If you are using the book as reference during this exercise, note that the figures in the book do not show the electrons once they make it to the conduction band. This is contrary to what we see in class. Normally, we show the electrons in the conduction band and the holes in the valence band. This approach seems more consistent, and less confusing, considering the electrons are entering the conduction band and leaving behind either an empty donor state or a hole. This is the format we will use for the class. (Hint: use this format on the homework, quizzes and tests, too!)
The illustrations represent the most likely situations. We all know electrons are always on the move because they are always gaining and losing energy which makes them move between energy states. When we draw band diagrams to visualize what is taking place within the semiconductor, we are actually taking a "snapshot" or a "still frame" of the electron in action. We are stopping time for a moment to analyze the situation.
We use band diagrams to visualize how the semiconductor responds to various stimuli, i.e. change in temperature, doping, light, etc., but they are only representations of what has the highest probability of occurring. In other words, if we watch the semiconductor for a long time and take 10,000 pictures, 9,998 will look the same and that is the one that we draw. Now you know what we mean by "snapshots".
(use the "back" button on your browser to get back to the problem)