Organic thin film transistors (OTFTs) were fabricated using pentacene as the active layer with two different gate dielectrics, namely SiO₂ and poly(methyl methacrylate) (PMMA), in top contact geometry for comparative studies. OTFTs with SiO₂ as dielectric and gold deposited on the rough side of highly doped silicon (n⁺ -Si) as gate electrode exhibited reasonable field effect mobilities. To deal with poor stability and large leakage currents between source/drain and gate electrodes in these devices, isolated OTFTs with reduced source/drain contact area were fabricated by selective deposition of pentacene on SiO₂/PMMA through shadow mask. This led to almost negligible leakage currents and no degradation in electrical performance even after 14 days of storage under ambient conditions. But, the field effect mobilities obtained were lower than 10^-3 cm² V^-1 s^-1, whereas by using PMMA as gate dielectric with chromium deposited on the polished side of n⁺ -Si as gate electrode, improved field effect mobilities (> 0.02 cm² V^-1 s^-1) were obtained. PMMA-based OTFTs also exhibited lower leakage currents and reproducible output characteristics even after 30 days of storage under ambient conditions.