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A silicon photodiode structure was studied for the spectral analysis of optical radiation. The structure consists of oppositely-directed barriers. We developed a model of the electronic processes occurring in the structure. The possibilities of the selection of separate waves from the integral flux of radiation, the wave absorption and the quantitative spectral analysis of the waves of the model for contaminated environment were investigated. An algorithm was developed for carrying out the spectral analysis without the preliminary calibration, and for promoting a possible creation of a new type of a portable semiconductor spectrophotometer.

The environmental crisis, as it relates to interaction between humans and the nature seems to be worsening with the progression of time. Depletion of ozone layer, acid rains, radioactive pollution, climate changes are some of the consequences on the environment. The environment is further burdened by mining, growing transportation and intentional activities as a result of global terrorism and poses some of the most precarious consequences. One of the most vulnerable problems is the pollution of fresh water with petroleum products, phenols, nitrogen, pesticides and heavy metals. Therefore, as scientists, an important task is to develop a large-scale real time monitoring of the water quality and the environment [

In the available methods of the spectral analysis, the spectral distribution of the electromagnetic radiation is obtained by light filters, a prism, a diffraction grating and high accuracy mechanical devices [

The effective solution to the above-stated problems is the development of such a semiconductor structure in which the electronic processes will provide high accuracy spectral analysis of the electromagnetic radiation.

There is research done in the area of multicolored photo receivers [

The registration accuracy of the above-mentioned structures is due to the necessity to provide similar absorption conditions and to create multiple photodiodes and multilayer structures with nanometric accuracy. The complicated technology of the development and the impossibility to control the spectral sensitivity by the external voltage impedes their development and usage.

To solve the problem, we propose a cheap, small-size, multi-functional primary sensor. In research papers [

The research work covers the modeling of the electronic process of the spectral analysis with the help of an electrically oppositely-directed silicon photodiode structure (

In

sorption along with the wavelength. The channels are electrically connected by a high conductivity p+ layer. The surface SiO_{2} film is reflective and is placed on the semitransparent metal film. The interior SiO_{2} film separates the upper active layer from the interior polycrystalline silicon passive platform. The latter transmits the unregistered part of the deeply penetrated radiation. At any polarization of the voltage, one of the n-p junctions is biased in the forward direction, and the other junction is biased in the opposite direction. The oppositely directed potential barrier gets wider with the increase of the bias voltage at the expense of the high ohmic n-layer. P^{+} layer is thin and the thickness d of the structure depends mainly on the n-layer, and in the point x_{p} the radiation intensity is taken to be equal to the surface intensity F_{0}.

Considering that the base is covered by the depleted regions of both barriers, we determine the distribution of the potential in the spatial charge region of the double-barrier structure (

In Poisson equation we pass from the potential

where

The boundary conditions for the given equation are

, (3)

By integrating, we obtain

When the boundary condition

When

Thus,

where:

If both barriers are identical, then

Consequently:

With the maximum applied radiation and when the depletion region of one junction (reverse biased) covers the whole d region, and the barrier of the second junction (directly biased) is compensated by the bias voltage, what can be determined from the conditions,

When

With the help of (7) we can determine the modulation depths of depleted layers, and the position of

And with the help of

In certain diodes, the photocurrent has diffusion and drift components. To determine the diffusion photocurrent, it is necessary to find p_{n} density of minority charge carriers―holes, in the n-base. For that, the following one-dimensional diffusion equation is solved,

where

electron-hole pairs; F_{0}: the total flux of incident photons;_{p}: the diffusion coefficient of holes in the n-region;_{n}_{0}: the equilibrium density of holes in the n-region.

Equation (9), at the boundary conditions of p_{n} = p_{n}_{0} when х = ¥ and p_{n} = 0 when х = x_{n}, can be solved as follows.

The relevant homogeneous equation considered is

_{1} ≠ k_{2}, the general solution for the homogeneous equation will be_{n}_{1} = ∞, and it has no physical interpretation. Hence

The specific solution of Equation (9) should be found in terms of the product

where the coefficients A and B of Equation (11) are determined by the method of undetermined coefficients. For that, Equation (3) is differentiated, and as a result we obtain:

If we substitute the values (3) and (4) into the given Equation (1), we will obtain:

Consequently

Thus Equation (11) takes the form:

The general solution for Equation (9), taking into account (10), will be

C_{1} is determined from the boundary conditions p_{n} = 0 when x = x_{n}, that is:

Hence

As a result, for the densities of minority charge carriers in the n-region of the semiconductor and the diffusion current, we will obtain the following equalities:

_{. }

At the point x = x_{n}_{ }

The density of the drift current has the form:

Under normal operating conditions, the member containing p_{n}_{0} is significantly smaller than the first member, and it can be neglected.

