This article presents an auto-calibration technique for current-based bandgap voltage references (BGRs), based on a digitally-assisted auto-calibration loop for calibration cost reduction. We first present a theoretical study of the process variation induced VEB and 1VEB variations in the BJT, which contribute to residual errors in the reference voltage (VREF) and its temperature coefficient (TC) after applying conventional one-point trimming. Based on the study, we further propose an automatic one-point trimming methodology using a current digital-to-analog converter (IDAC), which can simultaneously relax the VEB and 1VEB variations, resulting in a small drift in both VREF and its TC after calibration. Fabricated in 65 nm standard CMOS, the proposed auto-calibrated BGR achieves a measured TC of 22.3 ppm/◦C at 1.2 V supply within −40 ◦C to 120 ◦C. The line regulation is 1.26 mV/V or 0.13%/V from 1.2 to 2.5 V. Based on ten-chip measurement results, the achieved σ/µ variation in VREF improves from ±0.53% to ±0.12% within the entire temperature spectrum after applying the proposed auto-calibration technique at 27 ◦C.