A thorough performance analysis of large-area mu-PIC detectors has been made. Through systematic simulations and comparison with test results of the two detector versions, mu-PICI and mu-PIC3, we have identified that low primary electron collection was responsible for the degradation of measured gas gain of the mu-PIC3 device. The test of a new detector, mu-PIC5, fabricated with a narrower inter-cathode gap and anodes raised above the insulator surface, shows a threefold increase in gas gain reaching 104 in argon 80%-ethane 20% gas mixture, which agrees with simulations. The new detector demonstrates a stable long-term operation at high gains, above 6000 measured during more than 70 h with an X-ray source, with a moderate 6% gain increase due to dielectric polarisation. (c) 2004 Elsevier B.V. All rights reserved.