An ultra-wideband asymmetric spiral-stacked Marchand balun fabricated in the standard 0.18 μm CMOS process is demonstrated. An innovative technique named in-phase current return path is incorporated into the design to enhance the balun's balance metrics. The proposed balun has a measured insertion loss <5 dB (3 dB for an ideal balun) and a return loss better than 10 dB from 20.8 to 60 GHz. Significantly, it provides a measured amplitude imbalance <0.3 dB and a phase imbalance <2°within 10-51 GHz range. The balun occupies an area of only 0.043 mm 2 without pads. The resultant highly balanced CMOS balun is shown as a high-performing millimetre-wave component suitable for the full Ka and/or Q band applications. Introduction: Balun, which transforms a single-ended signal into two differential signals while maintaining equal signal amplitude, is widely used in antennas, push-pull amplifiers, balanced mixers and filters. The transformer based [1] and Marchand type [2-4] baluns with the merits of passive structures, which have no dc consumption and high operating frequencies, are perhaps two of the most prevalent structures for millimetre-wave applications. However, the transformer balun in [1] suffers from inferior performance in terms of insertion loss (IL) and return loss (RL) even though the 3.4 μm-thick top metal is used. A co-planar spiral-type Marchand balun with a slot pattern ground has been demonstrated in the standard 0.18-μm CMOS process [2]. The balun features an IL of 5 dB, an RL of 10 dB, a phase difference (PD) <5°and an amplitude difference (AD) <1 dB within a relative low-frequency range from 18.4 to 32.2 GHz. The bandwidth can be further extended using the broadside-coupled structures, which were exploited by Chiou et al. [3]. In [3], an asymmetric stacked balun was implemented by meander lines with a larger frequency range from 16.5 to 67 GHz. Nevertheless, the drawbacks are obvious that it has a relative high in-band IL of 7 dB, a PD of 5°and an AD of 1 dB, respectively. In our previous work [4], two asymmetric spiral-stacked baluns were developed with both amplitude and phase balance compensated. The measured results show that both of the baluns have a reduced PD of 2.5°, but without AD improvement. Here in this Letter, a new dual-asymmetric spiral-stacked Marchand balun is developed with the in-phase current return path (CRP) technique specifically for balance enhancement. The resultant balun achieves a superior performance of AD <0.3 dB and PD <2°within an ultra-wideband (UWB) from 10 to 51 GHz.