Module 9 : Photo Detectors
Lecture : Principle of Photo Detection
 

Using this in the expression for the responsivity, we get

$\displaystyle {\cal R} = \frac{I}{P_0}= \frac{q\eta}{h\nu}= \frac{q\eta}{hc}\lambda$

The responsivity, therefore, depends on the wavelength $ \lambda$. For an ideal photodetector, $ \eta =1$and $ {\cal R}$is linear with $ \lambda$.

Spectral Response :
The spectral response of a detector is is given by the manner in which the output signal of the detector varies with the change in the wavelength of the incident radiation. As the quantum efficiency depends on the wavelength, the response is not linear as would be the case if $ \eta =1$.
The energy of the photon must be sufficient to excite an electron across the energy barrier $ \Delta$. If $ \Delta$is in eV, the maximum wavelength that the detector would respond to is

$\displaystyle \lambda_{\rm max}\ \ ({\rm in \ nm}) = \frac{1240}{\Delta\ ({\rm ev})}$

However, the response does not fall off abruptly to zero for values of $ \lambda$above the threshold. This is because, due to thermal energy of the molecules, the absorption coefficent $ \alpha$of the material of the device is found to be given by

$\displaystyle \alpha = \alpha_0 e^{E/\Delta}$

where $ E$is the incident photon energy. For $ \lambda > \lambda_{\rm max}$, $ E<\Delta$so that the absorption of radiance becomes smaller.

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