A multi-layer a-Si:H based thermal neutron detector was designed, fabricated and simulated by Monte Carlo method. The detector consists of two a-Si:H pin detectors prepared by plasma enhanced chemical vapor deposition (PECVD) and interfaced with coated layers of Gd, as a thermal neutron coverter. Simulation results indicate that a detector consisting of 2 Gd films with thicknesses of 2 and 4 mum, sandwiched properly with two layers of sufficiently thick (approximately 30mum) amorphous silicon diodes, has the optimum parameters. The detectors have an intrinsic efficiency of about 42% at a threshold setting of 7000 electrons, with an expected average signal size of approximately 12000 electrons which is well above the noise. This efficiency will be further increased to nearly 63%, if we use Gd with 50% enrichment in Gd-157. We can fabricate position sensitive detectors with spatial resolution of 300 mum with gamma sensitivity of approximately 1x10(-5). These detectors are highly radiation resistant and are good candidates for use in various application, where high efficiency, high resolution, gamma insensitive position sensitive neutron detectors are needed.