Modeling and Characterization of Efficient Carrier Multiplication in Highly Co-doped Semiconductors and Disordered Materials

This thesis offers modeling of a newly discovered gain mechanism for various photodetection applications. Conventional avalanche photodetectors have been in use for the past four decades with impact ionization being the underlying carrier multiplication mechanism.However, tradeoff between sensitivity, dynamic range and bandwidth are some of the drawbacks of the present day photodetection technology. The newly discovered cycling excitation process (CEP) can be a potential candidate to address these issues with linear photo response, single photon sensitivity and high gain bandwidth product. The key feature of CEP is introduction of counter dopants in p-n junction silicon diode, with which the efficiency of auger excitation can be enhanced to great extent by facilitating relaxation of k selection rule. Higher uncertainty in k spaces dictates localization of carriers in real space. Hence, an initial hot carrier can excite electron-hole pair between localized states (e.g. from states closer to valence band to states closer to conduction band) at much lower bias. Another essential component of CEP is phonon/field assisted tunneling from localized states to mobile bands. Contrary to other photodetectors, phonons, actually, play a positive role in achieving gain. Experimentally gain of ~4000 at only 4V have been achieved in the CEP test structure along with photo response dependence on input light power, which is helpful for photon number resolving. Temperature dependent measurement also shows the positive role of phonons. Density functional theory calculation shows the change in band structure with doping bulk crystalline silicon with boron (B) and phosphorous (P) simultaneously. Comparison of density of states exhibits existence of states inside band gap. Furthermore, charge density plot clearly demonstrates electron and hole localization centered around P and B atoms respectively. Hence, highly counter doping with BP atoms turns the crystalline silicon into a quasi-disordered material. Since, highly counter doping introduces disorder in silicon, with this notion naturally disordered materials are explored as possible CEP gain media. Amorphous materials have low mobility due to their nature of disorder. Surprisingly, amorphous silicon (a-Si) photodiodes with thin a-Si layer (~40nm) have shown a gain-bandwidth product of over 2 THz with very low excess noise. To unveil the true gain mechanism, the thesis further delves into theoretical modeling and numerical analysis along with experimental data at different frequencies. Evidence of highly effective carrier multiplication process within a-Si as the primary gain mechanism, especially at high frequency is shown. There is also trap-induced junction modulation at much lower frequency. The analysis further suggests that the carrier multiplication process in thin a-Si can be much more efficient than in thick a-Si, even stronger than single crystalline Si in some cases. Although seemingly counter intuitive, this is consistent with the proposed cycling excitation process (CEP) where the localized states in the bandtails of disordered materials such as a-Si relax the k-selection rule and increase the rate of carrier multiplication. A more rigorous quantum mechanical scattering rate calculation also demonstrates the increase of strength of carrier multiplication with the presence of localized states and the increase of ionization coefficient with decreasing thickness of gain medium. A theoretical framework is offered to calculate the carrier multiplication process in a-Si or other disordered materials involving donor acceptor pairs (DAPs) and to answer several key and seemingly counter intuitive questions such as why amorphous silicon can be more efficient carrier multiplication material than single crystal silicon, why low carrier mobility of amorphous material helps rather than hurt carrier multiplication process, and why thin a-Si is more efficient than thick a-Si in carrier multiplication

Self-Consistent C-V Characterization of Depletion Mode Buried Channel InGaAs/InAs Quantum Well FET Incorporating Strain Effects

We investigated Capacitance-Voltage (C-V) characteristics of the Depletion Mode Buried Channel InGaAs/InAs Quantum Well FET by using Self-Consistent method incorporating Quantum Mechanical (QM) effects. Though the experimental results of C-V for enhancement type device is available in recent literature, a complete characterization of electrostatic property of depletion type Buried Channel Quantum Well FET (QWFET) structure is yet to be done. C-V characteristics of the device is studied with the variation of three important process parameters: Indium (In) composition, gate dielectric and oxide thickness. We observed that inversion capacitance and ballistic current tend to increase with the increase in Indium (In) content in InGaAs barrier layer.Comment: 5 pages, ICEDSA conference 201

Self Consistent Simulation of C-V Characterization and Ballistic Performance of Double Gate SOI Flexible-FET Incorporating QM Effects

