Carrier injection refers to the process of introducing charge carriers (either electrons or holes) into a semiconductor material. Semiconductors are materials with electrical conductivity between that of conductors and insulators, and their electrical properties can be modified by controlling the number of charge carriers within the material.
In the context of semiconductor devices, carrier injection is often associated with the movement of charge carriers across a junction or interface. Here are two common scenarios related to carrier injection:
PN junctions and diodes: In a semiconductor diode, which typically consists of a p-type (positive) region and an n-type (negative) region separated by a PN junction, carrier injection occurs when external voltage is applied across the diode.
When the diode is forward-biased (positive voltage applied to the p-type side and negative to the n-type side), it facilitates the injection of majority carriers (holes in the p-type and electrons in the n-type) across the junction, allowing current to flow.
When the diode is reverse-biased (opposite polarity), carrier injection is suppressed, and only a small leakage current flows.
Transistors: In transistors, carrier injection is a fundamental aspect of their operation. For example, in a bipolar junction transistor (BJT), which has three layers of semiconductor material (emitter, base, and collector), carriers are injected from the emitter into the base.
In an NPN transistor, electrons (minority carriers in the p-type base) are injected from the emitter (n-type) into the base (p-type). The injection of these carriers controls the flow of a much larger current between the collector and emitter, amplifying the signal.
Carrier injection is crucial for the functioning of various semiconductor devices, such as diodes, transistors, and other electronic components. The controlled injection and movement of charge carriers form the basis for the operation of semiconductor devices in electronic circuits.