diff --git a/src/14_doping_and_devices.md b/src/14_doping_and_devices.md
index 074aae7189574a8da0f682ab53be74065c97b623..3059a7c85b0dcbc820c72344da6f1a476c947650 100644
--- a/src/14_doping_and_devices.md
+++ b/src/14_doping_and_devices.md
@@ -238,6 +238,14 @@ Let us now work out what happens when the semiconductor is at the border between
 
 ### Exercise 2: Donor ionization
 
+Previously we have assumed that all dopants are ionized.
+Let us examine when this is a good assumption.
+For that we consider a doped semiconductor in the extrinsic regime.
+
+1. Assume that all dopants are ionized, determine the position of the Fermi level.
+2. Write down the concentration of dopants that are not ionized.
+3. Determine at what donor concentration one cannot assume anymore that all donors are ionized in germanium at room temperature.
+
 ### Exercise 3: Performance of a diode
 
 ### Exercise 4: Quantum well heterojunction in detail