18.7 A 0.7V, 2.35% 3σ-Accuracy Bandgap Reference in 12nm CMOS

Bandgap reference (BGR) circuits are widely used due to their stable output voltage over process, supply voltage and temperature variations. Reference voltage stability is critical for data-acquisition applications and lower supply voltages can reduce the power of mixed-signal systems. However BGR for analog circuits is one of the bottlenecks for sub-1V supply operation because BGR supply voltage is limited by VEB+VDS [1]. VEB refers to the emitter-base voltage of a pnp transistor which is limited to ~0.6 to 0.7V due to silicon junction cut-in voltage, while VDS is the drain-source saturation voltage of a current-mirror. The BGR temperature dependence is decided by the weighted sum of proportional-to-absolute-temperature(PTAT) and complementary-to-absolute-temperature (CTAT) terms. An alternative PTAT generator can be implemented by dVGS (gate-to-source voltage difference) of a MOS pair in subthreshold [2]. The CTAT generator can be implemented by special devices or using the gate-source voltage VGS of subthreshold MOSFETs. Although the VGS of a subthreshold MOSFET is smaller than emitter-base voltage of a pnp transistor, the MOSFET model inaccuracy in the subthreshold region and high process-dependent characteristic of MOSFET gate-source voltage induces high variation for voltage reference circuits.