Turn-on Characteristics Research Articles

A novel thin-film transistor with self-aligned field induced drain

In this letter, a novel thin-film transistor with a self-aligned field-induced-drain (SAFID) structure is reported for the first time. The new SAFID TFT features a self-aligned sidewall spacer located on top of the drain offset region to set its effective length, and a bottom gate (or field plate) situated under the drain offset region to electrically induce the field-induced-drain (FID). So, unlike the conventional off-set-gated TFTs with their effective FID length set by two separate photolithographic masking layers, the new SAFID is totally immune to photomasking misalignment errors, while enjoying the low off-state leakage as well as high turn-on characteristics inherent in the FID structure. Polycrystalline silicon TFTs with the new SAFID structure have been successfully fabricated with significant improvement in the on/off current ratio.

Advanced semiconductor switches for EM launchers

Electric launchers may be energized by switching pulses of variable magnitude and time from a pulsed alternator or a pulse forming network. In order to achieve the desired muzzle energy, the switches must withstand the action of the current pulse: the square of current integrated over time. High pressure spark gap switches and vacuum switches of various architectures have been utilized in most of the launcher development programs. Advances in semiconductor technology hold a promise of replacing these large and bulky switches with solid state switches which may become leading candidates for future combat systems applications. The US Army Research Laboratory has been testing and evaluating solid state switches of various types for several different applications. One type of semiconductor switch that was tested and evaluated is a thyristor that is 125 mm in diameter made by Silicon Power Corporation (SPCO). When a thyristor with this large diameter is turned on, it is necessary to develop and spread plasma uniformly and quickly across the wafer. This ensures that the current from the anode to the cathode is uniformly distributed, and not concentrated in local hot spots where an excessive current density could cause overheating and damage. This plasma spread can be controlled by the design of the gate structure and the triggering technique. Thyristors with different gate structures were tested and compared. The experimental findings of the turn-on characteristics and the maximum current capabilities for each design are presented. The status of futuristic wide band gap semiconductor switch technology is also reviewed.

Simulation of Drastic Lag Phenomena in GaAs-Based FETs for Large Voltage Swing

Turn-on characteristics of GaAs MESFETs and HEMTs are simulated when the gate voltage is changed abruptly. The gate-lag or slow current transient becomes more pronounced when the off-state gate voltage is more negative, because the surface-state effects or substrate-trap effects become more significant. Changes of I–V curves of GaAs MESFETs, when the drain voltage is swept with different speeds, are also simulated. When the swept time is short, the curve shows overshoot-like behavior and the kink disappears, indicating that the I–V characteristics should be quite different between DC and RF conditions.

An analytical model for turn-on characteristics of short channel polycrystalline-silicon thin-film transistor for circuit simulation

Short channel effects are incorporated to investigate the impact of the channel length on the turn-on characteristics of a polycrystalline silicon thin-film transistor (poly-Si TFT). The developed threshold voltage and field effect mobility are the key parameters in analysing the above-threshold characteristics. The expressions for the device transconductance and drain conductance are then developed, from which the channel resistance has also been extracted. The expressions so developed are simple and can be extensively used in modeling the short channel TFT. The predicted results are compared with available experimental data, and excellent matching confirms the validity of the model.

CMOS shallow-trench-isolation to 50-nm channel widths

The applicability of shallow-trench-isolation (STI) for CMOS to 50-nm channel widths has been explored. Transistors with channel width to 50 nm and trench width to 200 nm have been fabricated. A comparison of several oxide-filled and polysilicon field-plate-filled STI structures is presented including processing, device performance, and isolation leakage. It is shown that V/sub th/ roll off as a function of channel width can be made as small as 65 mV and 145 mV at 100 nm channel width for polysilicon and oxide filled STI, respectively. Off-state currents less than 5/spl times/10/sup -12/ A//spl mu/m and subthreshold slope around 80 mV/dec have been reached. Isolation breakdown voltages are about 8 V. Poly-filled STI effectively reduces channel edge effects, and provides excellent off-state, on-state, and turn-on characteristics all the way to 50-nm channel widths.

ZnS-based visible-blind UV detectors: Effects of isoelectronic traps

The aim of this study is to reveal the underlying cause of the gradual turn-on characteristic of low Te containing ZnSTe Schottky barrier photodiodes. The results of photoresponse studies on ZnS, ZnSSe and ZnSTe diodes indicate that the Te isoelectronic trapping effect is responsible for the gradual turn-on characteristic of low Te containing ZnSTe Schottky barrier photodiodes. The results also reveal that the ZnSSe diode, having the advantage of being free of isoelectronic centers, is a more suitable choice for applications requiring high visible rejection power. It is demonstrated that highly UV sensitive responsivity with an abrupt long wavelength cutoff tailored to lie between 340–400 nm can be achieved in the ZnSSe diode system.

