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A device to protect electronic circuitry from high voltage transients is constructed from a relatively thin piece of conductive composite sandwiched between two conductors so that conduction is through the thickness of the composite piece. The device is based on the discovery that conduction through conductive composite materials in this configuration switches to a high resistance mode when exposed to voltages above a threshold voltage.
Silicon-oxide-nitride-oxide-silicon (SONOS) nonvolatile semiconductor memories (NVSMS) have many advantages. These memories are electrically erasable programmable read-only memories (EEPROMs). They utilize low programming voltages, endure extended erase/write cycles, are inherently resistant to radiation, and are compatible with high-density scaled CMOS for low power, portable electronics. The SONOS memory cell erase cycle was investigated using a nonquasi-static (NQS) MOSFET model. The SONOS floating gate charge and voltage, tunneling current, threshold voltage, and drain current were characterized during an erase cycle. Comparisons were made between the model predictions and experimental device data.
A 3-micron CMOS timing sampler is described which is a test circuit designed into the Jet Propulsion Labs' CRRES chip to be flown on the Combined Release and Radiation Effects Satellite (CRRES). The timing sampler consists of 64 inverter-pair stages with sampling latches and decoder circuitry. The sampler is used to measure inverter-pair propagation delays, which are nominally 2.5 ns, with a resolution of 100 ps. A simple model was developed to explain the radiation-induced inverter-pair delay shifts in terms of radiation-induced MOSFET threshold-voltage shifts and effective modal capacitances. The magnitude of the shift in pair delay with radiation was estimated at the point where the n-MOSFET threshold voltage became zero. For a 0.7-V-threshold shift, the pair delay increased from its preradiation value by 360 ps for a rising step input and decreased by 190 ps for a falling step input.
The characteristics of electrical discharges induced on silverbacked Teflon samples irradiated by a monoenergetic electron beam have been studied under controlled laboratory conditions. Measurements of breakdown threshold voltages indicate a marked anisotropy in the electrical breakdown properties of Teflon: differences of up to 10 kV in breakdown threshold voltage are observed depending on the sample orientation. The material anisotropy can be utilized in spacecraft construction to reduce the magnitude of discharge currents.
The addressable inverter matrix consists of 222 inverters each accessible with the aid of a shift register. The structure has proven useful in characterizing the variability of inverter transfer curves and in diagnosing processing faults. For good 3-micron CMOS bulk inverters investigated in this study, the percent standard deviation of the inverter threshold voltage was less than one percent and the inverter gain (the slope of the inverter transfer curve at the inverter threshold voltage) was less than 3 percent. The average noise margin for the inverters was near 2 volts for a power supply voltage of 5 volts. The specific faults studied included undersize pull-down transistor widths and various open contacts in the matrix.
The threshold voltages of a carbon-nanotube (CNT) field-effect transistor (FET) are studied. The CNT channel is so thin that there is no voltage drop perpendicular to the gate electrode plane, and this makes the device characteristics quite unique. The relation between the voltage and the electrochemical potentials, and the mass action law for electrons and holes are examined in the context of CNTs, and inversion and accumulation threshold voltages (V(sub Ti), and V(sub Ta)) are derived. V(sub Ti) of the CNTFETs has a much stronger doping dependence than that of the metal-oxide- semiconductor FETs, while V(sub Ta) of both devices depends weakly on doping with the same functional form.
An electrostatic shield for complementary MOS integrated circuits was developed to minimize the adverse effects of stray electric fields created by the potentials in the metal interconnections. The process is compatible with silicon gate technology. N-doped polycrystalline silicon was used for all the gates and the shield. The effectiveness of the shield was demonstrated by constructing a special field plate over certain transistors. The threshold voltages obtained on an oriented silicon substrate ranged from 1.5 to 3 V for either channel. Integrated inverters performed satisfactorily from 3 to 15 V, limited at the low end by the threshold voltages and at the high end by the drain breakdown voltage of the n-channel transistors. The stability of the new structure with an n-doped silicon gate as measured by the shift in C-V curve under 200 C plus or minus 20 V temperature-bias conditions was better than conventional aluminum gate or p-doped silicon gate devices, presumably due to the doping of gate oxide with phosphorous.
