# 1. The diode¶

A semiconductor diode is an electronic device that allows current to flow easily in one direction and does not allow current to flow in the opposite direction.

The diode conducts current when positive voltage is applied to the anode and negative voltage to the cathode. If voltage is applied to the opposite, it is said to be reverse polarized and will not conduct current.

Semiconductor diode symbol.

The diode allows current to flow from the anode to the cathode. The arrow on the diode symbol indicates the direction of current from the positive pole to the negative pole.

Forward bias threshold voltage

The threshold voltage is the forward bias voltage above which the current begins to increase rapidly.

In rectifier diodes this voltage is about 0.65 volts. In LED type diodes this voltage increases up to 1.8 volts for red LEDs and up to 3 volts for white LEDs.

In special rectifier diodes (schottky diodes) this voltage can drop to 0.2 volts.

## Simulation¶

In the following simulation you can see a diode subjected to a positive voltage and a negative voltage of 1 volt.

The graph below left represents the voltage and current of the diode.

The graph below on the right represents the current flowing through the diode (vertical Y axis) as a function of the voltage received by the diode (horizontal X axis). It is a curve with a knee that begins to rise at 0.65 volts of the diode threshold voltage.

## Exercises¶

1. Draw the symbol of a semiconductor diode with the name of its terminals.

2. What function does a diode perform? When does it conduct current?

3. What is the threshold voltage and what is it for different diodes?

4. Draw a graph with the current flowing through a diode (Y axis) as a function of the diode voltage (X axis). Draw the voltage marks every 0.1 volts and the current marks every 0.1 amperes on the graph with sufficient separation between them.

Draw a line at the threshold voltage curve.

Remember to enlarge the simulator with the option File... Switch to Full Screen to be able to see the graph better.

5. What happens in the diode when the anode voltage is negative?