Capacitance-Voltage (C-V) & Ballistic Current- Voltage (I-V) characteristics of Double Gate (DG) Silicon-on- Insulator (SOI) Flexible FETs having sub 35nm dimensions are obtained by self-consistent method using coupled Schrodinger- Poisson solver taking into account the quantum mechanical effects. Although, ATLAS simulations to determine current and other short channel effects in this device have been demonstrated in recent literature, C-V & Ballistic I-V characterizations by using self-consistent method are yet to be reported. C-V characteristic of this device is investigated here with the variation of bottom gate voltage. The depletion to accumulation transition point (i.e. Threshold voltage) of the C-V curve should shift in the positive direction when the bottom gate is negatively biased and our simulation results validate this phenomenon. Ballistic performance of this device has also been studied with the variation of top gate voltage.Comment: 4 pages, ICEDSA 2012 conferenc

In_xGa_{1-x}Sb MOSFET: Performance Analysis by Self Consistent CV Characterization and Direct Tunneling Gate Leakage Current

In this paper, Capacitance-Voltage (C-V) characteristics and direct tunneling (DT) gate leakage current of antimonide based surface channel MOSFET were investigated. Self-consistent method was applied by solving coupled Schr\"odinger-Poisson equation taking wave function penetration and strain effects into account. Experimental I-V and gate leakage characteristic for p-channel InxGa1-xSb MOSFETs are available in recent literature. However, a self- consistent simulation of C-V characterization and direct tunneling gate leakage current is yet to be done for both n- channel and p-channel InxGa1-xSb surface channel MOSFETs. We studied the variation of C-V characteristics and gate leakage current with some important process parameters like oxide thickness, channel composition, channel thickness and temperature for n-channel MOSFET in this work. Device performance should improve as compressive strain increases in channel. Our simulation results validate this phenomenon as ballistic current increases and gate leakage current decreases with the increase in compressive strain. We also compared the device performance by replacing InxGa1-xSb with InxGa1-xAs in channel of the structure. Simulation results show that performance is much better with this replacement.Comment: 7 pages, EIT 2012 IUPUI conferenc

A Physically Based Analytical Modeling of Threshold Voltage Control for Fully-Depleted SOI Double Gate NMOS-PMOS Flexible-FET

In this work, we propose an explicit analytical equation to show the variation of top gate threshold voltage with respect to the JFET bottom gate voltage for a Flexible Threshold Voltage Field Effect Transistor (Flexible-FET) by solving 2-D Poisson's equation with appropriate boundary conditions, incorporating Young's parabolic approximation. The proposed model illustrates excellent match with the experimental results for both n-channel and p-channel 180nm Flexible-FETs. Threshold voltage variation with several important device parameters (oxide and silicon channel thickness, doping concentration) is observed which yields qualitative matching with results obtained from SILVACO simulations.Comment: 4 pages, EIT 2012-IUPUI conferenc

Modeling and Characterization of Efficient Carrier Multiplication in Highly Co-doped Semiconductors and Disordered Materials

This thesis offers modeling of a newly discovered gain mechanism for various photodetection applications. Conventional avalanche photodetectors have been in use for the past four decades with impact ionization being the underlying carrier multiplication mechanism.However, tradeoff between sensitivity, dynamic range and bandwidth are some of the drawbacks of the present day photodetection technology. The newly discovered cycling excitation process (CEP) can be a potential candidate to address these issues with linear photo response, single photon sensitivity and high gain bandwidth product. The key feature of CEP is introduction of counter dopants in p-n junction silicon diode, with which the efficiency of auger excitation can be enhanced to great extent by facilitating relaxation of k selection rule. Higher uncertainty in k spaces dictates localization of carriers in real space. Hence, an initial hot carrier can excite electron-hole pair between localized states (e.g. from states closer to valence band to states closer to conduction band) at much lower bias. Another essential component of CEP is phonon/field assisted tunneling from localized states to mobile bands. Contrary to other photodetectors, phonons, actually, play a positive role in achieving gain. Experimentally gain of ~4000 at only 4V have been achieved in the CEP test structure along with photo response dependence on input light power, which is helpful for photon number resolving. Temperature dependent measurement also shows the positive role of phonons. Density functional theory calculation shows the change in band structure with doping bulk crystalline silicon with boron (B) and phosphorous (P) simultaneously. Comparison of density of states exhibits existence of states inside band gap. Furthermore, charge density plot clearly demonstrates electron and hole localization centered around P and B atoms respectively. Hence, highly counter doping with BP atoms turns the crystalline silicon into a quasi-disordered material. Since, highly counter doping introduces disorder in silicon, with this notion naturally disordered materials are explored as possible CEP gain media. Amorphous materials have low mobility due to their nature of disorder. Surprisingly, amorphous silicon (a-Si) photodiodes with thin a-Si layer (~40nm) have shown a gain-bandwidth product of over 2 THz with very low excess noise. To unveil the true gain mechanism, the thesis further delves into theoretical modeling and numerical analysis along with experimental data at different frequencies. Evidence of highly effective carrier multiplication process within a-Si as the primary gain mechanism, especially at high frequency is shown. There is also trap-induced junction modulation at much lower frequency. The analysis further suggests that the carrier multiplication process in thin a-Si can be much more efficient than in thick a-Si, even stronger than single crystalline Si in some cases. Although seemingly counter intuitive, this is consistent with the proposed cycling excitation process (CEP) where the localized states in the bandtails of disordered materials such as a-Si relax the k-selection rule and increase the rate of carrier multiplication. A more rigorous quantum mechanical scattering rate calculation also demonstrates the increase of strength of carrier multiplication with the presence of localized states and the increase of ionization coefficient with decreasing thickness of gain medium. A theoretical framework is offered to calculate the carrier multiplication process in a-Si or other disordered materials involving donor acceptor pairs (DAPs) and to answer several key and seemingly counter intuitive questions such as why amorphous silicon can be more efficient carrier multiplication material than single crystal silicon, why low carrier mobility of amorphous material helps rather than hurt carrier multiplication process, and why thin a-Si is more efficient than thick a-Si in carrier multiplication

