The Zak transform and some counterexamples in time-frequencyanalysis

It is shown how the Zak transform can be used to find nontrivial examples of functions f, g∈L²(R) with f×g≡0≡F×G, where F, G are the Fourier transforms of f, g, respectively. This is then used to exhibit a nontrivial pair of functions h, k∈L²(R), h≠k, such that |h|=|k|, |H |=|K|. A similar construction is used to find an abundance of nontrivial pairs of functions h, k∈L² (R), h≠k, with |A_h |=|A_k| or with |W_h|=|W _k| where A_h, A_k and W_h, W_k are the ambiguity functions and Wigner distributions of h, k, respectively. One of the examples of a pair of h, k∈L²(R), h≠k , with |A_h|=|A_k| is F.A. Grunbaum's (1981) example. In addition, nontrivial examples of functions g and signals f₁≠f₂ such that f₁ and f₂ have the same spectrogram when using g as window have been found     

Multiterminal source encoding with one distortion criterion

The authors unify earlier investigations concerning the encoding of two correlated sources {X_k}, {Y_k } by means of separate encoders. Decoding is done by a single decoder which receives the outputs from both encoders. The reconstruction of {X_k} is required to be perfect in the usual Shannon sense. The authors determine the admissible rate region R (D), where D is the distortion of the reconstruction of {Y_k}. The binary Hamming case is investigated explicitly     

A simple characterization method for silicon-on-insulator materialsusing a depletion-mode MOSFET

A simple method is proposed for extracting the electrical parameters of a silicon-on-insulator (SOI) material from a depletion-mode MOSFET. It is based on an analysis of static input current-voltage I_D(V_G) and transconductance-voltage g_m(V_G) characteristics in the linear region. Functions varying linearly with gate voltage are constructed from I_D(V_G) and g_m(V_G) functions. These new functions allow a straightforward determination of the parameters usually obtained from a capacitance-voltage measurement (doping level, oxide charge, etc.) and also the bulk-layer and accumulation-layer carrier mobility     

Bayes estimation of reliability under a random environment governedby a Dirichlet prior

The reliability function of a component whose lifetime is exponentially distributed with a known parameter λ>0 is R (t|λ)=exp (-λt). If an environmental effect multiplies the parameter by a positive factor η, then the reliability function becomes R(t|η,λ)=exp(-ηλt). The authors assume that η itself is random, and its uncertainty is described by a Dirichlet process prior D(α) with parameter α=MG₀, where M>O represents an intensity of assurance in the prior guess, G₀, of the (unknown) distribution of η. Under squared error loss, the Bayes estimator of R(t|η,λ) is derived both for the no-sample problem and for a sample of size n. Using Monte Carlo simulation, the effects of n, M, G₀ on the estimator are studied. These examples show that: (a) large values of n lead to estimates where the data outweigh the prior, and (b) large values of M increase the contribution of the prior to the estimates. These simulation results support intuitive ideas about the effect of environment and lifetime parameters on reliability     

Free-electron laser operation in the intermediate gain region

The modification induced on the well-known antisymmetric gain function of a free-electron laser when the gain coefficient g ₀ ranges up to 10 is studied. Deviations from the linear regime for g₀⩾0.5 are presented, and a simple perturbative analysis which accounts remarkably well for the corrections to the linear gain formula even for g₀=10 is discussed. The effect of inhomogeneous broadening is also discussed     

Asymptotic distortion spectrum of clipped, DC-biased, Gaussiannoise [optical communication]

Consider a zero-mean, stationary Gaussian process g(t ), to which a large positive constant A has been added. Define a distortion process h_A(t) as equal to g(t)+A when the latter is negative and equal to zero otherwise. The author calculates the power spectrum of the process h_A(t) asymptotically as A becomes large. The results have application for estimating the nonlinear-distortion power in the recovered signal when many frequency-multiplexed subcarriers collectively modulate a laser's output power, as would be the case for CATV transmission over an optical fiber. The process h_A(t) then models the nonlinear distortion caused by occasional clipping of the DC-biased laser input     

Importance of source and drain resistance to the maximum fT of millimeter-wave MODFETs

The usual approximate expression for measured f_T =[g_m/2π (C_gs+C _gd)] is inadequate. At low drain voltages just beyond the knee of the DC I-V curves, where intrinsic f _t is a maximum for millimeter-wave MODFETs, the high values of C_gd and G_ds combine with the high g_m to make terms involving the source and drain resistance significant. It is shown that these resistances can degrade the measured f_T of a 0.30-μm GaAs-AlGaAs MODFET from an intrinsic maximum f_T value of 73 GHz to a measured maximum value of 59 GHz. The correct extraction of maximum f_T is essential for determining electron velocity and optimizing low-noise performance     

