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--- courses:ast201:8 [2023/12/09 01:54] – [3.3 Temperature] asad
+++ courses:ast201:8 [2023/12/09 22:39] (current) – [4.2 Photoabsorbers or photoconductors] asad
@@ Line 33: / Line 33: @@
 The semiconductor $c$ has electrons in the conduction band without any corresponding holes in the valence band. These are extrinsic semiconductors widely used to make electronic devices. Another class of extrinsics exhibits holes in the valence band without any corresponding electrons in the conduction band.
-==== - Emancipation of electrons ====
 At room temperature, all elements of the column IVA are non-conductors, but they become capable of ionization (promotion of electrons to the conduction band) as temperature increases.
@@ Line 110: / 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 ====