11.2:
Diode: Forward bias
A diode is forward-biased when its p-type region is connected to the positive terminal of the supply voltage and its n-type region is connected to the negative terminal.
The forward current-voltage characteristic displays negligible current for biasing voltages below 0.7 volts, known as the cut-in voltage. However, as the biasing voltage slightly exceeds 0.7 volts, the current in the diode rises sharply.
As a result, fully conducting diodes with different current ratings will exhibit the 0.7-volt drop at different currents.
The diode's current-voltage relationship involves a saturation current that depends on temperature and the cross-sectional area.
The thermal voltage, involving the Boltzmann constant and the magnitude of the electronic charge, is expressed in terms of temperature.
For significant forward current, the diode equation simplifies to an exponential form and is expressed logarithmically in terms of thermal voltage.
Comparing currents for two diode voltages, the equation, using base-10 logarithms, indicates that a ten times change in current corresponds to about a 60-millivolt voltage drop across the diode.
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.