In this paper, radar transmitter circuits for next generation automotive radar sensors are presented. A 79 GHz radar transmitter with an output power of 14.5 dBm consuming only 165 mA (including frequency dividers) from a 3.3 V supply voltage clearly shows the advantage of using an improved SiGe technology with an fmax of 380 GHz. In addition, two radar transmitters for higher frequencies (around 150 GHz) based on frequency doubler circuits are showing the potential of SiGe technologies. The first transmitter achieves an output power of 3 dBm (single ended) at 144 GHz, whereas the second transmitters delivers a differential output power of 0 dBm at 150 GHz. Both transmitters achieve an ultra-wide tuning range of about 45 GHz.