This paper presents an on-off keying (OOK) wake-up transmitter (TX) suitable for long-term applications at 433MHz. The architecture utilizes a surface acoustic wave (SAW) oscillator, enabling a robust carrier frequency generation, and an inverse class E output stage. The power amplifier (PA) is fully integrated on the integrated circuit (IC) and manufactured in a 22nm fully depleted silicon on insulator (FD-SOI) process. Due to the low threshold voltage of the transistors available in this technology, the circuit can be operated with a low supply voltage of 550mV. The measured DC power of the TX in sleep mode is 74nW. In laboratory measurements, the PA achieves an output power of 4.1dBm and an efficiency of 56%. The entire TX reaches an efficiency of 48%, which is to the authors best knowledge the highest value compared to the state of the art.