added pn diode image and source

src/14_doping_and_devices.md

@@ -248,7 +248,15 @@ For that we consider a doped semiconductor in the extrinsic regime.
 
 ### Exercise 3: Performance of a diode
 
-Consider a pn-junction diode. The current generated by the pn diode as a function of applied bias voltage is given as follows
+Consider a pn-junction diode as follows 
+
+<img src="https://upload.wikimedia.org/wikipedia/commons/thumb/7/79/PN_diode_with_electrical_symbol.svg/640px-PN_diode_with_electrical_symbol.svg.png" width="50%" alt="pn diode"></img>
+
+??? info "source"
+
+    By Raffamaiden [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]), [Link](https://commons.wikimedia.org/wiki/File:PN_diode_with_electrical_symbol.svg)
+
+The current generated by the pn diode as a function of applied bias voltage is given by 
 
 $$ I(V) = I_0\left[exp(\frac{eV}{kT})-1\right]$$