The physical mechanisms involved in the extraction of H- ions from the negative ion source are studied with a PIC 2D3V code. The effect of a weak magnetic field transverse to the extraction direction is taken into account, along with a variable bias voltage applied on the plasma electrode (PE). In addition to previous modeling works, the electron diffusion across the magnetic field is taken into account as a simple one-dimensional random-walk process. The results show that without PE bias, the value of the diffusion coefficient has a significant influence upon the value of the extracted H- current. However, the value of this coefficient does not affect qualitatively the mechanism leading to the peak of extracted H- ion current observed for an optimum value of the PE bias.