The temperature dependent electrical characteristics of silicon carbide power transistors – 4H-SiC metal semiconductor field-effect transistors (MESFETs) and 4H-SiC bipolar junction transistors (BJTs) have been investigated through simulation and experimental approaches. Junction temperatures and temperature distributions in devices under large power densities have been estimated. The DC and RF performance of 4H-SiC RF Power MESFETs have been studied through two-dimensional electro-thermal simulations using commercial software MEDICI and ISE. The simulated characteristics of the transistors were compared with the measurement results. Performance degradation of transistors under self-heating and high operating temperatures have been analyzed in terms of gate and drain characteristics, power density, high frequency current gain and power gain. 3D thermal simulations have been performed for single and multi-finger MESFETs and the simulated junction temperatures and temperature profiles were compared with the results from electro-thermal simulations. The reduction in drain current caused by self-heating was found to be more prominent for transistors with more fingers and it imposes a limitation on both the output power and the power density (in W/mm) of multi-fingered large area devices. Thermal issues for design of high power multi-fingered SiC MESFETs were also investigated. A couple of useful ways to reduce the self-heating effects were discussed. Trap-induced performance instabilities of the devices were analyzed by carrying out DC, transient, and pulse measurements at room and elevated temperatures. Electrical characteristics of 4H-SiC BJTs have been measured. A reduction in current gain at elevated temperatures was observed. Based on the collector current-voltage diagram measured at three different ambient temperatures the junction temperature was extracted using the assumption that the current gain only depends on the temperature. Temperature measurements have been carried out for SiC BJTs. Thermal images of a device under operation were recorded using an infrared camera. 3D thermal simulations were conducted using FEMLAB. Both the simulations and the measurement showed a significant temperature increase in the vicinity of the device when operated at high power densities, thus causing the decrease of the DC current gain. The junction temperatures obtained from the thermal imaging, simulation and extraction agree well.

The superior characteristics of silicon carbide, compared with silicon, have suggested considering this material for the next generation of power semiconductor devices. Among the different power switches, the bipolar junction transistor (BJT) can provide a very low forward voltage drop, a high current capability and a fast switching speed. However, in order to compete on the market, it is crucial to a have high current gain and a breakdown voltage close to ideal. Moreover, the absence of conductivity modulation and long-term stability has to be solved. In this thesis, these topics are investigated comparing simulations and measurements. Initially, an efficient etched JTE has been simulated and fabricated. In agreement with the simulations, the fabricated diodes exhibit the highest BV of around 4.3 kV when a two-zone JTE is implemented. Furthermore, the simulations and measurements demonstrate a good agreement between the electric field distribution inside the device and the optical luminescence measured at breakdown. Additionally, an accurate model to simulate the forward characteristics of 4H-SiC BJTs is presented. In order to validate the model, the simulated current gains are compared with measurements at different temperatures and different base-emitter geometries. Moreover, the simulations and measurements of the on-resistance are compared at different base currents and different temperatures. This comparison, coupled with a detailed analysis of the carrier concentration inside the BJT, indicates that internal forward biasing of the base-collector junction limits the BJT to operate at high current density and low forward voltage drop simultaneously. In agreement with the measurements, a design with a highly-doped extrinsic base is proposed to alleviate this problem. In addition to the static characteristics, the comparison of measured and simulated switching waveforms demonstrates that the SiC BJT can provide fast switching speed when it acts as a unipolar device. This is crucial to have low power losses during transient. Finally, the long-term stability is investigated. It is observed that the electrical stress of the base-emitter diode produces current gain degradation; however, the degradation mechanisms are still unclear. In fact, the analysis of the measured Gummel plot suggests that the reduction of the carrier lifetime in the base-emitter region might be only one of the causes of this degradation. In addition, the current gain degradation due to ionizing radiation is investigated comparing the simulations and measurements. The simulations suggest that the creation of positive charge in the passivation layer can increase the base current; this increase is also observed in the electrical measurements. / QC 20120522