Investigating the In Vitro Regeneration Potential of Commercial Cultivars of Brassica

In vitro regeneration is a pre-requisite for developing transgenic plants through tissue culture-based genetic engineering approaches. Huge variations among different genotypes of the genus Brassica necessitate the identification of a set of regeneration conditions for a genotype, which can be reliably used in transformation experiments. In this study, we evaluated the morphogenesis potential of four commercial cultivars (Faisal canola, Punjab canola, Aari canola, Nifa Gold) and one model, Westar, from four different explants namely cotyledons, hypocotyls, petioles and roots on three different Brassica regeneration protocols, BRP-I, -II and -III. The regeneration efficiency was observed in the range of 6–73%, 4–79.3%, 0–50.6%, and 0–42.6% from cotyledons, petioles, hypocotyls and roots, respectively, whereas, the regeneration response in terms of average shoots per explant was found to be 0.76–10.9, 0.2–3.2, 0–3.4 and 0–2.7 from these explants. Of the commercial varieties tested, almost all varieties showed poorer regeneration than Westar except Aari canola. In comparison to Westar, its regeneration frequency from cotyledons was up to 7.5-fold higher on BRP-I, while it produced up to 21.9-fold more shoots per explant. Our data show that the explant has strong influence on the regeneration response, ranging from 24% to 92%. While the growth of commercial cultivars was least affected by the regeneration conditions provided, the effect on Westar was twice that of the commercial cultivars. After determining the optimal explant type and regeneration conditions, we also determined the minimum kanamycin concentration levels required to selectively inhibit the growth of untransformed cells for these cultivars. Regenerated shoots of Aari canola could be successfully grown to maturity within 16–18 weeks, with no altered phenotype noted and normal seed yields obtained. Therefore, the commercial variety, Aari canola, could be a good candidate for future genetic transformation studies

A Hybrid Genetic Wind Driven Heuristic Optimization Algorithm for Demand Side Management in Smart Grid

In recent years, demand side management (DSM) techniques have been designed for residential, industrial and commercial sectors. These techniques are very effective in flattening the load profile of customers in grid area networks. In this paper, a heuristic algorithms-based energy management controller is designed for a residential area in a smart grid. In essence, five heuristic algorithms (the genetic algorithm (GA), the binary particle swarm optimization (BPSO) algorithm, the bacterial foraging optimization algorithm (BFOA), the wind-driven optimization (WDO) algorithm and our proposed hybrid genetic wind-driven (GWD) algorithm) are evaluated. These algorithms are used for scheduling residential loads between peak hours (PHs) and off-peak hours (OPHs) in a real-time pricing (RTP) environment while maximizing user comfort (UC) and minimizing both electricity cost and the peak to average ratio (PAR). Moreover, these algorithms are tested in two scenarios: (i) scheduling the load of a single home and (ii) scheduling the load of multiple homes. Simulation results show that our proposed hybrid GWD algorithm performs better than the other heuristic algorithms in terms of the selected performance metrics