Module 9 : Photo Detectors
Lecture : Principle of Photo Detection
 

Consider a thin slab of semiconductor which is illuminated by a beam of light propagating along the direction of its length (x-direction). Let $ I$be the radiation intensity (in watts/m$ ^2$) at a position $ x$from one end of the semiconductor. If $ \alpha=$absorption coefficient per unit length, the power absorbed per unit length is $ \alpha I$. The change in the intensity with distance along the sample length is given by

$\displaystyle \frac{dI}{dx} = -\alpha I$

which has solution

$\displaystyle I = I_0e^{-\alpha x}$

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