如果三极管是完好的,那么对于NPN硅管,当基极电压高于发射极电压0.6V~0.7V、集电极电压高于基极电压时,处于放大区；当基极电压高于发射极电压0.6V~0.7V、集电极电压低于基极电压但高于发射极电压0.3V~0.6V时（带有集电极负载电阻的电路形式）,处于饱和区；当基极电压高于发射极电压0.5V以下或低于发射极电压时,处于截止区.
对于PNP硅管,当基极电压低于发射极电压0.6V~0.7V、集电极电压低于基极电压时,处于放大区；当基极电压低于发射极电压0.6V~0.7V、集电极电压高于基极电压但低于发射极电压0.3V~0.6V时（带有集电极负载电阻的电路形式）,处于饱和区；当基极电压低于发射极电压0.5V以下或高于发射极电压时,处于截止区.