A mid-infrared light-emitting diode (LED) operating at 3.6 mum with a pulsed output power as high as 2 mW at room temperature is reported. The device is based on a double heterostructure of InAs0.42Sb0.18P0.40/ GaxIn1-xAs1-ySby/InAs0.42Sb0.18P0.40 grown by liquid phase epitaxy. The active region composition was adjusted using different Ga content (x = 0.05-0.17) over which range the band gap is approximately constant. The electrical and optical operating characteristics are reported. The emission spectra measured with and without 100% formaldehyde have also been measured using a short-path optical cell to demonstrate the potential of these LEDs for use in solid state formaldehyde gas sensors.