typo fixes

src/14_doping_and_devices.md

@@ -264,7 +264,7 @@ where $I_s(T)$ is the saturation current.
 
 1. What is the significance of adding dopant atoms to an intrinsic semiconductor?
    Can two intrinsic semiconductors joined together make a diode?
-2. Discuss the which processes carry current in a diode under reverse biase.
+2. Discuss which processes carry current in a diode under reverse bias.
 3. Based on this, estimate how the saturation current $I_s$ depends on temperature.
 
 ### Exercise 4: Quantum well heterojunction in detail