Current of the diode depends upon the voltage across the diode. The diode current can be expressed in the form of diode current equation. The current equation of the diode relates the current with the voltage across the diode. The current flowing through the diode does not vary linearly with increase in the applied voltage. The V-I characteristics of the diode has the exponential relationship. The resistance of the diode varies with increase in the temperature that is why the current does not vary linearly with voltage. The diode is non ohmic and non linear semiconductor device.

The thermal voltage, or Vt of the junction, is referred to as the term kT/q describes the voltage produced within the P-N junction as a result of the action of temperature. This amounts to around 26 millivolts at ambient temperature. A "nonideality" coefficient of 1 are assumed.

To study basic diode circuits, we don't need to be conversant with the "diode equation." Just be aware that, although the voltage dropped across a current-conducting diode does vary with the amount of current passing through it, this variation is often minimal and occurs over a wide range of currents. Because of this, a lot of textbooks just state that the voltage drop across a conducting, semiconductor diode stays constant at 0.7 volts for silicon and 0.3 volts for germanium.