Semi-two-dimensional (semi-2D) benzo[1,2-b: 4,5-b']-dithiophene (BDT)-based donor-acceptor copolymers have been regarded as one of the most successful organic semiconductors for photovoltaic applications. In this work, the structure-mobility relationship of four semi-2D BDT-based polymers bearing different kinds of conjugated side chains, including alkyl-monothienyl (T), alkyl-dithienyl (2T), branched alkyl-trithienyl (3T), and alkyl-benzotrithienyl (B3T), is thoroughly investigated. Because of the lower rigidity and the higher spatial mobility, the polymers owning lineally extended conjugated side chains, like PBDT-T and PBDT-2T, still possess certain crystallinity after thermal annealing. However, the crystallinity becomes evanescent as the side-chain conjugation further increases. PBDT-3T and PBDT-B3T bearing a large, branched conjugated side chain become nearly amorphous even after thermal annealing. Despite the low crystallinity, all these semi-2D polymers can still deliver decent hole mobility (mu(h)) of >0.1 cm(2) V-1 s(-1) after thermal annealing. More interestingly, the amorphous PBDT-3T can deliver a maximum mu(h) value of 0.4 cm(2) V-1 s(-1), outperforming the value of reference semicrystalline PBDT-T that owns a typical thienyl side chain. Based on the analyses of film morphology and solid-state crystallinity, the hyper-conjugated side chain with intense aggregation tendency is suggested to facilitate the intermittent interchain hopping between polymer chains, which compensates the short-range structural order of low-crystalline polymer to ensure the laudable charge carrier transport property. This work systematically explores the charge carrier transport behaviors of semi-2D polymers with varied biaxially extended conjugated side chains and enlightens a new design strategy to improve the charge carrier transport of low-crystalline polymers.