Shallow donor

A shallow donor refers to a specific energy level present in semi-conductors indicating a lower level of impurities.

Overview

Introducing impurities in a semiconductor which are used to set free additional electrons in its conduction band is called doping with donors. In a group IV semiconductor like silicon these are most of often group V elements like arsenic or antimony. However these impurities introduce new energy levels in the band gap affecting the band structure which may alter the electronic properties of the semiconductor to a great extent.

Having a shallow donor level means that these additional energy levels are not more than $3\; k\_b\; T$ (0.075 eV at room temperature) away from the lower conduction band edge. This allows to treat the original semiconductor as unaffected in its electronic properties having the impurity atoms only increasing the electron concentration. A limit to donor concentration in order to allow treatment as shallow donors is approximately 10¹⁹ cm^-3.

If there are energy levels due to impurities deeper in the bandgap they are called deep levels.