This paper presents a low-power 140 kHz relaxation oscillator (ROSC) for low-frequency clock generators and timers. In voltage-mode ROSCs, unavoidable shunt current consumption results from voltage slewing at the integration capacitor. The proposed circuit employs CMOS thyristor-based decision elements which effectively reduce shunt currents by exploiting internal positive feedback. A complementary-to-absolute temperature (CTAT) current reference compensates for the frequency’s temperature sensitivity. In order to achieve high negative temperature coefficient with small area and power overhead, the circuit reuses parts of the positive-to-absolute temperature (PTAT) bias generation block. Moreover, a modified start-up circuit with 3 times faster oscillator power-on is presented. The 0.09 mm ² oscillator consumes 6.5 nW/kHz at 1.5 V to 2.5 V supply if only the CTAT source is considered, resulting in a power consumption of 907.4 nW at 140 kHz. The measured temperature coefficient of - 514.7 ppm/K in the range of −40 °C to 85 °C shows an improvement of 5.5 times compared to the uncompensated case. A supply sensitivity of 2.62 %/V, a frequency resolution of 2.67 kHz/step, and an average clock jitter of 6.02 ns are achieved. The oscillator is embedded in an ultra-low power system-on-chip for autonomous environmental sensing.