Measurements of the current-voltage characteristics of a Langmuir probe in the near wake of a disk immersed in a pulsed flowing plasma were made. A 1 cm diameter biasable sphere was placed in the ion-free near wake region of a 10 cm diameter disk immersed in a Mach 8 pulsed flowing plasma. The current-voltage characteristic of the sphere was observed as a function of time as the sphere bias was scanned from -5000 V to +1000 V. The collected current is found to be monotonically increasing with increasing positive bias voltage but exhibits a threshold voltage for current collection as the bias voltage becomes more negative. Potential measurements in the wake region were made for a sphere bias voltages below, at, and above the current collection threshold for a number of times during the wake formation period. The time evolution of the potential profile is shown to change as the sheath around the biased sphere is established. Predictions from the particle trajectory code SIMION are compared with data, showing excellent agreement in the prediction of the current collection threshold.
We have introduced in a simple and efficient manner quantum mechanical corrections in our 3D 'atomistic' MOSFET simulator using the density gradient formalism. We have studied in comparison with classical simulations the effect of the quantum mechanical corrections on the simulation of random dopant induced threshold voltage fluctuations, the effect of the single charge trapping on interface states and the effect of the oxide thickness fluctuations in decanano MOSFETs with ultrathin gate oxides. The introduction of quantum corrections enhances the threshold voltage fluctuations but does not affect significantly the amplitude of the random telegraph noise associated with single carrier trapping. The importance of the quantum corrections for proper simulation of oxide thickness fluctuation effects has also been demonstrated.
A 3D statistical 'atomistic' simulation technique has been developed to study the effect of the random dopant induced parameter fluctuations in aggressively scaled MOSFETs. Efficient implementation of the 'atomistic' simulation approach has been used to investigate the threshold voltage standard deviation and lowering in the case of uniformly doped MOSFETs, and in fluctuation-resistant architectures utilising epitaxial-layers and delta-doping. The effect of the random doping in the polysilicon gate on the threshold voltage fluctuations has also been thoroughly investigated. The influence of a single-charge trapping on the channel conductivity in decanano MOSFETs is studied in the 'atomistic' framework as well. Quantum effects are taken into consideration in our 'atomistic' simulations using the density gradient formalism.
With the onset of quantum confinement in the inversion layer in nanoscale MOSFETs, behavior of the resonant level inevitably determines all device characteristics. While most classical device simulators take quantization into account in some simplified manner, the important details of electrostatics are missing. Our work addresses this shortcoming and provides: (a) a framework to quantitatively explore device physics issues such as the source-drain and gate leakage currents, DIBL, and threshold voltage shift due to quantization, and b) a means of benchmarking quantum corrections to semiclassical models (such as density- gradient and quantum-corrected MEDICI). We have developed physical approximations and computer code capable of realistically simulating 2-D nanoscale transistors, using the non-equilibrium Green's function (NEGF) method. This is the most accurate full quantum model yet applied to 2-D device simulation. Open boundary conditions, oxide tunneling and phase-breaking scattering are treated on equal footing. Electrons in the ellipsoids of the conduction band are treated within the anisotropic effective mass approximation. Quantum simulations are focused on MIT 25, 50 and 90 nm "well- tempered" MOSFETs and compared to classical and quantum corrected models. The important feature of quantum model is smaller slope of Id-Vg curve and consequently higher threshold voltage. These results are quantitatively consistent with I D Schroedinger-Poisson calculations. The effect of gate length on gate-oxide leakage and sub-threshold current has been studied. The shorter gate length device has an order of magnitude smaller current at zero gate bias than the longer gate length device without a significant trade-off in on-current. This should be a device design consideration.
