Proceedings of the Estonian Academy of Sciences

CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Engineering

Volume 10 No. 3 September 2004

Special issue on electronics: wide band-gap semiconductor structures, measurement and processing of sensor signals BEC 2004

Guest editorial; 155–156

Mart Min

Comparison of photon recycling effect in GaAs and GaN structures; 157–172

Enn Velmre and Andres Udal

Abstract. Extending earlier studies of photon recycling effect in GaAs, estimations are found for a new promising wide band-gap semiconductor GaN. Relying on available data, approximately one order higher radiative recombination coefficient and absorption coefficients than in GaAs and equal or greater Urbach energies than in GaAs are valid for GaN. The distance-dependent radiative recombination transfer functions are introduced and studied for typical charge carrier distribution cases. In spite of high absorption rates of GaN, the estimations show approximately one order of magnitude higher photon recycling efficiency than in the case of GaAs. By numerical simulations, the possibility of appearance of the S-shape forward I/V characteristics of p-i-n structures due to strong photon recycling is shown.

Key words: photon recycling, recombination radiation reabsorption, Gallium arsenide, Gallium aitride, radiative recombination transfer function, one-dimensional drift-diffusion simulations, p-i-n structures.

Comparison of the dynamic behaviour of complementary 6H- and 4H-SiC Schottky structures using numerical simulation; 173–178

Toomas Rang and Raido Kurel

Abstract. In this paper we compare, using numerical simulation, the dynamic behaviour of complementary Schottky structures based on 6H- and 4H-SiC substrates. Clear differences in the behaviour of these two Schottky structures at different temperatures has been established. The calculations show that the turn-off time is about the same for both Schottky structures, but it varies with the temperature about 1.5 times.

Key words: SiC, Schottky structures, turn-off time, high temperature behaviour, numerical simulation.

Interpretation of some physical parameters of SiC Schottky interfaces manufactured by diffusion welding technology; 179–184

Toomas Rang, Oleg Korolkov, and Jevgeni Ljutov

Abstract. The goal of this paper is to show some new problems introduced by the diffusion welding technology for SiC Schottky structures concerning the current transport through the metal–semiconductor interface using classical thermionic-diffusion theory. The experimental results show clearly that the classical Schottky current model parameters like Richardson coefficient, on-state series resistance and ideality factor differ from their usual values. The calculations show that the current transport, based on classical description of the ideal Schottky interface, does not give a true picture of the situation at the interface. Namely, during the manufacturing process, between the metal and semiconductor appears an extra thin silica layer, which, we guess, has an influence on the electrical charactersitics of the device.

Key words: SiC, diffusion welding technology, current transport, Schottky interface.

Improvement of lock-in signal processing for applications in measurement of electrical bioimpedance; 185–197

Toomas Parve and Raul Land

Abstract. Improvement of the switching mode lock-in signal processing, based on narrowing of the pulse of the half wave of the rectangular form signal, is discussed. The solution is aimed to the diminishing of the phasor errors caused by the sensitivity of the switching mode lock-in signal converters to the odd higher harmonics of the input signal. This improvement is technically realizable without significant increasing of circuit complexity and energy consumption. The method is foreseen for electrical bioimpedance measurement units used in implantable and portable medical devices for making medical measurements in vivo.

Key words: lock-in signal processing, measurement errors, electrical bioimpedance, implantable medical devices.

Theoretical development and computer simulation of a bioimpedance measurement system; 198–208

Toivo Paavle

Abstract. In this paper, theoretical conceptions of a novel bioimpedance measurement system (MS) are discussed. The proposed MS couples the lock-in measurement principles and a method of undersampling in digital signal processing. The MS is a mixed-signal system incorporating an analogue phase-locked loop (PLL) as the source of reference signal, demodulators for amplitude and phase demodulation, and a digital signal processor. It is shown that using a single PLL, simultaneous demodulation of two signals with different frequencies is possible. The theoretical conceptions of lock-in measurement and efficiency of the whole MS have been verified by means of special computer simulators, structures of which are presented. The results of this work can be considered as a preliminary study for the practical design.

Key words: bioimpedance, biomodulation, lock-in measurement, phase-locked loop, undersampling, signal-domain model, phase-domain model.

Transients in adaptive Fourier analysers; 209–226

Ants Ronk and Ülle Voolaine

Abstract. In the paper several different modifications of an adaptive Fourier analyser are considered presenting simulation results on transients of the analyser’s fundamental frequency and output signal. The results permit to compare convergence properties of these modifications and to choose and optimize a proper modification for different applications, including such an extension of the analyser, which processes composite signals consisting of several periodic signals of different waveforms and (not harmonically related) frequencies.

Key words: adaptive filters, observers, spectral analysis, waveform analysis, frequency estimation, transient processes.

Instructions to authors; 227–230