Polysilicon thin-film transistors with channel length and widthcomparable to or smaller than the grain size of the thin film

Poly-Si thin-film transistors (TFTs) with channel dimensions (width W, and length L) comparable to or smaller than the grain size of the poly-Si film were fabricated and characterized. The grain size of the poly-Si film was enhanced by Si ion implantation followed by a low-temperature anneal and was typically 1 to 3 μm in diameter. A remarkable improvement was observed in the device characteristics as the channel dimensions decreased to W=L =2 μm. On the other hand, TFTs with submicrometer channel dimensions were characterized by an extremely abrupt switching in their I_D versus V_GS characteristics. The improvement was attributed to a reduction in the effect of the grain boundaries and to the effect of the device's floating body     

Analysis of N×N passive optical starcoupler based on the normal modes of N input waveguides

An integrated passive N×N optical star coupler on silicon wafer is described. Antiresonant reflecting optical waveguides (ARROWs) are analyzed and utilized as the input and output waveguides of the N×N coupler. Combining the exact solutions of the slab ARROW waveguide with the effective index method, a 5×5 coupler is analyzed. In the slab waveguide analysis, the input waveguides are coupled to their neighbors. The interaction of the waveguides is described in terms of the normal modes of propagation. The resultant field distribution is then diffracted into the free space region which separates the input and output sections. The radiation illuminates the receiving aperture from which the receiving N waveguides branch out, each output element obtaining equal power levels. Different types of loss such as spillover loss and mismatch loss were analyzed and estimated for N=5. A 5×5 star coupler with a transmission efficiency of 56% at a wavelength of 1.3 μm is achievable     

A new technique for measuring MOSFET inversion layer mobility

An experimental technique for accurately determining both the inversion charge and the channel mobility μ of a MOSFET is presented. With this new technique, the inversion charge is measured as a function of the gate and drain voltages. This improvement allows the channel mobility to be extracted independent of drain voltage V_DS over a wide range of voltages (V_DS=20-100 mV). The resulting μ(V_GS) curves for different V_DS show no drastic mobility roll-off at V _GS near V_TH. This suggests that the roll-off seen in the mobility data extracted using the split C- V method is probably due to inaccurate inversion charge measurements instead of Coulombic scattering     

1/f noise in MODFETs at low drain bias

The 1/f noise in normally-on MODFETs biased at low drain voltages is investigated. The experimentally observed relative noise in the drain current S_I/I² versus the effective gate voltage V_G=V_GS-V_off shows three regions which are explained. The observed dependencies are S_I/I²∝V_G ^m with the exponents m=-1, -3, 0 with increasing values of V_G. The model explains m =-1 as the region where the resistance and the 1/f noise stem from the 2-D electron gas under the gate electrode; the region with m=0 at large V_G or V_GS≅0 is due to the dominant contribution of the series resistance. In the region at intermediate V_G , m=-3, the 1/f noise stems from the channel under the gate electrode, and the drain-source resistance is already dominated by the series resistance     

Design of a single-mode tapered waveguide for low-losschip-to-fiber coupling

The novel waveguide structures described in this paper have nonlinearly tapered shapes that result in low radiation losses despite their relatively short lengths. The core at the waveguide endface connected with the fiber has a very small cross section and an expanded mode field with a non-Gaussian shape. The taper structures are analyzed by using an improved step-transition method. This method is a based on the theory of enclosing a waveguide within electrical walls and that can therefore treat the radiation modes in a tapered waveguide as discrete mode spectra. Analyzing the relationships between the lengths and shapes of the tapers and the radiation loss due to the tapers show that appropriately tapered semiconductor waveguides operating at an optical wavelength of 1.55 μm and having a taper length of less than 0.7 mm can have a radiation loss of only 0.1 dB and a coupling loss with a conventional single-mode fiber of less than 0.5 dB     

Capacity of the Gaussian arbitrarily varying channel

The Gaussian arbitrarily varying channel with input constraint Γ and state constraint Λ admits input sequences x=(x₁,---,X_n) of real numbers with Σx_i²⩽nΓ and state sequences s=(S₁,---,s_n ) of real numbers with Σs_i²⩽nΛ; the output sequence x+s+V, where V=(V₁,---,V_n) is a sequence of independent and identically distributed Gaussian random variables with mean 0 and variance σ². It is proved that the capacity of this arbitrarily varying channel for deterministic codes and the average probability of error criterion equals 1/2 log (1+Γ/(Λ+σ²)) if Λ<Γ and is 0 otherwise     

