Gallium nitride epitaxial layers were grown by rf-plasma MBE on different buffer layer structures. Type I buffer layer consists of a conventional AlN high-temperature buffer layer (HTBL). Type II buffer layer consists of a GaN intermediate-temperature buffer layer (ITBL) grown on top an AlN HTBL. Measurement of flicker noise in metal-semiconductor-metal (MSM) structures on type II buffer layers exhibited close to two orders of magnitude reduction in the noise level compared to those fabricated on type I buffer structures. This shows that GaN thin films grown with the use of ITBL have significantly lower number of interface traps at the metal-semiconductor interface, which is attributed to be the main cause of the observed improvements in the optical properties of the devices. We also performed systematic studies on hot-electron degradation of the devices through the application of a large voltage bias. The data demonstrate substantial improvement in the hot-electron hardness for devices fabricated on type II buffer layer structures. (C) 2003 Elsevier Science B.V. All rights reserved.