TABLE OF CONTENTS The allowed states in the band gap of semiconductors are traditionally divided into "shallow" and "deep." This division is extremely arbitrary and shallow states are usually assumed to be those with ionization energies E i<0.1 eV and deep states, those with E i >0.1 eV1. For silicon carbide, this division is even more arbitrary, since even the ground donor and acceptor levels have ionization energies E i ?0.1 eV. Thus, strictly speaking, all the levels that have been studied in silicon carbide are deep. However, we regard, in what follows, the levels that determine the resistance of the base regions as shallow, compared with the deeper levels that make no noticeable contribution to the concentration of the majority charge carriers.
Undoped SiC layers have n-type conductivity. This is accounted for by the uncontrolled doping of the growing layer by nitrogen; in addition, nitrogen has a rather high solubility in SiC ( ?10 21cm -3)2 and the lowest ionization energy among all the impurity donor levels. By implanting N ions, it is also possible to obtain thin, heavily doped layers of SiC in order to form ohmic contacts3 ,4.
The existence of nonequivalent sites in the SiC lattice shows up most clearly in the energy position of the deep centers associated with nitrogen atoms 5 8. In all the main polytypes of SiC, nitrogen atoms have been observed...