Velocity saturation in the collector of Si/GexSi1-x/Si HBT's

The effects of velocity saturation on the unity gain-bandwidth product f_t and transconductance g_m of n-p-n and p-n-p heterojunction bipolar transistors (HBTs) with Ge_xSi_1-x bases are described and simulated. For the n-p-n device, velocity saturation combined with a valence-band offset at the base-collector junction causes accelerated g_m and f_t rolloff for current densities greater than the knee current for the Kirk effect. For the p-n-p device, the g_m and f_t are degraded for all current densities. These limitations combine with the limits imposed by dislocation formation due to strain in the pseudomorphic layer to impose constraints on the design of Si/Ge_x Si_1-x/Si HBTs     

p-channel FET based on p/n double-quantum-well heterostructure

An experimental demonstration of a p-channel FET based on a heterostructure having vertically integrated p- and n-type quantum-well channels is discussed. The AlGaAs/GaAs heterostructure consists of a quantum well with an underlying p-region positioned above a second quantum well with an underlying n-region. The p-FET is fabricated with self-aligned p⁺ regions formed by zinc diffusion. Electrical characteristics for 1.5-μm gate lengths are nearly ideal in appearance with a maximum I_d of 90 mA/mm, a g _m of 80 mS/mm, and a g_m/g _d ratio of 140 at 77 K. The results demonstrate the viability of such stratified structures for the development of complementary integrated circuits or other circuits requiring integration of multiple device types     

Improved hot-carrier immunity in submicrometer MOSFETs withreoxidized nitrided oxides prepared by rapid thermal processing

The fabrication of 0.8-μm MOSFETs using 7.7-nm-thick nitrided oxides reoxidized by rapid thermal processing at 900-1150°C for 15-200 s is described. The hot-carrier-induced degradation was studied in terms of subthreshold swing, threshold voltage V_T, and transconductance g_m voltage characteristics. Results indicate that rapid reoxidation markedly improves hot-carrier immunity; lifetimes reaching 30-mV V _T shift and 10 percent g_m degradation are improved by 3 and 1.5 orders of magnitude compared with those for thermal oxides, respectively. A degradation characteristic inherent to the (reoxidized) nitrided-oxide system is found, based on the gate-voltage dependence of g_m degradation     

Low-temperature DC characteristics of pseudomorphic Ga0.18In0.82P/InP/Ga0.47In0.53As HEMT

A high-gap strained GaInP material chosen to increase Schottky barrier height on InP is discussed. This material has been used for the first time in high electron mobility transistor (HEMT) fabrication on InP. For these devices the best g_m of a 1.3-μm gate HEMT is 300 mS/mm. Transistors of 3-μm gate length are studied at low temperature (100 to 293 K). Their DC electrical characteristics improve upon cooling. The best improvement is measured at the lowest temperature (+54% for g_m at 105 K). The structure is stable and does not present any g_m or I_ds collapse at low temperature, unlike AlGaAs/GaAs heterostructures     

Sequential imperfect preventive maintenance policies

Improvement factors in hazard rate and age for a sequential preventative maintenance (PM) policy are introduced. Two imperfect PM models are analyzed: (1) PM reduces the hazard rate while it increases with the number of PMs, and (2) PM reduces the age. The PM is done at intervals x_k (k=1,2, . . . , N) and is imperfect. The optimal policies to minimize the mean cost rates are discussed. The optimal PM sequences {x_k} are computed for a Weibull distribution     

NOTFET: a high-frequency InP nonlinear transistor

A device called the nonlinearly optimized transconductance field-effect transistor (NOTFET) has been developed for nonlinear circuit applications. An InP NOTFET with two g_m peaks based on InGaAs and InAlAs channels was designed fabricated, and tested for harmonic generation up to 12 GHz, demonstrating the device concept and its advantages compared to conventional FETs and HEMTs. Multipeak g_m/V_GS characteristics also can be obtained by using a multiheterojunction material structure with uncoupled quantum wells, a possibility which is under investigation     

Optimal probability-of-error thresholds and performance for twoversions of the sign detector

Expressions are obtained for specifying the optimal error probability (minimum P_e) thresholds λ₀₁ and λ₀₂ for the traditional and modified sign detectors, respectively. These thresholds are shown to depend on the parameters p, P₁, and M where: M is the number of observations z_i used in the test statistic; P₁=P(H₁ ) is the prior probability for hypothesis H₁ that signal s₁ is present and 1-P₁ =P(H₀) corresponds to the hypothesis H₀ that signal s₀ is present; and p=Pr{z_i⩾0|H₁} with s₀=0 for the traditional sign detector and p=Pr{z_i⩾λ|H₁ }=Pr{z_i<λ|H₀} with λ =(s₀+s₁)/2 for the modified sign detector. The expressions for λ₀₁ and λ₀₂, are given explicitly, and shown to be independent of P₁ for sufficiently large M. Optimal P_e versus M performance curves, corresponding to both versions of the sign detector, are obtained for a representative range of values for p and P₁     