Guided wave modulators in Ti ion implanted LiNbO3waveguides

Electrooptic modulators in Ti-ion-implanted LiNbO₃ waveguides are discussed. Low loss (<1-dB/cm) planar and channel waveguides were fabricated and compared to indiffused waveguides. Higher Δn values are obtained, allowing smaller waveguide geometries and tighter mode confinement. Wavelengths of 0.85 and 1.3 μm are used. The small mode profiles resulting from the Ti doses up to 4×10¹⁷ Ti/cm² resulted in V-L products of 8.8 V-mm at 0.85 μm and 20 V-mm at 1.3 μm. These values are lower than any previously reported for a Mach-Zehnder modulator using a buffer layer. Comparison of diffused and implanted waveguide modulators indicated that modular efficiency can be optimized by electrode gap spacing and enhanced with smaller mode profiles achievable in implanted guides     

Modeling of nonhyperbolic sine I-Vcharacteristics in poly-Si resistors

Poly-Si resistors with an unimplanted channel region (and with n-type source/drain regions) can exhibit a nonhyperbolic sine (non-sinh) I-V characteristic at low V_DS and an activation energy which is not simply decreasing monotonically with increasing V_DS. These phenomena are not explained by conventional poly-Si resistor models. To describe these characteristics, a self-consistent model which includes the effects of a reverse-biased diode at the drain end is presented. Numerical simulation results show excellent agreement with experiment in regard to the shape of the I -V characteristic and of the effective activation energy as a function of V_DS     

A dielectric model of the vegetation effects on the microwaveemission from soils

A layer of vegetation over the soil surface absorbs some of the radiation emitted from the soil and emits at its own temperature. This results in a reduction of the information in the microwave radiation about the soil surface. To study this problem further the authors use the model of F.T. Ulaby and M.A. El-Rayes (1987) for the dielectric constant of vegetation to estimate the absorption loss and optical depth, τ, of plant canopies for frequencies between 1 and 40 GHz. The authors treated τ as the product of a vegetation parameter b and vegetation water content, VW. They compared both the linear and square root (refractive) mixing models with the observed data in terms of the b parameter. These data were obtained from published reports on the values of τ and VW for crops ranging from prairie grass to corn and soybeans. The data fit the curve for the refractive model quite well. For the refractive model the value of b was independent of VW, while for the linear model there was some dependence on VW. For both models b is roughly proportional to the frequency     

Coupling of modes in bent biconically tapered single-mode fibers

Biconically tapered single-mode fibers are fabricated by heating a single-mode fiber while applying tension. As a result of this tapering, cladding modes are excited in the tapered region of the fiber where the V parameter of the fiber goes down below 1.0. These cladding modes couple to one another, leading to fluctuations of the optical throughput. Since the index difference between the cladding and the external medium is rather high (~0.5), the cladding modes will be of the exact type, namely, TE, TM, HE, and EH modes. The coupling of these modes in the tapered region is analyzed using the exact mode formalism when the tapered region is bent. The theoretical results agree very well with experimental results obtained on bent tapers which show strong fluctuations of the optical power as a function of the bending angle     

A complete model of the I-V characteristics fornarrow-gate MOSFETs

A unified and process-independent MOSFET model for accurate prediction of the I-V characteristics and the threshold voltages of narrow-gate MOSFETs is discussed. It is based on several enhancements of the SPICE2 LEVEL3 MOS model and the author's previous subthreshold I-V model. The expressions achieved for the drain current hold in the subthreshold, transition, and strong inversion regions. A continuous model is proposed for the transition region, using a scheme that ensures that both the current and conductance are continuous and will not cause convergence problems for circuit simulation applications. All of the modeled parameters are taken from experimentally measured I-V characteristics and preserve physical meaning. Comparisons between the measured and modeled I-V characteristics show excellent agreement for a wide range of channel widths and biases. The model is well suited for circuit simulation in SPICE     

Light-to-input nonlinearities in an R-LED series network

The light-to-current (L-I) and light-to-voltage (L-V) differential nonlinearities in the simple network of a customary LED and an external resistor R in series are analyzed and calculated theoretically and compared with experimental data. Particular emphasis is placed on the influence of the log-arithmetic slope ν of the L-I characteristic and the bias current I upon the ratio of the corresponding nonlinearity parameters. It is thus deduced that, for a given optical power P, over superlinear portions of the L-I curve (ν>1) the L-I linearity is typically better than its corresponding L-V linearity. On the contrary, when the L-I dependence is sublinear (ν<1) the voltage driving scheme may ensure for the R-LED network, or the LED alone, a local L-V response much more linear than the L-I response, provided that appropriate (optimum) I and/or R values are chosen     

A new technique to determine the average low-field electronmobility in MESFET using C-V measurement

