This report is about designing and implementing an on-chip lower power and area temperature sensor for an ultrasonic probe. The sensor was designed, implemented in schematic and simulated using the Virtuoso software for analysis and verification. The design of the circuit is based on another published paper , but with more smaller transistor dimensions in 90 nm CMOS technology. The sensor works in the sub-threshold region with only 600 mV of power supply. It operates over the temperature range of 0-120 °C, with maximum pulled current of 218.66 nA and mere power consumption of 131.2 nW at the highest temperature of 120 °C. It has an inaccuracy range of -3.3 to +7.6 °C with very high linearity and stability performance. It is based on measuring the temperature using the differences in gate-source voltages of a MOSFET which is proportional to the temperature in the sub-threshold region.