A 4 mW tunable C.W. single-frequency operation of a diode laser coupled to an external dispersive cavity has been achieved at 300° K. With the appearance of low-frequency pulsations (l.f.p.) at the laser output the single-frequency oscillation is eliminated. The l.f.p. give rise to a few longitudinal modes of the external cavity. On the basis of self focusing a model for l.f.p. phenomenon is proposed
Various models for calculation of refractive indices in semiconductor laser materials are discussed and the results for GalnAsP compounds are compared. It is shown that the modified single-oscillator model appears to be the most reliable. The group index is also considered since it can be found easily from measurements of laser spectra. The value of the group index is more model sensitive than the refractive index, but it is also a function of the active-layer thickness in double heterostructures as a consequence of waveguide dispersion. A simple formula connecting group and phase indices for a symmetrical dielectric slab waveguide is derived and used to calculate the effective group index for double-heterostructure lasers
The V/I characteristics of a gallium arsenide dual-gate field-effect transistor have been simulated numerically, using the Monte Carlo method. The transconductances extracted from these characteristics agree with those from a real device of the same geometrical and physical description.
A detailed experimental investigation of a SiO2insulated GaAlAs laser is performed. The results are compared with theoretical calculations, including strain from the SiO2layer, and good agreement is found. The theory can account for a kink seen in the light/cunent characteristic.
The growth and properties of GaAs1- xSbx/Ga1-yAlyAs1-xSbxdouble-heterostructure (d.h.s.) lasers have been investigated. We have obtained lasers with an antimony content up to x = 0.17 emitting at wavelengths up to 1.12 ¿¿m at room temperature for pulsed currents as low as 120 mA. The d.h.s.s were grown by liquid-phase epitaxy on GaAs substrates. The lattice mismatch problem was overcome by growing up to ten ternary GaAs1-xSbxlayers of graded Sb composition. A study of quaternary GaAlAsSb layer growth has shown the effect of initial saturation of the melt on the Al content of the resulting solid. To overcome this problem a special epitaxial process has been developed to grow quaternary layers with controlled aluminium content. The relative amounts of Al and Sb were determined by a photoluminescence technique which also allowed us to evaluate the band-gap energy step between the active and confinement layers. A study of the temperature dependence of the threshold current was also made for d.h.s having different aluminium compositions. Threshold current density was found to vary as exp T/To, where Toreaches 150 K for the highest aluminium content.
A theory of large-signal Impatt oscillator operation, including the effects of both constant and modulated optical carrier generation, is developed. This theory is related to easily measurable parameters of the particular device and predicts useful oscillator locking ranges for modest modulated optical-power levels. Computer simulations of practical device structures, under optical control, supplement the analytical work. The paper concludes with some recent results from an experiment in which optical subharmonic locking of an X-band Impatt oscillator has been achieved.
Microwave properties of GaAs and InP punchthrough diodes in the millimeter-wave range are investigated in this paper. By means of an exact large-signal computer simulation, taking into account the carrier velocity-field dependence, diffusion phenomena and a realistic doping profile, the influence of the negative differential mobility on oscillation performance is studied. Large negative resistances can then be obtained with well-designed n+¿p¿n¿n+structures. At a frequency of 40 GHz, optimum output powers of 20mW and 120mW have been calculated for GaAs and InP materials, respectively. The corresponding non-linear resistances are ¿7¿ and ¿7.5 ¿. An interesting use can then be expected as a self-oscillating mixer or as Doppler radar in the millimetre-wave range.
GalnAsP/InP double-heterostructure lasers were grown by liquid-phase epitaxy on (100) InP substrates for the wavelength region around 1.3 μm. The preparation and the properties of the wafers are described in detail. The characterisation includes the determination of the band-gap energy, the lattice mismatch of the GalnAsP active layer to InP as a function of the melt composition, and the measurement of the optical-gain spectra. Oxide-stripe geometry lasers, prepared from the wafers, lase at around 1.3 μm with pulsed thresholds between 300 and 400 mA. Some samples have been tested in c.w. operation at room temperature for more than 1400 hours. During the first 1000 hours a threshold increase of typically 2% has been observed.
The scanning electron microscope (s.e.m.) is widely used for the topographical examination and X-ray elemental analysis of integrated circuits, and is increasingly being used in electrical contrast modes. The voltage-contrast (v.c.) mode is particularly important because of the small dimensions of the devices found in current l.s.i. circuits which preclude the use of mechanical probes, and because of the minimal circuit loading which the electron beam imposes in high-frequency applications. Voltage contrast can now give a fair degree of measurement accuracy both in the static and dynamic cases. The dynamic situations often require the use of stroboscopic and sampling techniques, where the primary electron beam is chopped at a frequency locked to that of the signal being examined.
