Charles A. Lee

Interband scattering effects on secondary ionization coefficients in GaAs

Abstract The ionization rates of holes and electrons in GaAs were measured experimentally over a wide doping range covering field values from 2.2×10 5 V/cm to 4.7×10 5 V/cm. As opposed to most experimemental measurements in GaAs, no assumption of equal ionization rates of the two carriers has been made. By using the conventional theoretical relationship between carrier ionization coefficients and multiplication data, the effective α is observed to be larger than β in lightly doped diodes while β is larger than α in heavily doped diodes. Previous theories of ionization rates utilizing just the normal conduction and valence bands do not show any possibility of such a crossover. It is suggested that electron transitions to higher conduction bands, which effectively increase the equilibration time of the electron distribution function, offer a resolution of this difficulty. The dependence of the effective electron ionization rate on doping can be explained by the requirement that electrons must make an interband transition before reaching the ionization threshold energy. This interband transition time estimated by this experiment is of the order of 10 −13 sec and is comparable to the transit time of electrons in the avalanching region. The breakdown voltages extrapolated from the measured α and β are consistent with those observed experimentally.

Deep level transient spectroscopy analysis of spatially dependent doping profiles

The study of deep level semiconductor defects frequently employs deep level transient spectroscopy (DLTS), a technique developed for structures with constant doping concentration profiles. In this paper, the theory of the DLTS measurement is extended to the more general case in which both the doping profile and the trap profile vary with depth. This analysis yields a method of calculating exact DLTS transients for the general case, and by its application to several examples, demonstrates that the doping profile shape and the measurement conditions are both factors which determine the degree to which the DLTS transient can be approximated by an exponential. The structures which do exhibit approximately exponential DLTS transients include several which arise in the course of device processing. This study shows that in such structures, a knowledge of the doping profile and the general form of the trap profile can be combined with the standard DLTS boxcar sampling method to obtain trap profile information in ...

A new integrable optical modulator‐switch optimized for speed and power consumption

A new structure of planar waveguide directional coupler/switch composed of layered GaAs/AlGaAs is proposed and analyzed. This new configuration maximizes the change of coupling between adjacent guides while retaining their phase synchronism. This scheme provides, theoretically, a depth of modulation of 100%, a smaller device length with low drive power requirement, and an ability to compensate for dimensional offset by simple voltage tuning. Most importantly, this design is monolithically integrable with heterojunction laser sources. Projected operation limited by phase mismatch of the electrical and optical signals is above 12 GHz and a power per unit bandwidth of 280 μW/MHz is expected.

Verification of generalized telegraphist’s equations applied to dielectric waveguide problems

A simple numerical method based on generalized telegraphists equations as a full vector-wave analysis tool for dielectric waveguide problems is presented. The method is applied to various guiding structures for single-mode and multimode computation. The generalized telegraphists equation formulates the problem as a matrix eigenvalue equation whose solution spectrum of eigenvalues directly gives the modal propagation constants. Accuracies of better than 0.08% are possible for calculating the propagation constants.

Effect of thermal etching on silicon epitaxial growth by vacuum sublimation

Abstract Silicon epitaxial growth by vacuum sublimation was studied with emphasis on the effect of thermal etch of the substrate on the surface concentration of carbon. A calculation of surface concentration of carbon on a Si substrate during thermal etch in the vacuum system shows that prolonged thermal etch at a higher temperature progressively enhances a pile-up of involatile impurity on the substrate surface and formation of precipitates above solid solubility limits, which would cause generation of interfacial defects. This effect was experimentally confirmed with various temperatures and periods of thermal etch and with the use of LOPEX material. Experimentally optimized temperature and period of thermal etch with the use of LOPEX Si yielded a good quality epitaxial P / N junction; low defect density 0–100 over the area of 10 −4 cm −2 , low reverse saturation current of less than 1 × 10 −4 A / cm 2 at half of breakdown voltage of 90 V. Uniformity of multiplication factor M over the reverse biased P / N junction was measured with a laser probe. The variation around the average value over the area of a mesa junction structure 600 μm diam. was less than 10 per cent. Static I – V characteristics of PN junction, Schottky barrier NN + diodes and P + NN + diodes fabricated from grown layers by vacuum sublimation were also demonstrated.

Design and fabrication of the self-aligned opposed gate-source transistor

The authors report on the modeling, design, and successful realization of the self-aligned opposed gate-source transistor (OGST). An advanced distributed traveling-wave mathematical model of the OGST is used to determine the optimal gate width for 94-GHz operation. A novel fabrication process is described that utilizes a new lithography technique, resonant refractive index lithography, to self-align a 0.15- mu m source to a 0.3- mu m gate on opposite sides of a 0.2- mu m-thick GaAs active layer. The authors report on the DC operating characteristics of the self-aligned OGST and compare these results with theoretical models. >

Choice of boundary conditions for rectangular dielectric waveguides using approximate eigenfunctions separable in x and y.

In examining approximate eigenfunction solutions of a rectangular dielectric waveguide that are separable in x and y, we have found that a particular choice of field components of E and H for satisfying the boundary conditions results in orthogonal eigenfunctions for the different modes, while other choices do not.

Contact and thin film problems of submicron device structures

Abstract A review and assessment is given of selected problems associated with submicron structures and devices. In conjunction with these problems, thin film fabrication techniques are reviewed with the aim of exploiting the most recent developments.