Electronics Handbook/Components/Diodes/PN

Conduction
When connect Voltage source's Polarities the same with Diode's Polarities. Diode does not conduct. No current can flow

When connect Voltage source's Polarities different with Diode's Polarities. Diode conducts. current can flow

Biasing Voltage
Diode does not conduct current with voltage below Conducting Voltage. Conducting voltage is the voltage at current equals to 1mA. For Si the conducting voltage is 0.3 v. For Ge the conducting voltage is 0.6 v. Diode's Conducting Voltage is denoted as Vd
 * V > Vd . Diode conducts current, I ≠ 0
 * V < Vd . Diode does not conduct current, I = 0

With this characteristic Diode can be used like a mechanical switch. Switch opens equivalent to V < Vd. Switch closes equivalent to V > Vd

Current
With voltage greater than Diode's Conductng Voltage. The change in current with change in voltage can be expressed as an exponential of Euler's constant e
 * $$I = I_o ( 1 - e^\frac{V_d}{V} t )$$

When the value of V >> Vd ≈ 100v the I = Io