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--- courses:ast201:8 [2023/12/09 01:55] – asad
+++ courses:ast201:8 [2023/12/09 22:39] (current) – [4.2 Photoabsorbers or photoconductors] asad
@@ Line 109: / Line 109: @@
 As seen above, the diamond allotrope of carbon is an insulator because its band gap is huge (5.48 eV). The allotropes graphite and carbon nanostructures are conductors.
-{{:courses:ast201:semiconductors.webp?nolink&750|}}
-==== - Intrinsic photoabsorbers ====
+==== - Photoabsorbers ====
+So far we have talked about the emancipation of electrons via lattice vibration and collision, but an a photon with a wavelength larger than the **curoff wavelength** can also emancipate an electron. The cutoff wavelength
+$$ \lambda_c = \frac{hc}{E_G} = \frac{1.24 \ \mu\text{m}}{E_G \text{ eV}}. $$
+For Si, the value of 1.1 $\mu$m. In astronomy we use semiconductors as photoabsorbers. A simple example is shown below.
+{{:courses:ast201:photoabsorber.jpg?nolink&400|}}
+The photon stream promotes electrons to the conduction band leaving behind an equal number of holes in the valence band. This is a basic **detector** or **sensor** or **receiver** that converts energy into matter, photons into electrons. The greater the stream the higher the conductivity of the detector. If the voltage across the semiconductor is constant, the electrical current $i$ through the resistor $R_L$ would depend on the number of photons absorbed per second.
+So the voltage measured at $V_o$ will be directly related to the **intensity** of light.
 ==== - Extrinsic semiconductors ====