Vera Gradišnik

Study of the color detection of a-Si:H by transient response in the visible range

A new technique of color detection based on time-resolved charge collection during transient response measurement using a hydrogenated amorphous silicon detector has been proposed and experimentally demonstrated. The method based on the material intrinsic wavelength-filtering properties is implemented in color sensing by transient response of a detector to a combination of basic color light and voltage pulse. Resulting transient responses under different illumination and bias voltage conditions confirm considerable accuracy of proposed method. In particular, the best agreement of measured and calculated transient response curves has been obtained by using the spectral response of all three primary colors in calculations. It was observed that the presence of blue light drastically reduces the transient response to a chromatic light illumination. Finally, a voltage pulse influence to the basic colors transient response was found.

Transient response times of a-Si : H p-i-n color detector

Transients and relevant response times of a-Si : H p-i-n photodetector under various illuminations (in the visible range) and bias voltage conditions were studied. The model/method for possible color detection using on and off transient response times is proposed. Depending on illumination and bias pulse types, one or two processes are found to be involved in the conduction mechanism, including transition and trapping of both charge carrier types. Characteristic photocurrent transients and response times under modulated monochromatic and chromatic visible light illumination enable color recognition. The peculiar behavior of the blue light transients has been also plausibly explained by means of the proposed model

The Fourier analysis of a-Si:H photodiode transient response

In this paper, color detection based on the transient response of n-i-p a-Si:H photodiodes to light and voltage pulses was described. The responses to a voltage pulse at constant light and to a light pulse at constant reverse bias voltage were also measured. From the measured dark current as well as the photocurrent, which are both proportional to the instantaneous charge change, the charge was calculated using the FFT method. In the case of transient response to light and voltage pulses, the changeable capacitance as a measure of wavelength dependent light absorption is introduced. The obtained results show the charge increments dependence on wavelength and confirm the p-i-n a-Si:H photodiode as an efficient color detector.

Color detection using a capacitance of np silicon photodiode

This paper describes color detection using a capacitance of np silicon photodiode, which is part of a standard CMOS technological process. When a voltage step is applied to a photodiode, the incremental charge distribution in the device is separated into positive and negative components, which are assigned to the respective contacts. This instantaneous change of charges is equal to the dark current and photocurrent. It corresponds both to the depletion capacitance charge of np junction and to the diffusion capacitance charge. Hence, since the photogenerated charge within the photodiode structure is dependent on the wavelength of absorbed light, the photodiode capacitance is also wavelength dependent. Emphasizing the physical mechanism, the capacitance behavior observed in a one-junction Si photodiode is analyzed using one-dimensional numerical modeling. The interpretation of a color detection was based on the analysis of the transient current in response to a small voltage step at constant illumination. The analysis included quasi-neutral charge density and space-charge charge density components. Different transient current response (charge and discharge) times to a small voltage step can be ascribed to light absorption. Using Fourier analysis dependent on light wavelength, can be translated from time domain to frequency domain. This enables use of np photodiode in colour detection.

The transient photo-dark current ratio of a-Si:H p-i-n photodiode

In this paper the transient photo-dark current ratios (PDCR)of p-i-n a-Si:H amorphous silicon photodiode on voltage pulse at constant light and voltage pulses were measured and compared. The results show opposite PDCR behaviour of monochrome and chromatic transients in these two cases. From the measured dark current and the photocurrent, which are both proportional to the instantaneous charge change, the charge was calculated using the FFT method. The obtained results show that the depletion charge increments are much smaller than the stored ones.

The a-Si:H p-i-n color detector response time on modulated illumination

Presented in this paper is a study of the visible light detection by the hydrogenated amorphous silicon p-i-n sensor under low reverse bias, where linear increase of photocurrent under different illumination condition is observed. The method based on the material intrinsic wavelength-filtering properties is implemented in color sensing. The response times after switching on and off monochromatic and chromatic illumination at constant reverse bias voltage are analyzed. The results of transient response under different illumination conditions confirm the trap controlled conduction mechanism.

Influence of shallow impurity on steady-state probability function of multilevel deep impurity

The deep impurity added into the n- or p-type semiconductor is partially ionised. The probability function used to describe the occupation of a deep energy level, is the Fermi-Dirac function into which the entropy factor is introduced; /spl chi//sub p/ for donor level or /spl xi//sub n/ for acceptor level. The entropy factors are used to adjust the calculated and measured values. An effective deep energy level was defined depending on the predicted position of a deep level and on obtained entropy factor. Comparing the calculated and measured values for gold and platinum added into the n- and p-type silicon, we can see that the same predicted energy level is described with a quite different entropy factor in the n- and p-type semiconductor. According to the obtained positions of the effective deep energy levels, it can be concluded that in the compensation between shallow and deep impurity a deep level, which is nearest to the shallow level, must be considered. The other levels are neutral. It might happen that in the n-type semiconductor the higher acceptor level of platinum is occupied, while the lower one is empty. It seems that such a neutral energy level does not exist in the n-type, while in the p-type it does, and it is partially occupied.