In this letter, we describe a four thin-film-transistor (TFT) pixel circuit based on hydrogenated amorphous silicon (a-Si:H) technology for the active-matrix organic light-emitting diode (AMOLED) display applications. The circuit uses current-writing mechanism and can automatically adjust the threshold-voltage shifts of both the organic light-emitting diodes (OLEDs) and the TFTs induced by the circuit aging or process variations. Experimental results indicate virtually no variation of the output driving current after long-term bias-temperature-stress (BTS).

We present here a study on the propagation characteristics of two-dimensional periodic structures. The method of mode matching is employed to formulate the boundary-value problem in an exact fashion, and a perturbation analysis is carried out to explain the wave phenomena associated with photonic band gap structures. The dispersion curves of 2D periodic medium and 2D periodic impedance surface are investigated in detail.

We introduce the characteristics for continuous wave operation at room temperature of InGaN laser diodes fabricated on SiC substrates. The threshold current was 60 mA, the threshold voltage was 8.3 V, and the oscillation wavelength was 404.4 nm. The lifetime of the laser diodes with a constant light output of 1 mW at 25 was 57 hours. The heat dissipation of the devices mounted p-side-up on a stem without using a heat sink was shown to be as good as that of devices mounted p-side-down with an external heat sink, owing to the high thermal conductivity of SiC substrates.

We studied Si and Mg doping characteristics in cubic GaN and fabricated a light emitting diode of cubic GaN on a GaAs substrate by metalorganic vapor-phase epitaxy. The diode structure consisted of undoped and Mg-doped GaN stacking layers deposited on Si-doped GaN and AlGaN layers. The electron-beam-induced-current signal and current injection characteristics of this diode structure were measured. There was a peak at the interface between the Mg-doped and undoped GaN in the electron-beam-induced-current signal. This shows successful growth of the p-n junction. Light emitting operation was achieved by currents injected through the conducting GaAs substrate of this diode at room temperature. We observed electroluminescence below the bandgap energy of cubic GaN with a peak at 2.6 eV.

Investigations were carried out on metalorganic-chemical-vapor-deposition (MOCVD)-grown strained AlGaN/ GaN/sapphire structures using X-ray diffratometry. While AlGaN with lower AlN molar fraction (< 0.1) is under the in-plane compressive stress, it is under the in-plane tensile stress with high AlN molar fraction (> 0.1). Though tensile stress caused the cracks in AlGaN layer with high AlN molar fraction, we found that the cracks dramatically reduced when the GaN layer quality was not good. Using this technique, we fabricated a GaInN multi-quantum-well (MQW) surface emitting diodes were fabricated on 15 pairs of AlGaN/GaN distributed Bragg reflector (DBR) structures. The reflectivity of 15 pairs of AlGaN/GaN DBR structure has been shown as 75% at 435 nm. Considerably higher output power (1.5 times) has been observed for DBR based GaInN MQW LED when compared with non-DBR based MQW structures.

Laser diodes in the (Al, Ga, In) N system are attractive for many applications. Due to the wurtzite crystal structure, cleaved facets are not easily produced. We have investigated distributed feedback (DFB) laser diodes employing embedded dielectric gratings with the grating located above the active region. The dielectric gratings are incorporated via epitaxial lateral overgrowth. The DFB laser diodes had reduced threshold current densities over the etched cavity devices, with a minimum of 15 kA/cm2. The spectral emission width was considerably reduced for the DFB devices. Voltages for the DFB devices were high due to the presence of the Si3N4 grating within the p-type material.

Continuous-wave operation at room-tempera-ture has been demonstrated for InGaN multi-quantum-well (MQW) laser diodes (LDs) grown on FIELO GaN substrates with a backside n-contact. This was made possible by introducing important new concept of reducing threading dislocations that occur during the growth of the GaN substrates. We found that InGaN active layers grown on FIELO GaN are superior to those grown on conventional sapphire substrates in terms of their growth mode and the resultant In compositional fluctuation. The fabricated laser diode shows the threshold current, the threshold current density and the threshold voltage were 36 mA, 5.4 kA/cm2 and 7.5 V, respectively, with the lasing wavelength of 412 nm and internal quantum efficiency as high as 98%.