Silicon carbide is a promising wide bandgap semiconductormaterial for high-temperature, high-power, and high-frequencydevice applications. However, there are still a number offactors that are limiting the device performance. Among them,one of the most important and critical factors is the formationof low resistivity Ohmic contacts and high-temperature stableSchottky diodes on silicon carbide. In this thesis, different metals (TiW, Ti, TiC, Al, and Ni)and different deposition techniques (sputtering andevaporation) were suggested and investigated for this purpose.Both electrical and material characterizations were performedusing various techniques, such as I-V, C-V, RBS, XRD, XPS,LEED, SEM, AFM, and SIMS. For the Schottky contacts to n- and p-type 4H-SiC, sputteredTiW Schottky contacts had excellent rectifying behavior afterannealing at 500 ºC in vacuum with a thermally stableideality factor of 1.06 and 1.08 for n- and p-type,respectively. It was also observed that the SBH for p-type SiC(ΦBp) strongly depends on the choice the metal with alinear relationship ΦBp= 4.51 - 0.58Φm, indicating no strong Fermi-level pinning.Finally, the behavior of Schottky diodes was investigated byincorporation of size-selected Au nano-particles in Ti Schottkycontacts on silicon carbide. The reduction of the SBH isexplained by using a simple dipole layer approach, withenhanced electric field at the interface due to the small sizeof the circular patch (Au nano-particles) and large differenceof the barrier height between two metals (Ti and Au) on both n-and p-SiC. For the Ohmic contacts, titanium carbide (TiC) was used ascontacts to both n- and p-type 4H-SiC epilayers as well as onAl implanted layers. The TiC contacts were epitaxiallydeposited using a co-evaporation method with an e-beam Tisource and a Knudsen cell for C60, in a UHV system at low substrate temperature(500 ºC). In addition, we extensively investigatedsputtered TiW (weight ratio 30:70) as well as evaporated NiOhmic contacts on both n- and p-type epilayers of SiC. The bestOhmic contacts to n-type SiC are annealed Ni (&gt;950ºC)with the specific contact resistance of ≈ 8× 10-6Ω cm2with doping concentration of 1.1 × 10-19cm-3while annealed TiW and TiC contacts are thepreferred contacts to p-type SiC. From long-term reliabilitytests at high temperature (500 ºC or 600 ºC) invacuum and oxidizing (20% O2/N2) ambient, TiW contacts with a platinum cappinglayer (Pt/Ti/TiW) had stable specific contact resistances for&gt;300 hours. <b>Keywords</b>: silicon carbide, Ohmic and Schottky contacts,co-evaporation, current-voltage, capacitance-voltagemeasurement, power devices, nano-particles, Schottky barrierheight lowering, and TLM structures.

For roughly fifteen years the military has sought to use the properties of ceramics for armor applications. Current high-performance ceramics have extremely high compressive strengths and low densities. One ceramic that has been shown to be highly resistant under ballistic impact is silicon carbide (SiC). It has been found that even within the silicon carbides, those manufactured by certain methods and those with certain microstructural properties have advantages over others. In order to understand the microstructural reasons behind variations in ballistic properties, plate impact tests were conducted on two sintered silicon carbides with slightly different microstructures. Two variations of a silicon carbide with the trade name Hexoloy SA were obtained through Saint Gobain. Regular Hexoloy (RH) and Enhanced Hexoloy (EH) are pressureless sintered products having exactly the same chemistries. EH went through additional powder processing prior to sintering, producing a final product with a slightly different morphology than RH. Samples of each were characterized microstructurally including morphology, density, elastic wavespeeds, microhardness, fracture toughness, and flexure strength. The characterization revealed differences in porosity distribution and flexure strength. It was determined that the porosity distribution in EH had fewer large pores leading to an 18% increase in flexural strength over that for RH. The focus of the mechanics of materials community concerning dynamic material behavior is to pin down what exactly is happening microstructurally during ballistic events. Several studies have been conducted where material properties of one ceramic type are varied and the dynamic behavior is tested and analyzed. Usually, from one variation to the next, several properties are different making it hard to isolate the effect of each. For this study, the only difference in the materials was porosity distribution. Plate impact experiments were conducted at the Army Research Laboratory (ARL) using the gas gun facilities within the Impact Physics Branch. A VISAR was utilized to measure free surface velocities. Tests were performed on each material to determine the Hugoniot Elastic Limit (HEL) and spall strength. Spall strength was measured as a function of impact stress, and pulse duration.

The structure and the energy band gap of hydrogenated amorphous silicon carbide are theoretically revised. In the light of defect pool model, density of states distribution is investigated for various regions of mobility gap. The films are deposited by plasma enhanced chemical vapor deposition system with various gas concentrations at two different, lower (30 mW/cm2) and higher (90 mW/cm2), radio frequency power densities. The elemental composition of hydrogenated amorphous silicon carbide films and relative composition of existing bond types are analyzed by x-ray photoelectron spectroscopy measurements. The thicknesses, deposition rates, refractive indices and optical band gaps of the films are determined by ultraviolet visible transmittance measurements. Uniformity of the deposited films is analyzed along the radial direction of the bottom electrode of the plasma enhanced chemical vapor deposition reactor. The molecular vibration characteristics of the films are reviewed and analyzed by Fourier transform infrared spectroscopy measurements. Electrical characteristics of the films are analyzed by dc conductivity measurements. Conduction mechanisms, such as extended state, nearest neighbor and variable range hopping in tail states are revised. The hopping conductivities are analyzed by considering the density of states distribution in various regions of mobility gap. The experimentally measured activation energies for the films of high carbon content are too low to be interpreted as the difference between Fermi level and relevant band edge. This anomaly has been successfully removed by introducing hopping conduction across localized tail states of the relevant band. In other words, the second contribution lowers the mobility edge towards the Fermi level.

