High-field transport and terahertz generation in GaN

The conduction-band structure of GaN suggests that electron transport at high fields should exhibit a negative differential resistance (NDR) either via the transferred-electron effect or as a consequence of the negative effective mass beyond the inflection point. In order to discuss these possibilities we use a simple model of the band structure and obtain analytical expressions for the density of states and scattering rates due to the interaction with polar-optical phonons. Estimates of the cutoff frequencies for the Gunn effect in GaN and AlN are obtained. The negative-mass NDR is described in terms of a hydrodynamic model and the result is compared with that of Esaki and Tsu [IBM J. Res. Dev. 14, 61 (1970)]. We explore the effect in short diodes of length and transit time on the frequency-dependent NDR associated with quasiballistic motion and conclude with a discussion of transport when the electrons are localized by Bragg reflection. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3032272]