％ 用于模拟PN结中载流子的分布、电场和电势 ％ 总长为 L 的半导体截面上，左侧为 N 区，掺杂浓度 NA；右侧为 P 区，掺杂浓度为 ND ％ 假设截面积足够大- Used to simulate the PN junction carrier distribution, electric field and electric potential L total length of the semiconductor section, left for N areas, doping