In electronics, a diode is a two-terminal device. Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional electric current property.

The unidirectionality most diodes exhibit is also called the rectifying property. The most common function of a diode is to allow an electric current in one direction (called the forward biased condition) and to block the current in the opposite direction (the reverse biased condition). Thus, the diode can be thought of as an electronic version of a mechanical one way valve.

Fig 1 shows a simple diode circuit and figure 2 is its equivalent circuit for analysis.

Using KVL, \begin{equation} V_B = IR + V_D \end{equation} where VD is the voltage across the diode.

Diode is OFF

For negative voltages or voltages below the forward voltage of the diode, the diode can be represented as an open circuit. Substituting I = 0 to equation 1

\begin{equation} V_B = V_D \end{equation}

The forward voltage VF depends on the particular type of diode technology.

Diode Type VF
Silicon diode 0.7V
Germanium diode 0.3V
LED (light emitting diode) 1.8V

Diode is ON

For voltages above the forward voltage, the diode is represented as a rechargeable battery with the forward voltage as the battery voltage. Subsituting VF for VD and rearranging equation 1, you obtain the current flowing through the diode.

\begin{equation} I = {{V_B-V_F}\over R} \end{equation}

It is important to keep this current below the maximum allowed for the diode. A rule of thumb is to limit the current to under 20mA.

  • Turn the knob and observe the circuit behaviour
  • The steps to analyse a circuit with a diode are:
    1. Assume first the diode as an open circuit. Calculate the voltage across its terminals
    2. If the calculated voltage is greater than the forward voltage, add the voltage source. Recalculate.

Real diodes do not display such a perfect on-off directionality but have a more complex non-linear electrical characteristic.

Diodes also have many other functions in which they are not designed to operate in this on-off manner. One example is the varicap diode which is used as an electrically adjustable capacitor.