A method to determine the average low-field mobility using the number of electrons available for the conduction based on C-V measurement is proposed. This technique requires neither information of the doping profile in the channel, nor the exact value of the threshold voltage. For a D-mode MESFET, the average electron mobility magnitude is compared with that of the C. Chen and D.K. Arch (1989) method. The technique to determine the average electron mobility in the channel described is much simpler. Based on C- V measurement, good agreement is obtained between experimental data and simulation calculation for the electron density in the channel. Using the proposed method, the dependence of average electron mobility on the gate voltage is also proposed. Using the proposed method for determining the average electron mobility, the effect of a p-buried layer on the mobility was investigated, and is in good agreement with the physical phenomena     

Counting the number of minimum cuts in undirected multigraphs

The problem of counting the number of cuts with the minimum cardinality in an undirected multigraph arises in various applications, such as testing the super-λ-ness of a graph, as described by F.T. Boesch (1986), and calculating upper and lower bounds on the probabilistic connectedness of a stochastic graph G in which edges are subject to failure. It is shown that the number |C( G)| of cuts with the minimum cardinality λ(G) in a multiple graph G=(V,E) can be computed in O(|E|+λ(G)|V|² +λ(G)|C(G)||V|) time     

Recombination current in forward-biased p-n junctions

An analytical expression for the recombination current in a forward-biased p-n junction is derived and it is shown that formulas given for the recombination current in most textbooks overestimate the recombination current by a large factor of the order of (V_bi-V)/V_th where V _bi is the built-in voltage, V is the applied forward-bias voltage, and V_th is the thermal voltage     

An efficient N×N passive optical starcoupler

A novel method is presented for configuring an N× N passive star optical coupler and eliminating the excess loss problem associated with such couplers. A space-varying refractive index slab is introduced as a key design element for such a coupler. The wave mixing method is used to implement the coupler     

Balancing sets of vectors

For n>0, d⩾0, n≡d (mod 2), let K(n, d) denote the minimal cardinality of a family V of ±1 vectors of dimension n, such that for any ±1 vector w of dimension n there is a v∈V such that |v- w|⩽d, where v-w is the usual scalar product of v and w. A generalization of a simple construction due to D.E. Knuth (1986) shows that K(n , d)⩽[n/(d+1)]. A linear algebra proof is given here that this construction is optimal, so that K(n, d)-[n/(d+1)] for all n≡d (mod 2). This construction and its extensions have applications to communication theory, especially to the construction of signal sets for optical data links     

The effect of channel hot-carrier stressing on gate-oxide integrityin MOSFETs

The correlation between channel hot-carrier stressing and gate-oxide integrity is studied. It is found that channel hot carriers have no detectable effect on gate-oxide integrity even when other parameters (e.g., ΔV_T and ΔI _D) have become intolerably degraded. In the extreme cases of stressing at V_G≈V_T with measurable hole injection current, however, the oxide charge to breakdown decreases linearly with the amount of hole fluence injected during the channel hot-hole stressing. This may limit the endurance of a nonvolatile memory using hot holes for erasing. This can also explain the gate-to-drain breakdown of a device biased in the snap-back region, since snap-back at low gate voltage is favorable for hole injection. Snap-back-induced oxide breakdown could be an ESD (electrostatic discharge) failure mechanism     

Buried optical waveguide polarizer by titanium indiffusion andproton-exchange in LiNbO3

Buried optical waveguide polarizers on LiNbO₃ have been realized by titanium indiffusion, followed by proton-exchange and annealing. The proton-exchange process decreases the ordinary refractive index and so modifies the index profile of the titanium indiffused waveguide. The measured intensity profile is in good agreement with calculation. An aluminum film absorbs the surface TM mode on z-cut LiNbO₃, leaving a buried nearly symmetric TE mode with lower optical loss than surface-guided TE modes. The extinction ratio obtained is estimated to be greater than 50 dB/cm at 0.633 μm     

Interpretation and control of C-V measurementsusing pattern recognition and expert system techniques

The authors demonstrate how a pattern-recognition system can be applied to the interpretation of capacitance-voltage (C-V ) curves on an MOS test structure. By intelligently sequencing additional measurements it is possible to accurately extract the maximum amount of information available from C-V and conductance-voltage (G-V) measurements. The expert system described, (CV-EXPERT), is completely integrated with the measurement, instrumentation, and control software and is thus able to call up a sequence of individually tailored tests for the MOS test structure under investigation. The prototype system is able to correctly identify a number of process faults, including a leaky oxide, as shown. Improvements that could be gained from developing rules to coordinate G-V, capacitance-time, and doping profile measurements simply by recognizing the important factors in the initial C- V measurement are illustrated