EF corresponding to the charge ratio taken from the previously reported work based on an organic Schottky diode. Finally, the estimated value of Fermi level energy is applied to calculate the number of trapped charges through MATLAB based simulations. The concentration of trapped charges was found to be 1.54×1016 cm−3 which is in good agreement with the reported experimental results.

Simulation and transient analysis of organic/inorganic CMOS inverter circuit

Finite element device level simulations were used in conjunction with SPICE modeling to design and optimize a complementary MOS inverter circuit with an organic p-type and an inorganic n-type transistor combination. The device characteristics of the p-type and the n-type transistors were generated through 2D finite element device level simulations. The drain current (ID) vs. drain-source voltage (VD) and the drain current (ID) vs. gate voltage (VG) characteristics were modeled through fitting procedures to extract the SPICE parameters. Also, the design and transient analysis of a hybrid CMOS inverter circuit modeled using these SPICE parameters is presented. The inverter works reliably within the range of supply voltage (10V).

Modeling and characterization for polarization hysteresis of ferroelectric polymers

We present a modeling and simulation based study for the polarization hysteresis of ferroelectric polymers. A 2-dimensional finite element device-level model was implemented using SILVACOs ATLAS device simulator to generate the polarization hysteresis characteristics for the recently reported experimental data on Au/Poly(vinylidene fluoride-trifluoroethylene)/Au metal-insulator-metal (MIM) device. The simulated polarization dependence characteristics in the P(VDF-TrFE) thin-film were predicted from the parameters coercive field, remnant polarization and spontaneous polarization which were extracted from fitting to the experimental characteristics. The mathematical model which is referred by SILVACO for ATLASs ferroelectric model was implemented in MATLAB to generate the hysteresis curve. Landau-Devonshire expression for electric field in terms of polarization is also implemented in the MATLAB to produce the polarization hysteresis curve of mono-crystalline and polycrystalline ferroelectrics. The polarization hysteresis characteristics obtained from the ATLAS and MATLAB simulations were in good agreement with the reported experimental hysteresis characteristics.

Observations on model based predictions for memristor power dissipation

Comparison is made between memristor linear, nonlinear drift models with Hewlett Packard (HP) experimental data using MATLAB mathematical model analysis. Average power predictions indicated that the experiment with 0.58mW and linear model with 0.86mW showed better agreement in comparison to nonlinear model with 1.17mW. Instantaneous power profile comparisons between both linear and nonlinear models with experimental data indicate that further modeling efforts are needed to capture details in instantaneous power profile for experimental data. Average power model-based predictions are nominally on the same order as the physical device.

Semi empirical cadmium sulfide transistor model combining grain defects and semiconductor thickness variation

Proposed and tested is a methodology for modeling polycrystalline thin film transistors which exhibit shifts in threshold voltage due to both grain boundaries and semiconductor thickness. The process involves a model, which uses in part standard-analytic terms. It also includes terms for grain defects and for thickness added in using numerical simulation testing. From this testing, the threshold voltage for the CdS transistor exhibited an optimum thickness for enhancement mode operation. The semi empirical model was then brought into alignment with experimental results for a CdS transistor by adjusting the interface charge. Predictions from the semi empirical model produced transistor output characteristic and transfer curves showed to be in good agreement with experimental data.

A practical model to analytically characterize the polarization hysteresis of ferroelectric capacitors

A simulation based comparative study of the polarization hysteresis of the ferroelectric capacitor using various ferroelectric models is presented. A 2-dimensional finite element device-level model was implemented using SILVACOs ATLAS device simulator to generate the polarization hysteresis characteristics for Au/Poly(vinylidene fluoride-trifluoroethylene)/Au metal-insulator-metal (MIM) device. Landau free energy expression for electric field in terms of polarization is also implemented in the MATLAB to produce the polarization hysteresis curve of polycrystalline ferroelectrics. The main drawback of these models was their inability to predict polarization saturation at the same electric field limits for which the experimental device was saturating. A new model for ferroelectric hysteresis based on curve fitting algorithm is derived that forces the polarization to be saturated at desired electric field. The MATLAB simulation of this model and the experimental hysteresis is compared which shows an excellent level of agreement.

Fermi-dirac analysis and simulation of an Organic Schottky Diode

Based on the numerical model and the analytical expression developed in our previous work, the location of Fermi level in an organic semiconductor is determined using charge neutrality principle. As in our previous work, a pentacene based Organic Schottky diode is considered for the simulation and comparison of the various physical and electrical properties with the reported experimental results. At first, the consistency checks between the two values of free carriers (holes) is performed, one using our own numerical model and the other using the well known Physics equation. Secondly, the same model is used in the charge neutrality principle to extract the Fermi level energy. This value of Fermi level energy is further applied to calculate the charge ratio and the number of ionized trapped charges. The density of ionized trapped charge is found to be 9.9425×1015cm−3, which is in close agreement with the experimental value. Finally, these parameters are used to simulate the I–V characteristics of the above mentioned device through Matlab and Silvaco Internationals ATLAS simulation software. The simulation results produce an excellent overlapping with the experimental. Besides this, the effect of traps in I–V characteristics of such device is also demonstrated.