The hydrogenated amorphous silicon (a-Si:H) holds good promise for radiation detection from its inherent merits over crystalline counterpart. For the application to alpha spectroscopy, the induced charge collection in a-Si:H pin detector diodes was simulated based on a relevant non-uniform charge generation model. From the simulation, the diode requires an intrinsic layer of thickness $\sim$ 10 micrometers of larger. Also the input equivalent noise charge (ENC) as well as the reverse current for the sample diodes was measured and analyzed into three sources; shot noise, flicker or 1/f noise, and thermal noise from the contact resistance. By comparing the measured ENC with the calculated signal charge, the signal-to-noise ratio (S/N) for the sample diodes was estimated as a function of operational parameters; the applied reverse bias and the shaping time of Gaussian-pulse shaper. The analysis of signal and noise is useful for the optimum design of a-Si:H pin diodes for various charged particle radiation spectroscopy.