High-performance AlInAs/GaInAs HBTs for low-power digital circuits have been demonstrated. Large-area devices exhibit high current gain (up to 600), observable down to low currents. Small-area devices display ƒt up to 78 GHz and ƒmax up to 45 GHz. Fitting S-parameter measurements to an equivalent circuit model shows that ƒmax is limited by a large RbCbc time constant in the base circuit. These transistors have been used to fabricate the first frequency divider demonstrated in this material system.