UV/blue/green InGaN and GaN single-quantum-well structure light-emitting diodes (LEDs) were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. The external quantum efficiency (EQE) of the UV InGaN LED on ELOG was much higher than that on sapphire only at high-current operation. At low-current operation, both LEDs had the same EQE. When the active layer was GaN, EQE of the LED on sapphire was much lower than that on ELOG even at low- and high-current operations due to the lack of localized energy states formed by alloy composition fluctuations. In order to improve the lifetime of laser diode (LD), ELOG had to be used because the operating current density of the LD is much higher than that of LED. A violet InGaN multi-quantum-well/GaN/AlGaN separate-confinement-heterostructure LD was grown on ELOG on sapphire. The LDs with cleaved mirror facets showed an output power as high as 40 mW under room-temperature continuous-wave (CW) operation. The stable fundamental transverse mode was observed at an output power of up to 40 mW. The estimated lifetime of the LDs at a constant output power of 10 mW was more than 2,000 hours under CW operation at an ambient temperature of 60.

Optical transverse-mode properties of the GaN-based semiconductor laser-diode is characterized by effective refractive index method. In order to stabilize a transverse-mode in the conventional ridge-waveguide structure, very precise control of ridge-height is found to be necessary. On the contrary, a novel 2-step grown structure with 2-dimensional index guiding has wide feasibility for device parameter, excellent stability of large confinement factor in transverse-mode, and small aspect ratio of beam divergence, under the condition that AlN molar fraction of 0.08 in AlGaN current blocking layer and stripe width of 1.5 µm are used.

A low-cost technique using commercial UV grade silica fibers to construct microlens array that is suitable for mass-production of long-period gratings is reported. The growth rate of gratings fabricated using these arrays is much faster than the conventional amplitude masks. Our previous work had shown that this technique was 400% more efficient than the metal mask technique. Further improvement of this grating writing technique using plano-convex microlens array is reported in this paper. Under the same writing conditions, long-period gratings with absorption peaks of 1.5 dB and 17 dB were fabricated by using a microlens array and a plano-convex microlens array, respectively.

Long-period fiber gratings (LPGs) using a high-silica core fiber are presented. A high-silica core fiber has a residual stress in the core, and the grating structure is formed by stress releasing of the core using a focused CO2 laser beam. The dependence of the transmission spectrum on temperature and tensile strength is measured, and low dependence compared with conventional LPGs is observed. These unique characteristics are caused by the difference of temperature and tensile strength changes of the effective indices for the fundamental propagation mode and the cladding mode in the high-silica core fiber.

A general analytical expression for describing the growth of the resonant peak wavelengths of long period gratings is derived. The theoretical calculations explain the shift of peak loss wavelengths in the direction of either shorter or longer wavelengths as the induced index change of grating increases. We have calculated and experimentally verified the sensitivity of the resonant peak wavelengths with respect to an overlay index for various grating periods. It is shown that the center wavelength shift of the claddding modes depends strongly on the grating period and the claddding mode order.

A directly modulated LED or SLD (super luminescent diode) is attractive for low-cost lightwave systems such as access networks. This letter experimentally studies a directly modulated SLD followed by a gain-saturated semiconductor optical amplifier (SOA), and shows that the modulation rate is expanded in effect by the use of the gain-saturated SOA. This results from the shortened response time of the SLD due to the ASE light from the SOA and a level-equalizing effect in the gain-saturated SOA.

As a new idea for analyzing the wave scattering and diffraction from a finite periodic surface, this paper proposes the periodic Fourier transform. By the periodic Fourier transform, the scattered wave is transformed into a periodic function which is further expanded into Fourier series. In terms of the inverse transformation, the scattered wave is shown to have an extended Floquet form, which is a 'Fourier series' with 'Fourier coefficients' given by band-limited Fourier integrals of amplitude functions. In case of the TE plane wave incident, an integral equation for the amplitude functions is obtained from the the boundary condition on the finite periodic surface. When the surface corrugation is small, in amplitude, compared with the wavelength, the integral equation is approximately solved by iteration to obtain the scattering cross section. Several properties and examples of the periodic Fourier transform are summarized in Appendix.

A novel temperature sensor device based on a conventional long-period fiber grating but having an improved sensing resolution is presented. By forming a reflector at one cleaved end of the fiber embedding a long-period grating, a fine interference fringe pattern was obtained within the conventional broadband resonant spectrum of the grating. Due to the fine internal structure of the reflection spectrum of the proposed device, the accuracy in reading the temperature-induced resonant wavelength shift was improved. The formation of the self-interference fringe is analyzed and its properties are discussed in detail. The performance of the proposed device is analyzed by measuring the resonant wavelength shift of the device placed in a hot oven under varying temperature. The rate of the fringe shift is measured to be 551 pm/. The rms deviation is 10 pm over a 100 dynamic range, which corresponds to 0.2 in rms temperature deviation. The thermal variation of the differential effective index of the fiber is calculated to be (0.3 0.1)10-6/ by comparing the analytic calculations with the experimental results. The interference fringe shift is revealed to be inversely proportional to the differential effective group index of the fiber, which implies that the shifting rate strongly depends on the type of fibers and also on the order of the involved cladding mode.

