11.2 : Diode: Forward bias

In semiconductor devices, diodes play a crucial role in directing current flow, and its operation is primarily categorized into forward bias and reverse bias. A diode is said to be forward-biased when its p-type region is connected to the positive terminal of a battery and its n-type region is linked to the negative terminal. This configuration reduces the potential barrier within the diode, allowing current to flow easily from the p to the n-type region.

The behavior of a diode in forward bias is governed by its I-V characteristics which is influenced by the diode's material, temperature, and physical dimensions. When forward-biased, a diode's current (I_D) can be described by the diode equation:

where I_S is the saturation current, q is the electron charge, V_D is the applied voltage across the diode, n is the emission coefficient, k is Boltzmann's constant, and T is the junction temperature. The thermal voltage V_T (kT/q) measures the energy required to move charge carriers across the diode and its value at room temperature is about 26 mV.

The diode shows a negligible current for voltages below the cut-in voltage, typically 0.7V for silicon diodes. In forward bias, for every decade change in the forward current, the diode voltage changes by approximately 60mV. The saturation current (I_S) varies with temperature and the cross-sectional area of the diode and doubles for every 10°C increase. Due to the temperature dependence of I_S and V_T, a diode's voltage drop decreases by roughly 2mV for each 1°C increase in temperature at a constant current, a property leveraged in temperature-sensing circuits like electronic thermometers. Understanding these properties is crucial for electronics where diodes are central components, such as rectifiers, signal mixers, and voltage regulators.