gamma ray detection for germanium detectors High purity germanium detectors based on a semiconductor and  For radiation detection, the germanium detector is the best, but I confused because The functioning of a Ge detector is based on the use of the depletion region formed between two different (p and n-type) semiconductor materials.The operation of the detector is based on the interaction of gamma rays and the semiconductor within this depletion region. Why gamma rays just interact with depletion region?
 A: Jon Custer has responded first in the comment, but doesnt continue with the answer.  I think the question should have an answer to keep the Physics SE complete and useful for later searches....
Here is a cut through the HPGe detector. Pay attention to 


*

*Al window layer

*top Ge dead layer



The usual configuration is that you put the source at a distance $d$ in the axis. The photons that you detect


*

*come directly from the source 

*come indirectly from the source - they are scattered back from surrounding material to the detector

*come from elsewhere, but not your source


Only those coming directly are usually interesting for you. But they can


*

*scatter on Al window or Ge dead layer and you will see just a remains of the original energy.

*pass through totally and you see nothing

*scatter in the active Ge and fly away and you see just a part of the original energy (single escape and double escape effects we can put also in this point)


So what remains in the full-energy peak are just those lucky photons that loose all their energy in active Ge by one or more step effects. Mention the term full-energy. The background in the picture comes from those unlucky photons.

