The minimum amount of voltage required for conducting the diode is called knee voltage and it is also known as cut-in voltage. before knee voltage diode behave like a high resistor(means it’s not conduct).
knee voltage is different for different elements for e.g for Si knee voltage is 0.7v, knee voltage is Ge 0.3v. To know more about knee or cut-in voltage, you may read below article.
Read More:What is knee or cut-in voltage of diode