Operation of bulk CMOS devices at very low temperatures

Low-temperature (77K, 4.2K) operation is proposed for bulk CMOS devices to be used in superfast VLSI applications. Symmetrical variation of the parameters of both n-channel and p-channel MOSFETs with respect to the temperature and latch-up immunity makes CMOS a very promising device technology at low temperatures. To demonstrate the performance advantage of circuit operation at low temperatures, inverter chains and 16-kb static random-access memories (RAMs) with 2-/spl mu/m gate length were measured. Average propagation delay for an inverter chain has been reduced to 175 ps (77K) and 104 ps (4.2K) from 296 ps at 300K without sacrificing power dissipation. The power-delay product is less 1 fJ, which is the smallest for silicon devices reported to date. The chip select-access time of the RAM has been reduced to 14.3 ns (77K) from 24 ns (300K).     

Energy relaxation of photoexcited hot electrons at very low temperatures

The scattering of electrons with energy $\text{?<}\text{1}\text{2}\text{ms}{}^2$ by acoustical phonons at very low temperatures T?ms² is investigated.     

Integrated PbS-Si IR detector read-out

A novel technique for direct integration of PbS-Si heterojunction IR detectors and read-out circuitry using planar Si technology has been developed and implemented. X-Y address read-out mode of a detector array has been demonstrated. Injection efficiency of close to unity has been obtained under optimum bias condition at room temperature.     

Influence of substrate freeze-out on the characteristics of MOS transistors at very low temperatures

The low temperatures current-voltage characteristics of N-channel MOS transistors have been analysed. An excess drain current is observed for intermediate values of drain voltage. This anomalous drain current is explained in terms of substrate freeze-out, since at very low temperatures the MOS structure has a type of floating substrate potential within the depletion region. Due to the increase of the majority carrier current, flowing through the substrate to the source at increasing drain voltage, this substrate potential increases and causes a change of threshold voltage. This change is observed in the current-drain voltage characteristics of the MOSFET. Various experiments, such as measurements of substrate current, effects of temperature, gate and substrate voltages, support this interpretation. MOS transistors with various geometries and various dopings are analysed.     

Electrical characteristics of very thin SiO2deposited at low substrate temperatures

Very thin (≲ 100-Å) films of SiO2have been deposited by a modified plasma-enhanced chemical-vapor deposition (PECVD) process at very low substrate temperatures (≲ 350°C). Low flow rates of reactive gases and a high flow of inert carrier gas were used to lower the deposition rate, ensuring improved dielectric properties and good control over film thickness. Measurements made on MOS capacitors of current-voltage characteristics, electrical breakdown, interface trap density, and mobile ion drift indicate that these very thin PECVD films are approaching thermally grown SiO2in quality and may be suitable as gate dielectrics in device applications.     

Transistor noise at low temperatures

It is shown that the shot noise theory of transistors holds for low temperatures in alloy junction transistors, provided that the effect of hole-electron pair recombination in the emitter transition region is taken into account. The discrepancy between theory and experiment reported by Lee and Kaminsky can be accounted for by the fact that this process was ignored.     

Hot-electron-induced MOSFET degradation at low temperatures

The n-channel LDD MOSFET lifetime is observed to followtau=(A/I_{d})(I_{sub}/I_{d})^{-n}from 77 to 295 K when the device is stressed near the maximum Isub. Here Idis the drain current andAis the proportionality constant. The experimental result indicates thatnis approximately 2.7 and is independent of temperature. However, the proportionality constantAfollowsA = A_{0} exp (-E_{a}/kT), withE_{a} = 39meV. The smaller proportionality constant at low temperatures suggests that hot-electron injection (HEI) degradation is caused by the electron trapping in the oxide.     

hFEfalloff at low temperatures

It is shown that for shallow planar bipolar transistors, tunneling, currents through the emitter-base junction can cause pronounced hFEfalloff at low temperatures.     

Visual communication at very low data rates

A study is reported of the extraction of two-level cartoons from moving grey-level images for visual communication in the range 4.8-19.2 kbits/s. It is postulated that perceptually significant features of the human face and hands, at which cartoon lines should be drawn in the image, occur wherever surfaces in object space are approximately tangential to the line of sight of the camera or viewer. The implications of this postulate are analyzed for smooth lambertian reflectors seen against a background of the same luminance factor; it is shown that the combined effects of surface gradient and illumination lead to the formation of luminance valleys at the significant features. An experimental comparison of several detectors is described which confirmed the advantage of valley-based detection for deriving economical but recognizable cartoons.     

Photoinitiated TEA CO laser at low temperatures

Photoinitiated discharges between continuous electrodes have been stabilized in high-pressure (CO, He, N2) laser mixtures at 100 K. A 1/2-1 device, with restricted output aperture, has yielded 1.9-J/I atm (160 torr, 100 K) in smooth 240-μs-long pulses with an efficiency of 2 percent, representing a sevenfold improvement over previous results. The unrestricted output is estimated at 5.7-J/1 atm with 6-percent efficiency.     

