Page 50 - Special Issue Modern Trends in Scientific Research and Development, Case of Asia

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traps are filled with injected holes and the hole With an increase in the external stress, this section expands,
concentration begins to grow. but at lower stresses, we can assume x << L, L is the i-layer
thickness, therefore we will integrate from 0 to L.
At some voltages, the injected holes concentration is much
higher than that deep traps. This will happen if the photo Considering the above conditions, we get the relationship
generated holes concentration is greater than the traps between the applied voltage current.
concentration. If these conditions are satisfied, then for this
case we have the following equations:
(6)
For current density
Thus, we got the Mott's quadratic law. As seen from (6), the
(1) photocurrent value depends only on the hole parameters.
Therefore, information on the hole transfer mechanism can
For the electric field distribution in the i-a-Si: H region, we be obtained. To show this expression reliability and
have Poisson equations applicability for the Vidicon photo-CVC target, we will
analyze the experimentally obtained graphs and carry out a
numerical calculation. In [6], photo-CVCs were obtained at
(2)
different temperatures (Fig. 2).

and the equation for the charge exchange kinetics

(3)

Here N is the hole traps concentration in the mobility gap, Х
is the concentration of injected photo generated holes, τp is
their lifetime, μp is the hole mobility, g is the thermal
generation rate of the i-regions majority carriers, f is the
filling factor of hole traps, and this value at thermodynamic
equilibrium is determined by the following expression:

Fig.2. Photo-CVC obtained at various temperatures.

As can be seen in the temperature dependence, the photo-
Here G is the photogeneration rate. CVC section, which obeys the "quadratic" law, is shifted
To solve these equations, we will evaluate some terms of almost parallel to the high voltage region. This photo-CVC
behavior for samples with different thicknesses satisfies (6)
these equations. Due to the diffusion smallness coefficient,
equation. The holes’ photoelectric parameters depend on
we neglect the second terms (1) of the equation. Due to the
dark current dependence on voltage [5], it can be seen that g temperature, therefore, from this section of the photo-CVC, it
is possible to obtain the temperature dependence of the
thermal generation is insignificant, so we will not take it into
holes’ photoelectric parameters. And this case requires
account. The quantities ∆N, ∆p, ε and f depend on the applied
voltage. As the voltage increases, the injected holes number additional research. To determine the holes’ photo
in the i-layer increases strongly and all traps become filled, generation rate, we used the following expressions.

as mentioned above, then the filling factor is f = 1 if
thermodynamic equilibrium occurs between the traps and Where is the incident light spectral density, αi (λ) is the
injected holes. Then the following condition is satisfied:
incident light coefficient in the i-a-Si: H layer, Tni is the n layer
transmittance into the i-layer, p and s are polarization
, taking into account the above-given conditions indices, to obtain a photo-CVC plot obeying the "quadratic
law", we will use the following values and estimates of some
parameters of i-a-Si: H.
from equation (3) we have or then

the equation (1) and (2) takes on the following:
When measuring the vidicon target photoconductivity, the
light falls normally and is completely absorbed in the i-layer,
in addition, the n-layer thickness is insignificantly relative to

the i-layer. Then Where nin is the

relative refractive index. If we take into account that the n
Solving together (4) and (5) the equation we have the and i-layer consists only of a-Si: H then nin ≈ nni =1 and T=1.
following expression for the relationship between
The coefficient values of defect absorption α(λ) ~1-10 сm ,
-1
the holes mobility μp=0,67 сm /(В∙с), m=34÷35 and di≈1 μm.
2
. To integrate this expression, it is When light is absorbed at the surface i-layer, as indicated
necessary to estimate the lower integral bounds. As is above, a carriers "reservoir", that is, holes, appears, and
known, in the vidicon mode, the incident light is absorbed at along with it, a potential difference appears between the i-
the i-layer surface to x0 depth. This thickness, in which the layer part that absorbed and not absorbed light. Photo
holes are photogenerated, can be called a "virtual" cathode. generated holes must overcome this potential barrier. The

ID: IJTSRD35835 | Special Issue on Modern Trends in Scientific Research and Development, Case of Asia Page 45