  Sensor nodes and IoT systems require blocks that not only consume low power
but also have good accuracy. Voltage reference generators are also considered
important building blocks in sensor interface circuits. This paper presents a
solution to increase the accuracy of low power subthreshold voltage generators
by lowering the circuit sensitivity to temperature and supply voltage
variations. The enhancement is achieved by using two separate stages for
temperature coefficient (TC) and line sensitivity (LS) correction. A 0.18
{\mu}m standard CMOS process has been used for the proposed structure. The
effects of parameter variations in the fabrication process are investigated
using post-layout simulation and Monte Carlo analysis. In the supply voltage of
0.4 V to 2 V, an LS of 143.8 ppm/V is obtained. In typical corner conditions,
the achieved TC is 7.45 ppm/{\circ}C over the temperature range of 0{\circ}C to
80{\circ}C. Due to process changes, and mainly affected by threshold voltage
variations, the average TC can change to 39.2 ppm/{\circ}C. The minimum power
consumption at 0{\circ}C and at a supply voltage of 0.4 V is 3.25 pW while the
power consumption increases to 2.84 nW in 80{\circ}C and at the maximum supply
voltage of 2 V.