Accordingly, the drift current for two diodes in the ranges of the potential barriers with the widths

Since the p^{+} layer is very thin, the absorption depths are taken to be equal to the widths of the depleted layers of photodiodes x_{n}_{1} = x_{m }and x_{n}_{1} = d-x_{m}_{ }(

Finally, expression (15) without the diffusion component will take the following form for the general photocurrent flowing through one barrier.

The difference of the photocurrents of two barriers will give the general photocurrent flowing throught the structure.

At the illumination by the integral flux of radiation, expression (18) will take the following form.

where (_{i})―the total flux of photons incident along the wavelength ∆λ_{i}.

At the application of the negative voltage to the right-side barrier (minus is applied to the right contact), the right p-n junction is biased in the opposite direction. As a result, the potential barrier gets wider (_{2} is conditioned by right-side barrier.At the same time, the left-side barrier of the n-p junction gets narrower, and the absorption capacity of the range ∆λ of the adjacent region and the photocurrent I_{1} conditioned by the n-p junction decrease.

With the help of the algorithm as stated, it is possible to register a separate wavelength λ_{i} and its intensity by changing the voltage ∆V, thus obtaining the spectral dependence for wave intensities.In the research we used the AM0 spectral distribution of the solar radiation covering the wavelengths from λ = 200 nm up to λ = 800 nm. With the increase of the wavelength, we observe the increase of the intensity [

The linear dependence of the current and the wave intensity in expression (18) allows us to reduce the spectral intensity and, thus, to comply the absorption depth of the waves with the depth of the registering environment. Under the conditions of the reduction of the solar spectrum for 1010 times, according to Lambert’s law, the spectral dependence of the absorption depth in the shortwave (

Now, let us consider some peculiarities of the selection of separate waves from the integral flux of radiation with the help of the oppositely directed diode structures, and find out the possibilities of detection of the required wavelength and its intensity under the conditions of the wave absorption and the wave emission (fluorescence) by the object.

For determination of the spectral composition of the integral flux of the electromagnetic radiation and the definition of its change, the radiation absorption of the Sun is investigated. The wave intensities of the radiation are equally reduced so that the absorption depth corresponds to the width of the registering environment (1 - 2 μm). The photocurrent corresponding to the biggest possible value of x is conditioned by the most deeply penetrated wave. Under these conditions, the process of the selection of separate waves and their intensities from the integral flux of radiation consists in the following:

Let us assume that the informative signal is the photocurrent. We have the biggest values x_{m}_{1} and x_{m}_{2} of x_{m} corresponding to the change of bias voltage 1 mV, and the relevant photocurrents I_{1} and I_{2}. They are obtained from the absorption of the most deeply penetrated wave λ_{i}. With the help of these photocurrents, from Lambert’s law on radiation absorption in the homogeneous environment, we will have the coefficient of wave absorption.

where

Then, with the help of

Thus, with the help of (8), (20) and (21), we determine the absorption coefficient of the most deeply penetrated wave, the wavelength and the intensity of the wave.

Then, with the help of expression (18), considering the absorption surface, we form the dependence I_{1} = f(V) and subtract it from the experimental dependence I = f(V) of the summed current. As a result, we obtain a new dependence of the summed photocurrent on V, without the dependence I_{1} = f(V). By means of relevant software, this method helps us successively determine the lengths and the intensities of all the waves in the radiation, and, finally, the following dependence for the radiation spectrum (

It can be kept in the memory of the device, and be applied in the process of the object identification. The mainparameters of the quantitative determination of hazardous substances in water by means of UV and visible spetroscopy are known [

1) A new model of the portable photo sensor is developed for the remote identification of optical information. Identification and quantitative analysis of hazardous substances in water is studied as well.

2) In the radiation spectrum, the waves have different absorption depths, and the process of the selection of waves is carried out via the gradual change of the external voltage applied to the structure, and the widening of the registering volume.

3) The algorithm developed here permits us to carry out the spectral analysis without any preliminary calibration.

SurikKhudaverdyan,TaronHovhannisyan,NazeliMeliqyan,NarineMehrabyan,StepanTsaturyan,ManeKhachatryan,AshokVaseashta, (2016) On the Model of Spectral Analysis of Optical Radiation. Journal of Electromagnetic Analysis and Applications,08,23-32. doi: 10.4236/jemaa.2016.82003