Diodes are made from semiconductors such as silicon or germanium. Their most basic role is to block current in one direction but allow it in another. A diode has two terminals, and when it is placed inside a circuit in a direction where it allows current flow, it is called forward-biased. When it is placed the opposite way so that the current is blocked, it is called reverse-biased. Real diodes do not follow this behavior precisely, and so this model is called ideal.
Ideal diodes offer perfect conductivity when they are forward biased, so that there is no resistance at all to currents. When they are reverse-biased, they have infinite resistance. This means the diode behaves like a switch that is open when forward-biased and closed when reverse-biased. Real diodes do not have perfect conductivity and infinite resistance.
Diodes do not begin to operate until a certain voltage is reached. The knee voltage is where a diode begins to conduct when it is forward-biased. For silicon, this is ideally 0.7 volts and for germanium, it is ideally 0.3 volts. The diode current beyond the knee voltage increases rapidly upward so that it has a positive exponential rate.
The breakdown voltage for an ordinary diode is the point where it will self-destruct. It may melt, crack or catch on fire. This happens when a voltage threshold is reached when the diode is reverse-biased in a circuit. What this voltage is depends on the diode type, and the manufacturer's specifications.
When forward-biased, a real diode will also self-destruct if there is too much current. For this reason, a resistor is often used to limit current flow in circuits. Ideal diode models may neglect the use of resistors.
Ideal Diode Curve
An ideal diode curve is a graph of current vs. voltage. In shape, it is similar to a "z" lying on its side, but with the middle part a straight line instead of a diagonal, and with curves on the bends of its two legs.
The negative side of the plot shows the reverse-biased behavior of the diode, and is called the reverse region. The downward line of the Z is the breakdown voltage, which is like a negative exponential curve. This shows that the diode has a negative current because it is conducting. The straight line is largely along the zero voltage line of the negative x-axis, to show that the diode is totally blocking current flow.
Once the line reaches the positive side of the plot, it shows forward-biased behavior. This is called the forward region. The line extends until it reaches the knee voltage, and then curves or points upward exponentially, to show that the diode is conducting. When the diode is conducting, the current is no longer zero.
As previously noted, in real life, the diode will become destroyed if the current is too large when it is forward-biased. This is not normally shown on ideal diode graphs.