This paper proposes a radiation-hardened-by-design (RHBD) technique targeting single-event transient (SET) mitigation in bandgap reference (BGR) circuits. The dual modular redundancy (DMR) technique used for BGR ensures correct output voltage to the subsequent circuits. The BGR output voltage is detected and clamped by two threshold voltages and time-delayed switches further isolate SET pulses. The proposed BGR circuit was fabricated in 28 nm bulk CMOS process. Laser experiments illustrate that SET perturbation is almost eliminated when laser energy is up to 1 nJ. The RHBD BGR is proven to be good substitute for space applications.