The challenge of teaching new subject matter is a familiar one for most teachers. This paper investigates the content knowledge gains made by seven teachers as they learn to teach the topic of semiconductors through a process of self-study. "Semiconductors" is a new topic in the curriculum which looks at the sub-microscopic properties of materials used in electronics and is important because of its applications in modern technology. We present a phenomenographic analysis of 21 concept maps produced by seven teachers at various stages of their self-study project. Four categories of description emerged in the analysis, viz. no meaning, orientation to understanding of subject matter (SMK), a pedagogical content knowledge (PCK) context focus, and a PCK strategy focus. The last category proved to be the most interesting as it suggested that the teacher saw the learning of SMK and learning to teach the SMK as a single process. This finding was considered as important in understanding how teachers learn new SMK for teaching. We found that experienced teachers make sense of their new SMK in different ways. There is a variation of orientation in their learning and sense-making. The data also showed progression of teachers' understanding of the topic through the self-study process. The findings suggest that learning SMK for the purposes of teaching is different to learning for fulfilling the requirements of a study programme.