P. De Visschere

The validity of Ramo's theorem

Abstract It is shown that the original formulation of Ramos theorem is still valid in the presence of a space charge. The generalized version of this theorem which has been around for a very long time, is proved to be wrong. The consequences for the theories of semiconductor detectors and generation-recombination noise in p-n junctions are described.

Observation and simulation of space-charge effects and hysteresis in ZnS:MN ac thin-film electroluminescent devices

We have shown from basic considerations that the negative resistance effect measured in thin‐film electroluminescent devices must involve some variation of space charge in the phosphor layer. Time‐delayed voltage pulse experiments indicate that positive space charge decreases spontaneously. By introducing the presence of deep donor levels which can be ionized by accelerated electrons and, when positively charged, have some typical decay time for the release of a hole, we have created a numerical model which is able to simulate negative resistance, time‐delayed pulse experiments and the influence of temperature and frequency on the hysteretic behavior.

How to measure and interpret the conduction current in a.c. thin-film electroluminescent devices

Abstract In this paper we derive a general formula for the current supplied to a thin-film electroluminescent device, with a variable applied voltage and moving charges in the semiconductor layer. This formula results in an equivalent electrical network, with a current source representing the average current in the phosphor layer. We show how this current can be measured accurately by using a bridge measurement technique.

Concentration quenching and luminescent decay in AC thin-film ZnS:Mn electroluminescent devices

Abstract A review is given of the experiments and theories dealing with the concentration quenching effect and the associated non-exponential decay in ZnS:Mn thin-film electroluminescent devices. These effects are usually attributed to the migration of the excitation among the Mn atoms. For describing this concept quantitatively, two limiting models are introduced, one based on hopping and one on diffusion. Both models predict almost the same Mn concentration dependence. The hopping model gives also a reasonable explanation for the empirical decay law proposed by Benoit et al. However the migration model still needs a killer concentration proportional with the Mn concentration. Possibly a hole trap associated with a Mn center could fulfill this role.

Analytical model for thin‐film electroluminescent devices

Up to now, models for the electrical behavior of thin‐film electroluminescent devices have been proposed by several authors. These models usually account for electron tunneling, multiplication and recombination and involve considerable numerical calculations. In this paper we use a simplified but realistic tunnel characteristic, which makes it possible to calculate analytical steady‐state solutions for a symmetrical device without space charge. Some interesting features of these solutions, like a waveform dependent threshold field and a double current‐voltage characteristic will be discussed.

Screening and early diagnoSiS of proState cancer: an update

Abstract Screening for prostate cancer has become a main controversial topic. First the currently used screening tools, PSA (Prostate Specific Antigen) and DRE (Digital Rectal Examination) have a low accuracy in the prediction of prostate cancer. Second, the benefit of screening in reducing the prostate cancer related mortality was not uniformly shown in older screening studies and there was concern about the risk of overdiagnosis and overtreatment of insignificant prostate cancers. Very recently, 3 major prospective, randomized screening studies have been published. This paper aims to provide an overview how the performance of the current screening tools can be ameliorated and evaluates the recently published screening studies with practical considerations for future screening protocols.

High-voltage polycrystalline CdSe thin-film transistors

The properties of self-aligned high-voltage CdSe thin-film transistors (TFTs) are described. By analyzing the different failure mechanisms, it is found that In-doped CdSe makes device operation up to 200 V feasible. Furthermore, the high-voltage CdSe TFT shows excellent low off currents and high transconductance. >

The influence of space charge and electric field on the excitation efficiency in thin film electroluminescent devices

In electroluminescent devices electrons move every half period from the cathodic to the anodic interface of the phosphor layer and excite atoms which can lose their energy by the emission of a photon. It has been found experimentally by others that the efficiency for excitation of the luminescent atoms is largest close to the cathodic interface. By comparing transient measurements with the steady state situation we show that in steady state the efficiency is reduced and the difference between cathodic and anodic efficiency is larger. Both phenomena could be ascribed to creation of positive space charge during the first current pulses.

Evaluation of a 64 x 64 CdSe TFT addressed ACTFEL display demonstrator

An operating ACTFEL (AC thin-film electroluminescent display), addressed with TFTs, has been demonstrated. To achieve this, the authors used self-healing electrodes, limited the power supplied to the display, and used a stepped annealing cycle for the TFTs. The display could be driven with 10-V external drivers at a frame rate of 50 Hz and an EL driving frequency of 100 Hz.<<ETX>>

Numerical simulation of stimulated emission and lasing in dye doped cholesteric liquid crystal films

Dye-doped chiral-nematic liquid crystal lasers have great potential as small size, low-cost, widely tunable lasers. We present a numerical model for stimulated emission and lasing in liquid crystal films based on thin film optics. The gain threshold is modelled and the results are confirmed experimentally. The effect of the orientation of dye molecules and the matching of the photonic bandgap to the dye spectrum on the threshold for lasing is discussed.