Abstract: As a worldwide issue, air pollution has a substantial impact on everyday human life. There are surging demands for convenient ways of detecting pollution gases. Tunable diode laser absorption spectroscopy (TDLAS), operating in the mid-infrared region, is gaining increasing attention for pollution detection because of its high sensitivity and fast response. However, the lack of tunable single-mode mid-infrared lasers has delayed the development of TDLAS techniques. In this research, a novel cavity of equilateral-triangle-resonator (ETR) laser structure is investigated for the development of a tunable single-mode mid-infrared laser. GaSb-based InGaAsSb materials are used for the laser active region material because of their excellent performance in the 2.0-3.0 μm wavelength range, which corresponds to one of the trace gas detection windows. InGaAsSb lasers at long wavelengths face the problem of performance degradation attributed to poor carrier confinement in the active region. Based on theoretical modeling, high Al-content AlGaAsSb layers are used as the barrier layer to overcome this problem. Fabricated 2.8 μm InGaAsSb laser diodes with an optimal active region design demonstrate improved performance and realize continuous-wave operation at room temperature. Long wavelength InGaAsSb lasers are demonstrated in pulsed mode up to 3.12 μm at room temperature, which is very close to the latest world record. Mid-infrared ETR microlasers are fabricated on a GaSb-based InGaAsSb laser structure using processing techniques of photolithography and inductively coupled plasma etching. A 1.9 μm single-mode ETR microlaser is demonstrated operating continuous-wave at 77 K for the first time in the world. The peak wavelength of the single-mode laser emission is successfully tuned continuously over 3.2 nm. This device serves as a prototype to study the feasibility of the ETR cavity structure for the development of the tunable single-mode mid-infrared lasers for gas sensing applications.