The ultimate control over chain self-assembly is key to unravel and optimize the relationship between film microstructure and charge carrier mobility in solution processable conjugated polymer semiconductors. Here we employ preparatory size exclusion chromatography to produce fractions of a poly(thieno)thiophene polymer, coded PBTTT-C-12, with varying number-average molecular weight, M-n, from 5.8 to 151 kDa and low polydispersity index of 1.1-1.4. Solution processing of these samples into bottom-contact, bottom gate, field effect transistors reveals a strong dependence of transistor performance on the molecular weight. Further analysis of the films' microstructure and crystallinity show three distinct regions: fiber formation (ca. 5-20 kDa), terrace formation (20-50 kDa), and a rough morphology (50-150 kDa). The performance of low-M-n films was found to increase rapidly with increasing chain length, and while the best transistor performance was found with the terrace morphology, films not exhibiting the terraced morphology (using 80 kDa polymer) were capable of similar performance. In addition, by blending only 5 wt % of a high molecular weight fraction into a low-M-n film, we demonstrate the ability to drastically increase the measured charge carrier mobility of the low-M-n material without attaining a terraced morphology. This illustration suggests a viable route to easily increase the processability and transistor performance of low molecular weight conjugated polymeric or oligomeric semiconductors. In addition, GIXRD and thermal analysis of select fractions further indicate that the films of higher molecular weight exhibit a reduced side-chain crystallinity due to chain entanglement; the degree of backbone crystallinity remains more constant.