In a diode, the saturation current, often denoted as ISI_SIS, is the small reverse current that flows through the diode when it is reverse-biased. This current is due to the thermal generation of electron-hole pairs within the semiconductor material. When the diode is reverse-biased, the majority carriers are swept out of the depletion region, leaving behind a small leakage current composed of minority carriers. This leakage current, which is very small in magnitude, is what constitutes the saturation current.
Saturation current exists because of the intrinsic properties of the semiconductor material used to manufacture the diode. Even when the diode is reverse-biased, thermal energy causes some electrons and holes to break free from their atoms and become minority carriers. These carriers can cross the junction, resulting in a small, constant current. This current is not dependent on the reverse bias voltage but rather on the temperature and material properties of the diode.
To find the saturation current of a diode, one can refer to the diode’s datasheet, which typically lists the ISI_SIS value. Alternatively, the saturation current can be determined experimentally by measuring the reverse current at a known reverse voltage, well below the breakdown voltage, and extrapolating to the condition where the voltage tends to zero. This process involves fitting the diode’s current-voltage characteristics to the Shockley diode equation and extracting the saturation current parameter.
The saturation current of a diode at room temperature varies depending on the type of diode and its material properties. For silicon diodes, the saturation current is typically in the range of nanoamperes (nA) to picoamperes (pA). For instance, a typical silicon diode might have a saturation current on the order of 10 nA at room temperature. This value can be significantly different for diodes made from other materials, such as germanium or gallium arsenide, which have different intrinsic carrier concentrations and thermal properties.
The current saturation point in a diode refers to the condition where the diode current stops increasing linearly with the applied voltage in the forward-bias region and starts to increase exponentially. This point is typically characterized by the onset of significant forward conduction, where the diode voltage is sufficient to overcome the built-in potential barrier of the p-n junction. In practical terms, this occurs when the forward voltage reaches about 0.7V for silicon diodes and about 0.3V for germanium diodes. Beyond this point, the current increases rapidly with a small increase in voltage, leading to the characteristic exponential I-V curve of the diode.