Upgrade for Phase II of the Gerda experiment

The Gerda collaboration is performing a sensitive search for neutrinoless double beta decay of ^76Ge at the INFN Laboratori Nazionali del Gran Sasso, Italy. The upgrade of the Gerda experiment from Phase I to Phase II has been concluded in December 2015. The first Phase II data release shows that the goal to suppress the background by one order of magnitude compared to Phase I has been achieved. Gerda is thus the first experiment that will remain “background-free” up to its design exposure ( 100 kg year ). It will reach thereby a half-life sensitivity of more than 10^26 year within 3 years of data collection. This paper describes in detail the modifications and improvements of the experimental setup for Phase II and discusses the performance of individual detector components.