Achieving light weight armor design has become an important engineering challenge in the last three decades. As weapons becoming highly sophisticated, so does the ammunition, potential targets have to be well protected against such threats. In order to provide mobility, light and effective armor protection materials should be used. In this thesis, numerical simulation of the silicon carbide armor backed by KevlarTM composite and orthogonally impacted by 7.62mm armor piercing (AP) projectile at an initial velocity of 850 m/s is analyzed by using AUTODYN hydrocode. As a first step, ceramic material behavior under impact conditions is validated numerically by comparing the numerical simulation result with the test result which is obtained from the literature. Then, different numerical simulations are performed by changing the backing material thickness, i.e. 2, 4, 6 and 8mm, while the thickness of the ceramic is held constant, i.e. 8mm. At the end of the simulations, optimum ceramic/composite thickness ratio is sought. The results of the simulations showed that for the backing thickness values of 4, 6 and 8mm, the projectile could not perforate the armor system. On the contrary, the projectile could penetrate and perforate the armor system for the backing thickness value of 2mm and it has still some residual velocity. From these results, it is inferred that the optimum ceramic/composite thickness ratio is equal to about 2 for the silicon carbide and kevlar configuration.

Porous carbon derived from an inorganic silicon carbide (SiC) precursor, termed SiC-derived carbon, is an attractive material for electrochemical energy storage applications, including electrodes for electrical double layer capacitors (EDLCs). The objective of this thesis is to investigate the effects that the carbide-derived carbon (CDC) microstructure and pore structure have on the energy and power characteristics of the EDLC electrodes. Conventional SiC CDC is produced from non-porous crystalline SiC powder at temperatures above 800 °C. Here we studied the performance of SiC CDCs produced by chlorination at 700-900 °C of an ordered mesoporous SiC precursor, which was synthesized via a 1000 °C pyrolysis of polycarbosilane infiltrated into an SBA-15 silica template having ordered mesopores. The SiC CDC was purified from chlorine impurities by annealing in ammonia. The surface area and pore size of the purified SiC CDC was characterized via N2 and CO2 sorption using density functional theory (DFT) and Brunnauer, Emmet, and Teller (BET) theory. The specific capacitance, power and energy densities were characterized via electrochemical measurements of the SiC CDC electrodes in 1 M tetraethylammonium tetrafluoroborate (TEABF4) acetonitrile solution. The SiC CDC exhibited a specific surface area (SSA) in excess of 2400 m2/g and gravimetric capacitance values of up to ~ 150 F/g, among the highest ever reported for any electrodes in this electrolyte. The ordered mesopores allowed for fast ion transport within each particle, resulting in excellent capacity retention under high current rates and ultra-fast frequency response, thus allowing for extremely high power and energy densities. The best overall performance was achieved in SiC CDC samples chlorinated at the lowest temperature of 700 °C.

