Figure 8.1: Schematic representation of an isolated n-type and p-type semiconductor and corresponding band diagrams.
When a p-type and an n-type semiconductor are brought together, a very large
difference in electron concentration between n- and p-type regions causes a diffusion
current of electrons from the n-type material across the metallurgical junction into the ptype material. The term “metallurgical junction” denotes the interface between the n- and
p-type regions. Similarly, the difference in hole concentration causes a diffusion current of
holes from the p- to the n-type material. Due to this diffusion process the region close to
the metallurgical junction becomes almost completely depleted of mobile charge carriers.
The gradual depletion of the charge carriers gives rise to a space charge created by the
charge of the ionized donor and acceptor atoms that is not compensated by the mobile
charges any more. This region of the space charge is called the space-charge region or
depleted region and is schematically illustrated in Fig. 8.2. Regions outside the depletion
region, in which the charge neutrality is conserved, are denoted as the quasi-neutral
regions.