Two-dimensional analysis of substrate-trap effects on turn-on characteristics in GaAs MESFETs

Effects of substrate traps on turn-on characteristics of GaAs MESFETs are studied by two dimensional (2-D) simulation. When the off-state gate voltage is much more negative than the threshold (pinch off) voltage and the surface-state effects are small, abnormal current overshoot and subsequent slow transients are observed for the case with undoped semi-insulating substrate including an electron trap: EL2. Even if the surface-state effects are pronounced to show the large gate-lag, the drain current may show the overshoot-like behavior at relatively early periods. The case of Cr-doped substrate with a hole trap: Cr is also discussed.

The problem and treatment of DC offsets in FDTD simulations

This paper discusses the causes of and some solutions to the commonly observed problem of DC field offsets in finite-difference time-domain (FDTD) simulations. DC electric and magnetic field offsets are shown to be valid calculated responses of the modeled systems, resulting from interaction between the turn-on characteristics of the source and the properties of the models. The DC offsets may be avoided in the time domain by tailoring the source waveforms or in the frequency domain by post-processing the FDTD output.

Photoresponse studies of ZnSSe visible–blind ultraviolet detectors: A comparison to ZnSTe detectors

This work focuses on the investigation of the difference between the photoresponse of ZnS, ZnSSe, and that of ZnSTe Schottky-barrier photodiodes, with a particular aim to reveal the underlying causes of the gradual turn-on characteristic of low-Te-containing ZnSTe Schottky barrier photodiodes. To form the bottom electrode layer for the newly developed ZnSSe diode, n-type doping of ZnSSe by incorporating Al flux during molecular beam epitaxial growth was studied. Excellent-to-good dopant activation is achieved for Se composition up to 50%. The measured photoresponse of the diodes clearly indicates that the Te isoelectronic trapping effect is responsible for the gradual turn-on characteristic of low-Te-containing ZnSTe Schottky-barrier photodiodes. The results also reveal that the ZnSSe diode, having a much better visible rejection power, is a more suitable choice for high-performance visible–blind ultraviolet detection applications.

Two-dimensional analysis of surface-state effects on turn-on characteristics in GaAs MESFETs

Effects of substrate traps on turn-on characteristics of GaAs MESFETs are studied by two dimensional (2-D) simulation. When the off-state gate voltage is much more negative than the threshold (pinch off) voltage and the surface-state effects are small, abnormal current overshoot and subsequent slow transients are observed for the case with undoped semi-insulating substrate including an electron trap: EL2. Even if the surface-state effects are pronounced to show the large gate-lag, the drain current may show the overshoot-like behavior at relatively early periods. The case of Cr-doped substrate with a hole trap: Cr is also discussed.

Numerical analysis of substrate effect on turn-oncharacteristics of GaAs MESFET

The effects of substrate trap ‘EL2’ on the turn-on characteristics of GaAs MESFETs are studied through two-dimensional simulation. It is found that abnormal current overshoot and subsequent slow transients can be observed due to EL2 when the off-state gate voltage is strongly negative and electrons are also depleted in the substrate.

Turn-on characteristics of polycrystalline silicon TFT's-impact of hydrogenation and channel length

Experimental measurements and two-dimensional (2-D) numerical simulation have been used to investigate the impart of the polycrystalline silicon (poly-Si) density of states (DOS) and channel dimensions on the transient response of poly-Si thin-film transistors (TFTs). TFTs exposed to different hydrogenation times were used to investigate the effect of the poly-Si DOS. The experimental results show that TFT turn-on time increases with increasing channel length and decreases with increasing hydrogenation time. For the first time, transient simulations were carried out using best fit poly-Si DOS distributions which were extracted numerically from DC transfer characteristics. The resulting simulations show excellent agreement with the experimental data. Degradation in the transient characteristic is thereby correlated with an increase In the poly-Si DOS.

A compact double-gate MOSFET model comprising quantum-mechanical and nonstatic effects

In this work, we investigate the electrical properties of the Double-Gate MOSFET (DG-MOSFET), which turn out to be very promising for device miniaturization below 0.1 /spl mu/m. A compact model which accounts for charge quantization within the channel, Fermi statistics, and nonstatic effects in the transport model is worked out. The main results of this investigation are: (1) the ideality factor in subthreshold is equal to unity, i.e., the slope of the turn-on characteristic is 60 mV/decade at room temperature; (2) the drain-induced barrier lowering is minimized by the shielding effect of the double gate, which allows us to reduce the channel length below 30 nm; and (3) the device transconductance per unit width is maximized by the combination of the double gate and by a strong velocity overshoot which occurs in response to the sudden variation of the electric field at the source end of the channel, and which can be further strengthened near the drain in view of the short device length. As a result, a sustained electron velocity of nearly twice the saturation velocity is achievable. The above results prove that the potential performance advantages of the double-gate device architecture may be worth the development effort.