A semi-synchronous circuit is a circuit in which every register is ticked by a clock periodically, but not necessarily simultaneously. In a semi-synchronous circuit, the minimum delay between registers may be critical with respect to the clock period of the circuit, while it does not affect the clock period of an ordinary synchronous circuit. In this paper, we discuss a delay insertion method which makes such a semi-synchronous circuit faster. The maximum delay-to-register ratio over the cycles in the circuit gives a lower bound of the clock period. We show that this bound is achieved in the semi-synchronous framework by the proposing gate-level delay insertion method.

Dynamical theory of cellular automata on groups is developed. Main results are non-Euclidean extensions of Sato and Honda's results on the dynamics of Euclidean cellular automata. The notion of the period of a configuration is redefined in a more group theoretical way. The notion of a co-finite configuration substitutes the notion of a periodic configuration, where the new term is given to it to reflect and emphasize the importance of finiteness involved. With these extended or substituted notions, the relations among period preservablity, injectivity, and Poisson stability of parallel maps are established. Residually finite groups are shown to give a nice topological property that co-finite configurations are dense in the configuration space.

The dynamics is investigated on the delayed regulation model under period-2 perturbation described as x(t+1)={A+(-1)te}x(t){1-x(t-1)}, t=1, 2, 3 . . . The e-dependences of the bifurcation points are analyzed through usual stability analysis of fixed point and periodic solution, however one of them is derived through the stability analysis of the "virtual" period-2 solution.

A resonant-tunneling-diode (RTD) based sense amplifier circuit design has been proposed for the first time to envision a very high-speed and low-power memory system that also includes refresh-free, compact RTD-based memory cells. By combining RTDs with n-type transistors of conventional complementary metal oxide semiconductor (CMOS) devices, a new quantum MOS (Q-MOS) family of logic circuits, having very low power-delay product and good noise immunity, has recently been developed. This paper introduces the design and analysis of a new QMOS sense amplifier circuit, consisting of a pair of RTDs as pull-up loads in conjunction with n-type pull-down transistors. The proposed QMOS sensing circuit exhibits nearly 20% faster sensing time in comparison to the conventional design of a CMOS sense amplifier. The stability analysis done using phase-plot diagram reveals that the pair of back-to-back connected static QMOS inverters, which forms the core of the sense amplifier, has meta-stable and unstable states which are closely related to the I-V characteristics of the RTDs. The paper also analyzes in details the refresh-free memory cell design, known as tunneling static random access memory (TSRAM). The innovative cell design adds a stack of two RTDs to the conventional one-transistor dynamic RAM (DRAM) cell and thereby the cell can indefinitely hold its charge level without any further periodic refreshing. The analysis indicates that the TSRAM cell can achieve about two orders of magnitude lower stand-by power than a conventional DRAM cell. The paper demonstrates that RTD-based circuits hold high promises and are likely to be the key candidates for the future high-density, high-performance and low-power memory systems.

An ultra-fast optoelectronic decision circuit using resonant tunneling diodes (RTD's) and a uni-traveling-carrier photodiode (UTC-PD) is proposed. The circuit employs two cascaded RTD's for ultra-fast logic operation and one UTC-PD that offers a direct optical input interface. This novel configuration is suitable for ultra-fast decision operation. Two types of decision circuits are introduced: a positive-logic type and a negative-logic type. Operations of these circuits were simulated using SPICE with precisely investigated RTD and UTC-PD models. In terms of circuit speed, 40-Gbit/s decision and 80-Gbit/s demultiplexing were expected. Furthermore, the superiority of the negative-logic type in terms of the circuit operating margin and the relationship between input peak photocurrent and effective logic swing were clarified by SPICE simulations. In order to confirm the basic functions of the circuits and the accuracy of the simulations, circuits were fabricated by monolithically integrating InP-based RTD's and UTC-PD's. The circuits successfully exhibited 40-Gbit/s decision operation and 80-Gbit/s demultiplexing operation with less than 10-mW power dissipation. The superiority of the negative-logic type circuit for the circuit operation was confirmed, and the relationship between the input peak photocurrent and the effective logic swing was as predicted.