This　paper　presents　a　modeling　and　analyzing　methodology　of　millimeter-wave　on-chip　transformer　balun　based　on　the　90nm　SiGe　BiCMOS　process.　The　physical　parameters　of　transformer　balun　including　turns　ratio,　coil　geometry,　tap　grounding　strategy,　and　geometric　center　misalignment,　are　investigated　for　adjusting　balun＇s　electrical　properties.　Experiments　show　that　increasing　the　turns　ratio　can　improve　its　energy　transmission　efficiency,　increases　the　coupling　coefficient　by　a　factor　of　+0.05　and　+0.24　respectively.　Adjusting　the　coil　spacing　can　increase　its　coupling　coefficient　by　a　factor　of　0.13.　Shifting　the　centers　of　the　primary　and　secondary　coils　can　increase　its　amplitude　and　phase　imbalance　and　deteriorate　CMRR　from　29.5dB　to　24dB.　These　findings　offer　a　systematic　guidance　for　millimeter-wave　balun　design,　enhancing　high-frequency　performance,　interference　resistance,　and　signal　integrity.　©　2025　IEEE.