SOLUTION

• A diode is a two-terminal device made of a p-n junction. It works like a unidirectional switch that allows current flow only in one direction.
• It is a conductor in the positive direction and an insulator in the negative direction.
• Once it starts conducting, its resistance is negligible. It is like a closed switch.
• The voltage and current relationship across a diode have three regions: the forward bias region, reverse bias region, and reverse breakdown region.

• When the voltage is larger than the forward bias voltage value (V_F = 0.3 V for germanium diode, VF = 0.6 V to 0.7 V for a silicon diode),
• V_D > V_F, the diode becomes a conductor and acts as a closed switch. Hence, the current increases very fast indicating very low resistance at this device. 
• In this region, the diode can be considered as a closed switch with negligible resistance and a small voltage drop of VF.
• When the diode is in the reverse bias region, V_Z < V_D < V_F, then the diode acts as an insulator or open switch. In other words, it acts like a very large resistor.
• When the VD < VZ, the diode operates in the breakdown region and acts as a closed switch.