https://blog.gongquiz.com/2021/09/14/%eb%b0%98%eb%8f%84%ec%b2%b4-%ec%b8%a1%ec%a0%95%ea%b8%b0%eb%82%b4%ec%97%90%ec%84%9c-%ec%a0%84%ec%9e%90-%ec%a0%95%ea%b3%b5electron-hole%ec%8c%8d%ec%9d%84-%eb%a7%8c%eb%93%9c%eb%8a%94%eb%8d%b0-%ed%95%84/