This work presents silicon germanium (SiGe) based millimeter-wave (mm-wave) circuits and radar sensors in package. The presented sensors achieved state-of-the-art performance in terms of equivalent isotropically radiated power (EIRP) and bandwidth. Furthermore, due to low power and compact form factor the sensors are suitable for variety of applications ranging from automotive, sense and avoid for UAVs and drones, cooperative radars, and other sophisticated MIMO systems. Innovation, optimization, and improvement in the mmwave circuits building blocks, antennas, and package design are described in detail which are instrumental in realization of the high performance mm-wave radar sensors. A special focus has been given to the design of power amplifiers (PAs) as it play an influential role on the system parameters such as range, resolution, and power consumption. Over the past decade advancements in the SiGe technology has made it possible to design circuits operating in the fundamental harmonic for frequency exceeding 100 GHz, however the available gain is still limited. In order to alleviate several stages are usually cascaded to achieve reasonable gain in the amplifier design. The design of multi-stage amplifier is explored by highlighting the importance of impedance transformation ratio which has significant effect on overall gain and bandwidth performance, and by employing techniques such as staggered-tuning, progressive lossy matching, and higher order matching network. Furthermore, in order to increase the output power a low-loss on-chip differential power combiner is presented. The power combiner is integrated with two differential PA modules to show the improvement in output power. The use of power combiner can increase output power, however the effect on power added efficiency (PAE) is always detrimental. Operating in the weak avalanche region has the potential to improve both output power and PAE simultaneously. Careful design measures are required not to exceed breakdown voltage during dynamic operation of the PA. Breakdown mechanisms are described and safe operation region in the periphery of weak avalanche is provided where higher output and PAE can be achieved without running into breakdown region of the device. Frequency multiplier based transceiver chips working in the D-band are designed where effects of multiplication ratio on system bandwidth are explained. Higher order matching networks are used for both frequency multiplication and power amplification. Furthermore, a high power wideband PA is designed in transmit chain to increase the range of the radar sensor. Different versions of the chip include binary phase-shift keying (BPSK) and in-phase and quadrature (IQ) modulator. Receiver chains are based on fully-differential IQ down-conversion where two orthogonal signals are generated by a differential branch-line coupler. The transceiver chips are packaged in an embedded wafer level ball grid array (eWLB) package. Different in-package antenna and arrays are designed for high EIRP and wideband operation. At these frequencies the package itself starts to radiate from the side walls which results in unwanted nulls in the beam pattern. This effect is more pronounced in wideband antennas. The lateral dimensions of the package are optimized along with the antenna to avoid any undesirable nulls in the beam pattern across the required frequency range. Furthermore, an aperture in the secondary metal layer is also utilized to mitigate this issue. The system in package (SiP) radar sensors are operated in frequency-modulated continuous-wave (FMCW) configuration and the performance is compared with the state-of-the-art.