In this paper, carrier reservoir semiconductor optical amplifiers (CR-SOAs) are employed for the first time to design an all-optical (AO) half adder at 120 Gb/s. The CR-SOAs are incorporated in Mach–Zehnder interferometers (MZIs) configured as XOR and AND gates to execute the required SUM and CARRY logic functions,respectively. The performance of the half adder is evaluated through the quality factor (Q-factor), whose variation against the CR-SOA’s critical operating parameters, including amplified spontaneous emission noise, is theoretically investigated. The simulation results show that MZIs with CR-SOAs can implement an AO half adder running at 120 Gb/s with a high Q-factor.