A series of donor acceptor (D A) conjugated polymers (CPs) comprising dithieno[2,3-b;7,6-b]carbazole (C1) or dithieno[3,2b;6,7-b]carbazole (C2) as D unit and thienopyrroledione (TPD), isoindigo (IID), or diketopyrrolopyrrole (DPP) as A unit were synthesized, and their semiconducting properties were investigated with organic field-effect transistors (OFETs). Because of different bonding geometry of Cl and C2, the CPs based on these isomeric D units have distinct backbone conformation. The CPs based on C1 unit and all three A units show strong backbone curvature. In consequence, these polymers all formed amorphous films and exhibited low OFET mobility in the level of 10(-3) cm(2)/(V s). P(TPD-C2) and P(DPP-C2), which comprise C2 and TPD or DPP, both display pseudo-straight-shaped backbones and formed ordered films with the polymer backbones adopting edge on orientation respective to the substrates. Accordingly, P(TPD-C2) and P(DPP-C2) exhibited the highest mobility of 0.31 and 1.36 cm(2)/(V s), respectively. However, P(IID-C2) could only form amorphous films probably owing to its highly stiff backbone, leading to a moderate OFET mobility (2.96 x 10(-2) cm(2)/(v s)). This implies that large fused aromatics are promising building blocks for high mobility D A CPs when polymer backbone conformation and rigidity are appropriately manipulated.