As one type of vertical thin-film transistors, permeable metal-base transistors (PMBTs) with a permeable metal film embedded between two semiconductor layers have been proposed for high gain current amplifier. In principle, compared with conventional bipolar transistors, PMBTs should have a higher speed and are easier to fabricate compatible with flexible and printed electronics. However, functional PMBTs are not realized due to low current gains (<50) and lack of output current saturation. In this paper, making use of the nano-textured surface of an organic semiconductor, we are able to fabricate devices with permeable metal base films having a pore size of about 20 nm and achieve current gains up to 476 with output current saturation. Correlations between the nano-scale porosity and the charge transmission/amplification behaviors in the device are explained with characterization of the metal base porosity. From our device simulation results, the small pore size is essential to achieve current saturation in the device due to the potential-pinning effect in the small pore regions. Finally, using a similar strategy, we also demonstrate a high gain (=260) solution-processed metal oxide-based PMBTs with output current saturation.