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 (ID) can be described by the diode equation:
where IS is the saturation current, q is the electron charge, VD 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 VT (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 (IS) varies with temperature and the cross-sectional area of the diode and doubles for every 10°C increase. Due to the temperature dependence of IS and VT, 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.
