A highly integrated K and Ka-band transceiver MMIC that integrates 33 circuits in a 3 mm 3 mm area using three-dimensional MMIC (3D-MMIC) technology is presented. It achieved a noise figure of 3.4 dB and a conversion gain of 39 dB at 25 GHz in the receive mode. It also achieved a high P1dB of 8 dBm and a conversion gain of 29 dB at 25 GHz in the transmit mode. The power dissipation of the MMIC is only 0.34 W in the receive mode and 0.38 W in the transmit mode. It also integrates a frequency doubler, a single-pole/double-through (SPDT) amplifier for local oscillator (LO) signals, and step attenuators with built-in inverters using N-channel depression HEMTs. The attenuators make it possible to control receiver gain and transmitting power, and thermal noise reduction to the receiver block.