Roles of low field mobility and its carrier-concentrationdependences in high electron mobility transistors and other field effecttransistors

A model is proposed for high-electron-mobility transistors (HEMTs) and other heterostructure FETs in which the dependence of low field mobility μ on carrier concentration N_s is taken into account. On the basis of this model, the influence of μ and its N_s dependence on drain current and transconductance g_m are clarified, In particular, high mobility (>10⁵ cm²/V-s) is shown to be effective in achieving and maintaining the intrinsic limit of g_m(=ε₂ν_s/d*) irrespective of bias conditions, where ν_s is the saturation velocity and ε₂ and d* are the dielectric permittivity and the effective thickness of the gate insulator, respectively. The N_s dependence of mobility is found to greatly affect the gate-voltage dependence of g _m and leads, in some cases, to an appreciable increase of g_m above its intrinsic limit     

New hot-carrier degradation mode in PMOSFETs with W gate electrodes

Hot-carrier degradation of W gate PMOSFETs, which are surface-channel devices because of the work function of W, has been investigated in comparison with polycide (WSi_x/n⁺ poly-Si) ones. In W gate PMOSFETs, transconductance g_m and threshold voltage V_th decrease on the drain avalanche hot-carrier (DAHC) stress, and Δg_m /g_m0 and ΔV_th become minimum at V_G≅V_D/2. By using the charge-pumping technique, it is found that, after stressing at the same stress condition, the interface state density of W gate devices is about 10 times larger than that of polycide ones but the densities of trapped electrons are almost equal. These results indicate that the difference of hot-carrier degradation between W and polycide gate devices is mainly caused by the difference of the interface state density     

AlInAs-GaInAs HEMT for microwave and millimeter-wave applications

The status of lattice-matched high-electron-mobility transistors (HEMTs) and pseudomorphic AlInAs-GaInAs grown on In substrates is reviewed. The best lattice-matched devices with 0.1-μm gate length had a transconductance g_m=1080 mS/mm and a unity current gain cutoff frequency f_T=178 GHz, whereas similar pseudomorphic HEMTs had g_m=1160 mS/mm and f_T=210 GHz. Single-stage V-band amplifiers demonstrated 1.3- and 1.5-dB noise figures and 9.5- and 8.0-dB associated gains for the lattice-matched and pseudomorphic HEMTs, respectively. The best performance achieved was a minimum noise figure of F_min=0.8 dB with a small-signal gain of G_a=8.7 dB     

A new two-mode channel FET (TMT) for super-low-noise and high-powerapplications

A super-low-noise two-mode channel FET (TMT) with high- and plateau-shaped transconductance (g_m) characteristics has been developed. It has two electron transport modes against the applied gate voltage (V_gs). That is, the electrons mainly drift in a highly doped channel region at a shallow V_gs. A plateau g_m region and the maximum g_m were achieved at a V_gs range of -0.25~+0.5 V and 535 mS/mm, respectively. The minimum noise figure and associated gain for the TMT were superior in the low-drain-current (I_ds) region and nearly equal in the middle and high I_ds region to those of an AlGaAs/InGaAs pseudomorphic HEMT fabricated using the same wafer process and device geometry     

MOSFET capacitance and conductance in the saturation regime

The relationship between capacitance and conductance of a MOSFET is examined in the region where velocity saturation dominates. In this domain it is shown that charge on the drain terminal (physically distinct from that in the channel) most be considered in order to keep the model from predicting the unphysical result that C_gd is negative. It is also shown that, under the same assumptions, g_m<C_gg/τ where g _m is the transconductance, τ is the transit time, and C_gg is the gate capacitance     

Improved transconductance under high normal field MOSFETs withultrathin nitrided oxides

The gate-voltage dependence of electron mobility at 298 and 82 K in MOSFETs with nanometer-range thin (reoxidized) nitrided oxides prepared by rapid thermal processing (RTP) at 900-1150°C for 15-300 s is discussed. Rapid nitridation improves the mobility and current derivability under high normal field over thermally grown oxides at both temperatures: the transconductance g_m at a gate drive of 3.5 V is improved by half an order of magnitude, whereas the peak g_m remains comparable to that of an oxide. Nitridation also avoids the negative g_m observed at 82 K for an oxide MOSFET. These improvements are substantially unchanged by additional reoxidations