A diode is a two-terminal electronic component that allows current to pass one direction, while blocking current going the other direction. This favoring of one direction is called bias. For semiconductor diodes (which will be our focus, as we will see shortly), electricity is typically not conducted until a certain threshold voltage in the forward direction is achieved. This is called forward bias.
Here are some important facts about diodes that will be relevant in solar cells:
- They have nonlinear resistance and conductance, or a nonlinear current-voltage (I-V) characteristic (see “Current-Voltage Characteristic”). The behavior of diodes depends on many interrelated factors in the circuit.
- Semiconductor diodes are temperature dependent, meaning the voltage drop (reduction in voltage) across a diode in a circuit changes with temperature.
As we will see in the section “p-n Junction Diodes”, solar cells function largely as semiconductor diodes that allow a usable electric current to be generated from manipulated slabs of material. Thus, diodes are the heart of solar electronics.
Above: This graph illustrates the I-V curve for a diode, where as the current is reversed, the diode experience reverse bias voltage. Note that the graph is nonlinear.
Source: “Diode curve”. allaboutcircuits.com. 4 March 2012. <http://www.allaboutcircuits.com/vol_3/chpt_3/1.html>