<p>Silicon carbide is a promising wide bandgap semiconductormaterial for high-temperature, high-power, and high-frequencydevice applications. However, there are still a number offactors that are limiting the device performance. Among them,one of the most important and critical factors is the formationof low resistivity Ohmic contacts and high-temperature stableSchottky diodes on silicon carbide.</p><p>In this thesis, different metals (TiW, Ti, TiC, Al, and Ni)and different deposition techniques (sputtering andevaporation) were suggested and investigated for this purpose.Both electrical and material characterizations were performedusing various techniques, such as I-V, C-V, RBS, XRD, XPS,LEED, SEM, AFM, and SIMS.</p><p>For the Schottky contacts to n- and p-type 4H-SiC, sputteredTiW Schottky contacts had excellent rectifying behavior afterannealing at 500 ºC in vacuum with a thermally stableideality factor of 1.06 and 1.08 for n- and p-type,respectively. It was also observed that the SBH for p-type SiC(Φ_Bp) strongly depends on the choice the metal with alinear relationship Φ_Bp= 4.51 - 0.58Φ_m, indicating no strong Fermi-level pinning.Finally, the behavior of Schottky diodes was investigated byincorporation of size-selected Au nano-particles in Ti Schottkycontacts on silicon carbide. The reduction of the SBH isexplained by using a simple dipole layer approach, withenhanced electric field at the interface due to the small sizeof the circular patch (Au nano-particles) and large differenceof the barrier height between two metals (Ti and Au) on both n-and p-SiC.</p><p>For the Ohmic contacts, titanium carbide (TiC) was used ascontacts to both n- and p-type 4H-SiC epilayers as well as onAl implanted layers. The TiC contacts were epitaxiallydeposited using a co-evaporation method with an e-beam Tisource and a Knudsen cell for C₆₀, in a UHV system at low substrate temperature(500 ºC). In addition, we extensively investigatedsputtered TiW (weight ratio 30:70) as well as evaporated NiOhmic contacts on both n- and p-type epilayers of SiC. The bestOhmic contacts to n-type SiC are annealed Ni (>950ºC)with the specific contact resistance of ≈ 8× 10^-6Ω cm²with doping concentration of 1.1 × 10^-19cm^-3while annealed TiW and TiC contacts are thepreferred contacts to p-type SiC. From long-term reliabilitytests at high temperature (500 ºC or 600 ºC) invacuum and oxidizing (20% O₂/N₂) ambient, TiW contacts with a platinum cappinglayer (Pt/Ti/TiW) had stable specific contact resistances for>300 hours.</p><p><b>Keywords</b>: silicon carbide, Ohmic and Schottky contacts,co-evaporation, current-voltage, capacitance-voltagemeasurement, power devices, nano-particles, Schottky barrierheight lowering, and TLM structures.</p>

<p>The temperature dependent electrical characteristics of silicon carbide power transistors – 4H-SiC metal semiconductor field-effect transistors (MESFETs) and 4H-SiC bipolar junction transistors (BJTs) have been investigated through simulation and experimental approaches. Junction temperatures and temperature distributions in devices under large power densities have been estimated. </p><p>The DC and RF performance of 4H-SiC RF Power MESFETs have been studied through two-dimensional electro-thermal simulations using commercial software MEDICI and ISE. The simulated characteristics of the transistors were compared with the measurement results. Performance degradation of transistors under self-heating and high operating temperatures have been analyzed in terms of gate and drain characteristics, power density, high frequency current gain and power gain. 3D thermal simulations have been performed for single and multi-finger MESFETs and the simulated junction temperatures and temperature profiles were compared with the results from electro-thermal simulations. The reduction in drain current caused by self-heating was found to be more prominent for transistors with more fingers and it imposes a limitation on both the output power and the power density (in W/mm) of multi-fingered large area devices. Thermal issues for design of high power multi-fingered SiC MESFETs were also investigated. A couple of useful ways to reduce the self-heating effects were discussed. Trap-induced performance instabilities of the devices were analyzed by carrying out DC, transient, and pulse measurements at room and elevated temperatures. </p><p>Electrical characteristics of 4H-SiC BJTs have been measured. A reduction in current gain at elevated temperatures was observed. Based on the collector current-voltage diagram measured at three different ambient temperatures the junction temperature was extracted using the assumption that the current gain only depends on the temperature. Temperature measurements have been carried out for SiC BJTs. Thermal images of a device under operation were recorded using an infrared camera. 3D thermal simulations were conducted using FEMLAB. Both the simulations and the measurement showed a significant temperature increase in the vicinity of the device when operated at high power densities, thus causing the decrease of the DC current gain. The junction temperatures obtained from the thermal imaging, simulation and extraction agree well. </p>

Silicon carbide (SiC) is one of the hardest known materials and is also, by good fortune, a wide bandgap semiconductor. While the application of SiC for high-temperature and high-power electronics is fairly well known, its utility as a highly robust, chemically-inert material for microelectrical mechanical systems (MEMS) is only beginning to be well recognized. SiC can be grown on both native SiC substrates or on Si using heteroepitaxial growth methods which affords the possibility to use Si micromachining methods to fabricate advanced SiC MEMS devices. The control of film stress in heteroepitaxial silicon carbide films grown on polysilicon-on-oxide substrates has been investigated. It is known that the size and structure of grains within polycrystalline films play an important role in determining the magnitude and type of stress present in a film, i.e. tensile or compressive. Silicon carbide grown on LPCVD polysilicon seed-films exhibited a highly-textured grain structure and displayed either a positive or negative stress gradient depending on the initial thickness of the polysilicon seed-layer. In addition a high-quality (111) oriented 3C-SiC on (111)Si heteroepitaxial process has been developed and is reported. SiC MEMS structures, both polycrystalline (i.e., poly-3C-SiC) and monocrystalline (i.e., 3C-SiC) were realized using micromachining methods. These structures were used to extract the stress properties of the films, with a particular focus on separating the gradient and uniform stress components.