Characteristics of Surface-Channel Strained Si1-yCyn-MOSFETS

AbstractThe first demonstration of n-MOSFETs fabricated using strained Si1-yCy surface channels is reported. Tensile-strained Si1-yCy layers with substitutional carbon contents up to 0.8 atomic percent were epitaxially grown on <100> Si substrates by rapid thermal chemical vapor deposition, using silane and methylsilane as the silicon and carbon precursors. n-MOSFETS were fabricated using standard MOS processing with reduced thermal exposure to minimize the possibility of strain relaxation. A remote plasma CVD oxide was employed to form the gate oxide. The Si1-yCy devices exhibit electrical characteristics that are typical for Si n-MOSFETs, with good turn-on and subthreshold characteristics. MOS capacitance-voltage analysis demonstrates comparable oxide interface qualities for the Si1-yCy and Si control devices. No carbon-related leakage current is observed in source and drain diode junctions. Characterization of the MOSFET electron inversion layer mobility at room temperature shows comparable mobilities, within the sensitivity of the measurement, for the Si1-yCy and Si control devices. This is in contrast to the mobility enhancement observed in n-MOSFETs fabricated using tensile- strained Si grown on relaxed Si1-xGex layers. At low temperatures, the inversion layer mobility of Si1-yCy devices is lower than that of the Si controls, and appears to be affected by Coulomb and possibly random alloy scattering.

Bis(dithienothiophene) organic field-effect transistors with a high ON/OFF ratio

A conjugated, fused-thiophene oligomer, bis(dithienothiophene) (BDT), has been synthesized and deposited by vacuum sublimation as the active layer in organic thin film transistors (TFTs). The TFTs show exceptionally high ON/OFF ratios up to 108 between accumulation and depletion with sharp turn-on characteristics comparable to that of amorphous silicon TFTs (subthreshold slope S=0.6 V/decade). Field-effect mobilities are 0.02–0.05 cm2/V s. The good performance is explained by the relatively high π-π* gap of the short-chain BDT molecule and the favorable coplanar π-π stacking in BDT, differing from the herringbone stacking in the oligothiophenes.

Study on the Diamond Field Emitter Fabricated by Transfer Mold Technique

ABSTRACTStrip-shaped diamond-tip field emitter array was fabricated by using the transfer mold technique. The sharp turn-on characteristic was observed from the current-voltage measurement of the fabricated diamond-tip field emitter array. The turn-on characteristic of the diamond-tip field emitter array was compared with that of a flat diamond film. High emission current density was obtained from the diamond-tip field emitter array. The threshold voltage of the diamond-tip field emitter array was lower than that of a flat diamond film.

Inversion channel edge in trench-isolated sub-1/4-μm MOSFET's

A comprehensive quantitative study of edge effects in the channel of trench isolated sub-1/4-/spl mu/m CMOS devices is presented. Model devices derived from CMOS technologies have been examined using three-dimensional (3-D) finite element device simulation. The off-state, turn-on, and on-state device characteristics of the channel edge and terminal currents and their sensitivity to channel edge geometry, doping, oxide thickness and interface charges have been quantified. Analysis of on-state current shows that edge-effect models should include comparable contributions from decrease in threshold voltage and increase in effective width. The edge effect is shown to interact with the short-channel effect. Methods for minimizing the trench edge effect are quantified including edge doping, channel edge rounding, and recessing the channel below the top of the trench. The placement of a fixed- or floating-potential field plate in the trench is shown to be an effective method for controlling the edge effect below 100-nm channel widths.

Improvement of Turn-on Characteristics of the High Voltage MOSFETs Switch with the Master/Slave Connection.

Improvement of Turn-on Characteristics of the High Voltage MOSFETs Switch with the Master/Slave Connection.

Low-dimensional resonant tunnelling and Coulomb blockade: a comparison of fabricated versus impurity confinement

Nanometre-scale fabrication techniques, combined with epitaxial resonant tunnelling structures, now routinely allow the study of quasi-0D confined electron systems. In addition to energy level separations that are tunable by the confining potentials, these systems can also exhibit Coulomb blockade. Surprisingly similar effects are also observed for conventional, unconfined resonant tunnelling devices. We have recently discovered that the turn-on characteristics of nearly all resonant tunnelling devices exhibit sharp peaks in conductance, attributable to tunnelling through single quantum well donor states. These unintentional donor states are distributed in energy, depending on position in the quantum well. We have performed electronic spectroscopy of these states, and consistently find binding energies approximately 10 meV greater than expected for a single quantum well donor due to quantum well fluctuations. We present detailed measurements of single-electron tunnelling through a single donor bound state utilizing simple non-confined heterostructures.

Improved turn-on characteristics of a hot electron transistor at 300 K

Through compositional grading of the collector, we have significantly reduced both the offset and saturation voltage of a tunneling hot-electron transfer amplifier relative to a similar transistor with a fixed composition collector structure. In the absence of a collector-base voltage, the electric field produced by the compositional grading improves the collection efficiency of ballistic electrons transported across the base. >