The Space Charge is simply the excess or deficiency of electrons/holes/ions that build up in one region of a material (such as at the interface or junction). The layer of this excess/deficiency found at the interface of a junction is called the space charge layer (or double layer). In a p-n junction, this is best described as a deficiency. When a p-type semiconductor is connected to an n-type semiconductor via a junction, an electrochemical driving force or potential is observed.

Another way to talk about the space charge layer at the junction. If we can talk about holes with respect to electrons, we can talk about the depletion region with respect to space charge layers. Opposites attract, right? Right--and then they recombine and there are no charges. As an analogy, imagine if you were to roll hot coals and ice cubes into one another--in the end you would have neither hot coals nor ice cubes (in fact, you get low energy mud). The same thing occurs for electron-hole pairs. They recombine and form a layer of no charge, which inhibits an more drift of electrons and holes across the junction. In reality this conjugate pouring tapers off (attenuates) and each material is left with a deficiency of electrons or holes (a space charge layer) that would not have existed without the junction.