We proposed a thermally-tuned distributed Bragg reflector (DBR) laser diode that has a high tuning efficiency over a wide wavelength tuning range. The laser diode is composed of a gain, a phase control (PC), and a DBR region, and its wavelength is tuned coarsely and finely by the micro-heaters on the DBR and PC regions, respectively. To improve the tuning efficiency, we developed a technique for fabricating a thermal isolation structure through a reverse mesa etching process, replacing the complex process that uses an InGaAs sacrificial layer. The DBR laser diodes (DBR-LD) fabricated using this method effectively confines heat generated by the heater, resulting in an approximate tuning range of 40 nm. This technology, which has achieved nearly four times larger wavelength tuning range than the thermally-tuned DBR-LDs without a thermal isolation structure, is considered suitable for the cost-effective development of wide-wavelength-tuning DBR-LD light sources.