A new versatile, coplanar‐contact device configuration has been developed for generation of coherent emission from indium antimonide. The devices, which generate coherent emission at frequencies from 6.5–30 GHz, exhibit three types of operation. Emission is generated (1) in a thin‐layer plasma formed by a strong transverse magnetic field, (2) in a thin‐layer plasma formed as a result of the coplanar‐contact configuration (zero magnetic field) and (3) in a thick‐layer plasma formed by forcing carriers away from the contact surface with a strong transverse magnetic field. For a resonance condition between the coplanar‐contacts, the emission frequency generated in a thin‐layer plasma is related to the contact spacing in accordance with two‐stream theory. The emission from the thin‐layer plasma in zero magnetic field and the noise emission generated at room temperature by the coplanar‐contact samples in a 10‐kG transverse magnetic field give further confirmation of the two‐stream theory. In the first and second types of operation, instantaneous power levels of 1 to 10 &mgr;W are emitted for input power levels of 0.2 to 1.5 W. In the third type of operation, with an input power level of 2 W, peak output power levels of 80 to 170 &mgr;W atX‐band frequencies have been measured. The low input power level, which is required for emission from the coplanar‐contact devices, permits cw operation in several samples. Low‐frequency instabilities generated in a thick plasma layer are frequency‐locked by a 160‐MHz external signal with a −3 dBm power level.