Practical behavior of a Diode
In practice, the diode current often deviates from the predictions of the ideal diode equation. This deviation is primarily due to the presence of resistance in the neutral regions of the diode. While the resistivity of these neutral zones is generally low (typically between 1 and 100 ohms, depending on doping levels), it becomes significant only for larger currents (greater than 1 mA). This effect can be represented by adding a series resistor in the circuit, accounting for the voltage drop across the neutral regions.
Furthermore, under reverse bias conditions, it was assumed that the reverse current remains nearly constant, approaching zero. However, when the reverse bias exceeds a critical level known as the breakdown voltage, a dramatic increase in reverse current occurs. In CMOS diodes, this phenomenon is typically caused by avalanche breakdown. This occurs as the increasing reverse bias leads to a higher electric field across the junction, accelerating carriers crossing the depletion region to high velocities. At a critical field strength (Ecrit), carriers gain sufficient energy to create electron-hole pairs during collisions with immobile silicon atoms. These newly created carriers, in turn, generate more carriers before leaving the depletion region. While avalanche breakdown itself is not destructive and subsides after removing the reverse bias, long-term operation in avalanche conditions is discouraged due to the potential for structural damage from high current levels and associated heat dissipation.
Why is the diode current in practice often less than what the ideal diode equation predicts?
The diode current is often less due to some voltage drop over the neutral regions of the diode, modeled by adding a resistor in series with the diode contacts.
What happens to the reverse current of a diode when the reverse bias exceeds a certain level known as the breakdown voltage?
When the reverse bias exceeds the breakdown voltage, the reverse current shows a dramatic increase due to avalanche breakdown, where carriers gain enough energy to create more carriers in the depletion region.
What is avalanche breakdown in diodes, and under what conditions does it occur?
Avalanche breakdown occurs when the reverse bias creates a high electrical field across the junction, causing carriers to gain energy and create electron-hole pairs. This happens at a critical field strength (Ecrit) and is more likely with higher impurity concentrations.
How does temperature affect the diode current, and what are the two temperature-dependent factors mentioned?
Temperature affects diode current in two ways:
- The thermal voltage (fT) increases linearly with temperature, causing the current to drop.
- The saturation current (IS) is also temperature-dependent, doubling approximately every 5°C theoretically and every 8°C experimentally.
What are the practical implications of temperature dependence on diode current for digital circuits?
Temperature dependence can substantially increase current levels and power consumption in digital circuits. Additionally, it affects the isolation quality of reverse-biased diodes, which are often used in integrated circuits