An energy storage system for a spacecraft is provided which employs a solar powered flywheel arrangement including a motor/generator which, in different operating modes, drives the flywheel and is driven thereby. A control circuit, including a threshold comparator, senses the output of a solar energy converter, and when a threshold voltage is exceeded thereby indicating the availability of solar power for the spacecraft loads, activates a speed control loop including the motor/generator so as to accelerate the flywheel to a constant speed and thereby store mechanical energy, while also supplying energy from the solar converter to the loads. Under circumstances where solar energy is not available and thus the threshold voltage is not exceeded, the control circuit deactivates the speed control loop and activates a voltage control loop that provides for operation of the motor as a generator so that mechanical energy from the flywheel is converted into electrical energy for supply to the spacecraft loads.
Due to the common use of Flash technology in many commercial and industrial Programmable Logic Devices (PLDs) such as FPGAs and mixed-signal microcontrollers, flash technology is being utilized in fuzed munition applications. This presents a long-term reliability issue for both DoD and NASA safety- and mission-critical applications. A thorough understanding of the data retention failure modes and statistics associated with Flash data retention is of vital concern to the fuze safety community. A key retention parameter for a flash cell is the threshold voltage (VTH), which is an indirect indicator of the amount of charge stored on the cells floating gate. Initial test results based on a study of charge loss in flash cells in an FPGA device is presented. Statistical data taken from a small sample set indicates quantifiable charge loss for devices stored at both room temperature and 150 C. Initial evaluation of the distribution of threshold voltage in a large sample set (800 devices) is presented. The magnitude of charge loss from exposure to electrostatic discharge and electromagnetic fields is measured and presented. Simulated data (and measured data as available) resultant from harsh-environment testing (neutron, heavy ion, EMP) is presented.
Due to the common use of Flash technology in many commercial and industrial Programmable Logic Devices (PLDs) such as FPGAs and mixed-signal microcontrollers, flash technology is being utilized in fuzed munition applications. This presents a long-term reliability issue for both DoD and NASA safety- and mission-critical applications. A thorough understanding of the data retention failure modes and statistics associated with Flash data retention is of vital concern to the fuze safety community. A key retention parameter for a flash cell is the threshold voltage (VTH), which is an indirect indicator of the amount of charge stored on the cells floating gate. Initial test results based on a study of charge loss in flash cells in an FPGA device is presented. Statistical data taken from a small sample set indicates quantifiable charge loss for devices stored at both room temperature and 150 C. Initial evaluation of the distribution of threshold voltage in a large sample set (800 devices) is presented. The magnitude of charge loss from exposure to electrostatic discharge and electromagnetic fields is measured and presented. Simulated data (and measured data as available) resultant from harsh-environment testing (neutron, heavy ion, EMP) is presented.
When MOSFETs are scaled to deep submicron dimensions the discreteness and randomness of the dopant charges in the channel region introduces significant fluctuations in the device characteristics. This effect, predicted 20 year ago, has been confirmed experimentally and in simulation studies. The impact of the fluctuations on the functionality, yield, and reliability of the corresponding systems shifts the paradigm of the numerical device simulation. It becomes insufficient to simulate only one device representing one macroscopical design in a continuous charge approximation. An ensemble of macroscopically identical but microscopically different devices has to be characterized by simulation of statistically significant samples. The aims of the numerical simulations shift from predicting the characteristics of a single device with continuous doping towards estimating the mean values and the standard deviations of basic design parameters such as threshold voltage, subthreshold slope, transconductance, drive current, etc. for the whole ensemble of 'atomistically' different devices in the system. It has to be pointed out that even the mean values obtained from 'atomistic' simulations are not identical to the values obtained from continuous doping simulations. In this paper we present a hierarchical approach to the 'atomistic' simulation of aggressively scaled decanano MOSFETs. A full scale 3D drift-diffusion'atomostic' simulation approach is first described and used for verification of the more economical, but also more restricted, options. To reduce the processor time and memory requirements at high drain voltage we have developed a self-consistent option based on a thin slab solution of the current continuity equation only in the channel region. This is coupled to the Poisson's equation solution in the whole simulation domain in the Gummel iteration cycles. The accuracy of this approach is investigated in comparison with the full self-consistent solution. At low drain voltage only single solution of the nonlinear Poisson equation is sufficient to extract the current with satisfactory accuracy. A pilot version of a hydrodynamic 'atomistic' simulator has been developed in order to study the effect of the nonequilibrium, non local transport in decanano MOSFETs on the random dopant induced current fluctuations. For the first time we have also applied the density gradient approach in 3D to investigate the effect of the quantum confinement on the threshold voltage fluctuations. The developed 'atomistic' simulation techniques have been applied to study various fluctuation resistant MOSFET architectures including epitaxial and delta doped devices.