Reliable detection of a phase-modulated optical carrier has been demonstrated over 500 m of graded-index optical fibre using a semiconductor-laser source and mode-mode interference. The system performance is limited by the noise and spectral properties of the optical source. Spectral measurements indicate that transmission over several kilometres should be possible. This has direct significance for amplitude-modulated systems with semiconductor-laser sources as it implies that modal noise can occur at fibre connections up to several kilometres from the optical source
The current/voltage characteristics of Si n-p-ntransistors, when the device is pulsed into second breakdown for several nanoseconds, are studied. The collector/base characteristics have limits given by Ie= 0 which can be modelled approximately by neglecting recombination, and by Ib= 0 which requires recombination to be fully understood. This later condition is closely analagous to the Kirk effect with a plasma of holes and electrons close to the base region. In all these second-breakdown states the electron multiplication factor is low, typically less than 1.33, so that there is little useful multiplication gain. Second breakdown thus prevents the transistor from being used as a 3-terminal avalanche transistor. Failure at high current levels occurs most readily in the Ie= 0 state. The cutback in voltage and increase in current can be limited by tailoring the collector impurity profile. These two features suggest ways in which transistors may be improved to protect them against short pulse burnout via current-mode second breakdown.
We analyse a theoretical model of self-sustained oscillations in D.H. injection lasers, where a single or two filaments feed on two distinct carrier reservoirs, and consider the simultaneous effect of optical coupling by saturable absorption, carrier diffusion, and by spontaneous emission. We derive conditions for instability valid for any form of the gain function and specialise the results to the case of logarithmic and linear gain. Numerical results are presented showing trajectories in the phase plane of the carrier densities as well as the time evolution of carrier and photon densities.
Conventional models of junction heating in semiconductor lasers have been considered, and are found to be inconsistent with the experimental observations on stripe-geometry double-heterojunction lasers. In particular, the onset of the thermal runaway of c.w. threshold current, induced either by increasing the heat-sink temperature or by degradation processes, is found to occur at a current much less than predicted theoretically. Part of the reason for these inconsistencies is that at high temperatures there is a departure from the commonly assumed exponential dependence of pulsed threshold on junction temperature. However, it is the observation that the temperature rise of the active region is not directly proportional to the input power which explains why the experimental results are so different from the theoretical predictions. The results are discussed with reference to a model of multiple heat sources, with sources assuming different relative importance as the input power increases.
A technique is described which utilises a long-gate m.o.s.f.e.t. structure to measure carrier density and drift mobility profiles in submicrometre epitaxial and ion-implanted layers. Both profiles are recovered right up to the semiconductor surface. Application of the method to studying carrier transport processes in layers for short-gate f.e.t.s indicates that mobility degradation due to diffuse surface scattering occurs, resulting in a surface-mobility/bulk-mobility ratio varying between 0.5 and 1
The noise characteristics of a semiconductor punchthrough structure are examined for operation under low-field conditions, i.e. constant mobility, and for trap levels in the bandgap of the semiconductor. It is shown that the noise measure is decreased under the influence of trapping of injected carriers, for the small-signal injection approximation. With the increase of the concentration of traps, the noise measure decreases, but the frequency band, where it takes place, narrows and is displaced to a lower frequency.
Switching characteristics and charge centroids of m.n.o.s. memory devices with different interface state densities were studied at 25°C and at low temperatures. Experimental evidence, showing that interface states with high density affect the charging process in m.n.o.s. memory devices, is presented.
A new phase-plane analysis of transients in injection lasers is presented, which leads to general conditions for no ringing in the light output. For operation not meeting the general conditions, it is shown that a double-step current modulation of the laser can reduce the amplitude of the ringing. The result of this analysis is then applied to a laser with absorbing sections. It is shown that the enhanced superluminescence in these lasers will produce an effective double-step drive when a single-step drive is externally applied. This result then helps to explain why various kinds of lasers with absorbing section show little or no transient photon oscillations.
This paper describes the growth and characterisation of double heterostructure multilayers of GalnAsP/InP for the fabrication of laser diodes emitting near 1-3μm. A single-phase growth technique has been employed enabling good uniformity, lattice match and wavelength control. The contact resistance profiling technique has been extensively used in the establishment of a growth procedure which effectively suppresses the diffusion of zinc from the heavily doped capping layer. Lasers fabricated from the material have shown good reproducibility, with pulsed room-temperature thresholds as low as 140 mA for 10 ¿m wide oxide stripe devices.
The possibility of making a shift register using an array of p+-i-n+diodes has been examined by investigating the transfer action of the on state of negative resistance characteristics between closely spaced p+-i-n+diodes. A gold-doped Si substrate is used to fabricate the p+-i-n+ structure. The possibility of the transfer of the on state has been examined. The main requirements for transferring are as follows: (a) The array of diodes must be located within about 500 µm in lateral length, (b) The operation time of the transfer action must be within about 10 µs. To clarify the transfer mechanism, experiments concerning the effect of carrier diffusion, Joule heat and recombination radiation due to on state current from one diode to the adjacent diode have been carried out. From the experimental results, it has been found that the transfer mechanism is dominated by carrier diffusion from the filamentary current path of one diode to the next.
