Diode: Forward bias

A diode is forward-biased when its p-type region connects to the positive terminal and its n-type region connects to the negative terminal of a supply voltage. This configuration reduces the potential barrier within the diode, allowing current to flow easily from the p to the n-type region. Forward bias enables the diode to conduct current with minimal resistance.

Below the cut-in voltage of approximately 0.7 volts for silicon diodes, the applied voltage is insufficient to overcome the potential barrier at the p-n junction. The diode remains in a non-conducting state until the voltage exceeds this threshold. Once the cut-in voltage is surpassed, current rises sharply as charge carriers gain enough energy to cross the junction.

Saturation current doubles for every 10-degree Celsius increase in temperature and depends on the diode's cross-sectional area. This temperature dependence causes the diode's voltage drop to decrease by approximately 2 millivolts for each 1-degree Celsius increase at constant current. This property is leveraged in temperature-sensing circuits like electronic thermometers.

For every decade change in forward current, the diode voltage changes by approximately 60 millivolts. This relationship is derived from the diode equation using base-10 logarithms and thermal voltage, which involves Boltzmann's constant and electronic charge. This predictable voltage-current relationship is fundamental to diode forward characteristics and circuit design.

Thermal voltage is expressed as kT/q, where k is Boltzmann's constant, T is junction temperature, and q is electronic charge. At room temperature, thermal voltage is approximately 26 millivolts. This parameter measures the energy required to move charge carriers across the diode and is essential for understanding the exponential relationship between current and voltage in forward bias.

The 0.7-volt drop represents the cut-in voltage threshold for silicon diodes, which is a material property independent of current rating. However, different diodes reach this voltage at different currents depending on their saturation current and physical dimensions. Once forward-conducting, all silicon diodes exhibit approximately the same voltage drop across the junction regardless of their current capacity.

For significant forward current, the diode equation simplifies to an exponential form and is expressed logarithmically in terms of thermal voltage. This simplified form eliminates less significant terms and reveals the exponential relationship between current and applied voltage. The logarithmic expression enables practical calculations of voltage changes corresponding to current variations in forward-biased diodes.