Widening the High-Entropy Operating Window of Perpendicular Magnetic Tunnel Junction TRNGs via Multibit xor Whitening

Perpendicular magnetic tunnel junctions (pMTJs) are promising entropy sources for hardware true random number generators (TRNGs) owing to their intrinsic stochastic switching and CMOS compatibility. However, it is challenging to tune the switching probability to the ideal 50% for uniform randomness, as the stochastic switching process is highly sensitive to device variations and environmental conditions. Here, we systematically evaluate the robustness of entropy generated by magnetic tunnel junction (MTJ) stochastic switching events under varying operating conditions and assess changes in randomness quality after multibit xor postprocessing. By sweeping pulse widths from 2 to 10 ns (corresponding to average switching probabilities from 0% to 90%), we show that increasing the xor width significantly expands the high-entropy operating range. The stable entropy window broadens from a point near 50% for raw outputs and 2-bit xor to a range of 16%-84% with 16-bit xor. In addition, overlapping xor windows can increase throughput by up to 2.67× without degrading entropy quality across the stable range. These results demonstrate that high-degree xor whitening enables bias-tolerant, feedback-free entropy generation and provides a practical route toward pMTJ TRNG designs for cryptographic applications.