MOS flat-band capacitance method at low temperatures

The expression C_FB=C_ox×(ϵ_si /L_D)/[C_ox+(ϵ_si /L_D)] (where L_D is the Debye length), commonly used for the flatband capacitance of the MOS structure, is invalid in the temperature range below 100 K. Consequently, significant error may be encountered when the flatband capacitance method is used to extract the flatband voltage, V _FB, which is of considerable interest for both the modeling and characterization of MOS devices. To extend this method to low-temperature CMOS applications one has to use a more general model that can be obtained by applying Fermi-Dirac statistics and taking into account the impurity freeze-out effect. It is shown that when the temperature dependence of V_FB is extracted using this approach, the experimental data for n⁺ polysilicon gate MOS capacitors are in good agreement with a simple method     

Some CMOS device constraints at low temperatures

Two of the CMOS device constraints at low temperatures have been identified, namely, the transconductance and the breakdown voltage roll-off. In the short channel devices, the transconductance first increases then decreases with the decreasing temperature. This transconductance roll-off phenomenon is likely caused by the parasitic series resistance in the source and drain regions. The breakdown voltage of the MOSFET's due to the parasitic bipolar transistor action decreases with the decreasing temperature, which is caused by the increase of the impact ionization rate at low temperatures.     

Electron-phonon processes in semiconductors at low temperatures

The thermal conductivity and thermopower are simultaneously studied in the semiconductors HgSe, HgTe, InP, GaAs, Cd _x H_{1 ? x} Te, and Ag₂S in which the effect of electron drag by phonons was detected at low temperatures T. It is found that x = 1–3 in the dependences K _ph ∝ T ^?x . It is shown that the underestimated values of x are caused by phonon scattering at certain defects. It is also found that the maxima of K _ph(T) and α_ph(T) coincide in all studied crystals, except for Ag₂S. For the first time, it is found that the strong effect α_ph ∝ T ^?3 is observed in Ag₂S, the α_ph maximum is at T = 16 K, and the K _ph maximum is at 27 K. The results obtained are in agreement with Callaway and Herring theory. The effect of magnetic field on the drag thermopower α_ph(T) in n-Cd _x H_1?x Te (in which the strong dependence α_ph ∝ T ^?3 and the strong effect of the magnetic field on it are detected) is considered for the first time. The results are compared with the Askerov theory. The force parameter A _ph(?) of the drag effect and its dependence on magnetic field are determined.     

Magnetic field integral equation at very low frequencies

It is known that there is a low-frequency breakdown problem when the method of moments (MOM) with Rao-Wilton-Glisson (RWG) basis is used in the electric field integral equation (EFIE); it can be solved through the loop and tree basis decomposition. The behavior of the magnetic field integral equation (MFIE) at very low frequencies is investigated using MOM, where two approaches are presented based on the RWG basis and loop and tree bases. The study shows that MFIE can be solved by the conventional MOM with the RWG basis at arbitrarily low frequencies, but there exists an accuracy problem in the real part of the electric current. Although the error in the current distribution is small, it results in a large error in the far-field computation. This is because a big cancellation occurs during the far field computation. The source of error in the current distribution is easily detected through the MOM analysis using the loop and tree basis decomposition. To eliminate the error, a perturbation method is proposed, from which a very accurate real part of the tree current has been obtained. Using the perturbation method, the error in the far-field computation is also removed. Numerical examples show that both the current distribution and the far field can be accurately computed at extremely low frequencies by the proposed method.     

单取代聚乙炔的荧光光谱在低温下的裂变

在18至300 K的温度范围内,对聚乙炔的一种单取代衍生物的荧光特性进行了研究.在室温时,这种聚乙炔的单取代衍生物的薄膜能够发出强的绿色荧光,其荧光光谱的主要荧光峰位于510 nm,而它的两个次要荧光峰分别位于440 nm和380 nm.位于510、440 nm的两个荧光峰分别是该高分子材料所形成的激发缔合物的主要和次要发光峰,而位于380 nm的荧光峰是单条高分子链的发光峰.当温度从300 K降到18 K的过程中,原荧光光谱发生中的激发缔合物的主要发光峰从510 nm逐渐红移到570 nm,而其激发缔合物的次要发光峰逐渐消失;与此同时,该高分子材料的380 nm的荧光峰逐渐与主荧光峰分开.这些光谱方面的变化可用该高分子在低温下所发生的结构上的变化来解释.     

Transport properties of microcrystalline silicon at low temperatures

The dark conductivity and photoconductivity along with pulsed electron spin resonance have been measured over a wide temperature range with a high crystallinity hydrogenated microcrystalline silicon (μc-Si: H) sample. The transport mechanism in μc-Si: H is discussed on the basis of these measurements. Striking similarities in the temperature dependences of the dark conductivity and photoconductivity between μc-Si: H and some well-studied materials, such as hydrogenated amorphous silicon, suggest that at low temperatures hopping of carriers between localized states dominates the transport properties of μc-Si: H. Fiz. Tekh. Poluprovodn. 32, 905–909 (August 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

Thermal effects in p-channel MOSFETs at low temperatures

Thermal effects due to device self-heating in p-channel enhancement MOSFETs operated at cryogenic temperatures are discussed. Device heating is observed through drain-current transients, and the drain current is used to monitor heating. Comparisons are made with earlier results for n-channel devices. Some implications for low-temperature CMOS operation are considered     

Performance of RF MEMS switches at low temperatures

The actuation voltage of microelectromechanical system (MEMS) metal switches was investigated at temperatures ranging from 10 to 290 K. The investigation shows a 50% increase in the actuation voltage at low temperature. A comparison has been made using a published model and showed similar increment of actuation voltage at low temperature     

Carrier heating effects in junctions at very low currents

The effects of carrier heating by the electric field are analysed in abrupt $\text{p-n}$ junctions at very small currents. Two features, which are usually neglected, are shown to be of importance. (a) The heat transported by the carriers plays an appreciable part in the energy balance equation, which determines the carrier temperature. (b) The current has, in the junction and in the neighbourhood, two important additional components, one being due to a correction of the diffusion current, and the other being proportional to the gradient in the temperature of the carriers.