The basis of most semi-conductor devices is the p-n junction. Here p-type material is brought into contact with n-type material by a suitable process so that between the two types of material we have a zone of transition — the depletion layer. This layer spans the region where purely p-type properties change to purely n-type. On one side of the layer there are many holes in the valence band and on the other there are many electrons in the conduction band. How we present this state of affairs to the student may be open to question, but, as electrons are the particles which move, it is considered better to represent the conduction electrons as possessing higher energy, i.e. to let electron energy be considered to be positive upward in any model or diagram produced to explain the p-n junction and its rectifying properties.