This paper presents an energy-efficient single-ended (SE) transmitter (TX) for memory interfaces, supporting non-return-to-zero (NRZ) and four-level pulse amplitude modulation (PAM-4) dual modes. The dual-mode TX fully reuses the output driver, allowing high-performance impedance matching at the “0”/“00” and “1”/“11” levels of the NRZ/PAM-4 mode, and enabling low-power relaxed impedance matching at the “01” and “10” levels of the PAM-4 mode. The pre-emphasis equalization is proposed with high energy efficiency to enlarge the eye-opening using impedance modulation and a half-rate charge pump (HRCP) for the ground-terminated dual-mode TX. The five-step three-point ZQ calibration with offset cancellation is performed to enhance the level separation mismatch ratio (RLM). Implemented in a 22-nm CMOS process, the TX achieves the data rate of 16 Gb/s/pin NRZ drawing 8.07 mW over a 12.9-dB loss channel and of 32 Gb/s/pin PAM-4 drawing 6.87 mW over a 4.6-dB loss channel with 99.0% RLM.