Juozas Vyšniauskas

ANALYSIS OF NOISE CHARACTERISTICS OF GaAs TUNNEL DIODES

An analysis and investigation of noises of GaAs tunnel diodes, which abrupt p+-n+ profile was obtained by using amphoteric nature of silicon, were performed. The main scope of this work was to verify the concepts of the explanation of white noise characteristics on the ground of shot noise and on the ground of the Gupta theorem of thermal noise in resistive elements. The other scope was to investigate the peculiarities of low frequency noise in p+-n+ junctions formed by using amphoteric silicon nature.

Noise characteristics and reliability of high power white light emitting diodes based on nitrides

High power white light emission diode reliability and aging processes have been investigated. Optical, electrical and noise characteristics have been carried out for initial devices and during their aging. Analysis of noise characteristics help revealing of light emission diode aging processes and reliability problems. It is found that optical and electrical noise spectra changes reflect light emission diode aging. Noise characteristics, especially correlation factor between optical and electrical fluctuations, and current-voltage characteristics at low bias reveal physical processes that take place during investigated device aging and rapid its degradation. It is shown that reason of high power light emission diode degradation is related with defects presence in the device structure. Additional defects appear during LED operation and lead to the leakage current and non-radiative recombination increase.

Investigation of High-Speed Transient Processes and Parameter Extraction of InGaAsP Laser Diodes

Fiber optic communication systems rely on the speed of the modulated light sources such as laser diodes (LDs). High speed direct current modulation of the optical signal, narrow spectral width, small dimensions and good light coupling efficiency to the optical fiber are the advantageous features of LD. Especially the long-haul optical links are critical to the chromatic dispersion and thus a single-mode and stable wavelength optical information carrier is needed. Distributed feedback (DFB) LDs are usually used in this situation. These DFB lasers have an almost single-mode optical emission spectrum, a stable optical frequency output, an optical modulation rate in excess of Gb/s and an uninterrupted operation time of up to 108 hours. However, the communication system with a high-speed direct modulation is limited by transient processes in the LD.

Dynamic characteristics and reliability of gain-coupled InGaAsP laser diodes

Dynamic characteristics of partly gain-coupled multiple-quantum-well InGaAsP distributed feedback laser diodes (LD) have been investigated. Relative intensity noise measurements were done and key dynamic parameters (differential gain, K-factor) were estimated. From time-resolved frequency chirp measurements, the chirp components and the chirp parameter were extracted. Several tens of devices were characterized. Although all the devices were of the same structure and dimension, their dynamic parameters were spread in values. It is believed, that the spread is due to the limited device processing accuracy. The correlation between the dynamic parameters was observed. The K-factor and differential lifetime were found to be correlated mutually and with the differential gain, which indicates that the latter plays an essential role in determining the variance of dynamic parameters. It was shown that the increase of the coupling coefficient results not only in both the oscillation and damping frequency increase, but also in the decrease of the amplitude of the time-resolved frequency chirp oscillation. LDs were subjected to the accelerated aging by applying at 100 °C the 150-mA bias for 1800 h. The degradation of the threshold current, the efficiency, and the amplitude ,of the time-resolved frequency chirp oscillation was observed.

Dynamic characteristics of gain-coupled InGaAsP laser diodes and their reliability

Dynamic characteristics of partly gain-coupled multiple-quantum-well InGaAsP distributed feedback laser diodes have been investigated. Basic dynamic parameters (differential gain, K-factor, differential carrier lifetime) were found out. It is shown that the increase of coupling coefficient results not only in both oscillation and damping frequency increasing, but also in time-resolved frequency chirp oscillation decreasing. Relation between K-factor and differential carrier lifetime and their correlation with deferential gain indicates essential differential gain role in dynamic parameter dispersion.

Simulation of dynamic characteristics of GaN p-i-n avalanche diode operating as particle detector with internal gain

An evolution of the transient characteristics of the GaN p-i-n diodes, operating in the avalanche mode and acting as particle sensors, has been simulated by using the Synopsys TCAD Sentaurus software package and the drift-diffusion approach. Profiling of the charge generation, recombination and drift-diffusion processes has been performed over a nanosecond time-scale with a precision of a few picoseconds and emulated through the photo-excitation of an excess carrier domain at different locations of the active volume of a diode. Shockley–Read–Hall (SRH), Auger and radiative recombination processes have been taken into account. Fast and slow components within a current transient have been analysed based on the consideration of the carrier spatial distribution at different instants of the avalanche process. The internal gain due to charge multiplication ensures the sufficient charge collection on electrodes of the relatively thin (5 µm) diode operating in the avalanche mode. It has been shown that the simulated evolution of the detector transient responses by employing the drift-diffusion approach reproduces properly the qualitative modifications of the main features of a detector with an internal gain, realized by induction of the avalanche processes governed by the applied external voltage.

Carrier density and mobility fluctuations due to carrier retrapping process in homogeneous semiconductors

It is shown that the free charge carrier capture-emission process causes both the charge carrier density and mobility fluctuations. In this report we present the calculation results in order to find out how the capture-emission process affects the free charge carrier mobility, and how the latter impacts the low-frequency noise level. The carrier mobility dependence on phonon, impurity and carrier-carrier scatterings and mobility change dependence on the electric field were taken into account. It is also shown that fluctuations of the charge carrier density and mobility due capture-emission process are completely correlated, and that their relaxation times are the same as for charge capture-emission process. The general expression for estimation of active capture center density in the sample from the low-frequency noise measurements is presented.

Hydrodynamic modelling of terahertz rectification in AlGaN/GaN high electron mobility transistors

We report on the numerical modelling of rectification in a gated two-dimensional electron gas. We demonstrate that drift-diffusion-based and energy-relaxation-based models predict different features of rectified terahertz radiation as a function of gate bias. Whereas the widely accepted mechanism for rectification is considered to be plasmonic-based, there are conditions when diffusion currents originating by non-local carrier heating can dominate the response. Moreover, diffusive contributions can substantially enhance the response becoming an important phenomenon, which has to be considered in future designs of efficient transistor-based terahertz rectifiers.

Low frequency noise characterisation of biased silicon CMOS terahertz detectors

An investigation of the low-frequency noise and responsivity characteristics of silicon 0.15-μm-MOSFET-based detectors for terahertz radiation under applied dc source-to-drain current has been carried out. It is shown that the base noise of the investigated transistors is thermal channel noise at higher frequencies, while at lower frequencies, generation-recombination and 1/f-type noise prevail over thermal fluctuations. Characteristic times of the observed generation-recombination processes are in the range from 80 μs to 1.6 ms.

Correlation Function Method for Semiconductor Laser Optical Noise Investigation and Fluctuations of Directivity Diagram of Laser Radiation

Correlation function method was used for the optical noise investigation in InGaAsP/InP multiple-quantum-well lasers. It was observed that correlation factor between laser optical noises measured by two photodetectors decreases with laser current increasing above threshold. It is found that this correlation factor decrease is determined by fluctuations of directivity diagram of laser radiation in free space. These excess fluctuations can be eliminated using aperture near the laser emission facet that passes only central part of the beam.