With the onset of quantum confinement in the inversion layer in nanoscale MOSFETs, behavior of the resonant level inevitably determines all device characteristics. While most classical device simulators take quantization into account in some simplified manner, the important details of electrostatics are missing. Our work addresses this shortcoming and provides: (a) a framework to quantitatively explore device physics issues such as the source-drain and gate leakage currents, DIBL, and threshold voltage shift due to quantization, and b) a means of benchmarking quantum corrections to semiclassical models (such as density-gradient and quantum-corrected MEDICI). We have developed physical approximations and computer code capable of realistically simulating 2-D nanoscale transistors, using the non-equilibrium Green's function (NEGF) method. This is the most accurate full quantum model yet applied to 2-D device simulation. Open boundary conditions and oxide tunneling are treated on an equal footing. Electrons in the ellipsoids of the conduction band are treated within the anisotropic effective mass approximation. We present the results of our simulations of MIT 25, 50 and 90 nm "well-tempered" MOSFETs and compare them to those of classical and quantum corrected models. The important feature of quantum model is smaller slope of Id-Vg curve and consequently higher threshold voltage. Surprisingly, the self-consistent potential profile shows lower injection barrier in the channel in quantum case. These results are qualitatively consistent with ID Schroedinger-Poisson calculations. The effect of gate length on gate-oxide leakage and subthreshold current has been studied. The shorter gate length device has an order of magnitude smaller current at zero gate bias than the longer gate length device without a significant trade-off in on-current. This should be a device design consideration.
Lightning transients were pin-injected into metal-oxide-semiconductor field-effect transistors (MOSFETs) to induce fault modes. This report documents the test process and results, and provides a basis for subsequent lightning tests. MOSFETs may be present in DC-DC power supplies and electromechanical actuator circuits that may be used on board aircraft. Results show that unprotected MOSFET Gates are susceptible to failure, even when installed in systems in well-shielded and partial-shielded locations. MOSFET Drains and Sources are significantly less susceptible. Device impedance decreased (current increased) after every failure. Such a failure mode may lead to cascading failures, as the damaged MOSFET may allow excessive current to flow through other circuitry. Preliminary assessments on a MOSFET subjected to 20-stroke pin-injection testing demonstrate that Breakdown Voltage, Leakage Current and Threshold Voltage characteristics show damage, while the device continues to meet manufacturer performance specifications. The purpose of this research is to develop validated tools, technologies, and techniques for automated detection, diagnosis and prognosis that enable mitigation of adverse events during flight, such as from lightning transients; and to understand the interplay between lightning-induced surges and aging (i.e. humidity, vibration thermal stress, etc.) on component degradation.
The current focus of NASA's space flight programs reflects a new thrust towards smaller, less costly, and more frequent space missions, when compared to missions such as Galileo, Magellan, or Cassini. Recently, the concept of a microspacecraft was proposed. In this concept, a small, compact spacecraft that weighs tens of kilograms performs focused scientific objectives such as imaging. Similarly, a Mars Lander micro-rover project is under study that will allow miniature robots weighing less than seven kilograms to explore the Martian surface. To bring the microspacecraft and microrover ideas to fruition, one will have to leverage compact 3D multi-chip module-based multiprocessors (MCM) technologies. Low energy CMOS will become increasingly important because of the thermodynamic considerations in cooling compact 3D MCM implementations and also from considerations of the power budget for space applications. In this paper, we show how the operating voltage is related to the threshold voltage of the CMOS transistors for accomplishing a task in VLSI with minimal energy. We also derive expressions for the noise margins at the optimal operating point. We then look at a low voltage CMOS (LVCMOS) technology developed at Stanford University which improves the power consumption over conventional CMOS by a couple of orders of magnitude and consider the suitability of the technology for space applications by characterizing its SEU immunity.