Three-section distributed Bragg reflector (DBR) tunable lasers emitting in the 1.55 mu m region are studied through a theoretical model which takes into account thermal effects and exploits an accurate calculation of the optical parameters. Static modal competition is discussed for transverse and longitudinal modes in operation at and above threshold. The static tuning characteristics are presented; a saturation of the wavelength shift is found for control current densities of approximately=20 kA-cm/sup -2/ due to Auger recombination processes and heating effects. The dependence of the tuning ranges on the section lengths, waveguide thickness results, and some design criteria are derived.>