We present interband cascade light-emitting devices with incoherent and broadband light emission peaked at a wavelength of around 3.7  μm. The substrate-side-emitting devices display higher wall plug efficiencies and maximum output powers than any earlier mid-infrared LEDs operating in continuous wave at room temperature. To reduce absorption losses, the epitaxial structures were grown on low doped (low absorbing) GaSb substrates. The nine active stages were positioned in different configurations to investigate the impact of constructive or destructive interference when reflected from the epitaxial-side metallization of the flip-chip mounted devices. A comparison shows improved electrical properties and outcoupling efficiencies when all active stages are centered within a single antinode of the optical field. The optimized voltage efficiency combined with low optical losses lead to a maximum wall plug efficiency of 0.7%. Flip-chip mounted devices with a 640-μm squared mesa reached output powers of up to 5.1 mW at ambient temperatures with driving current (voltage) of 0.6 A (5.1 V).