A new CML-type monostable/bistable logic element IC is fabricated using inonolithically integrated resonant tunnelling diodes (RTDs) and InP/InGaAs Heterojunction Bipolar Transistors (HBTs). The D flip-flop function of the fabricated circuit is confirmed up to 20 Gbps at room temperature. This result indicates the potential of the RTD/HBT technology for high-speed logic applications.