E8 Advanced Semiconductor Devices

Topic

Transistors are the fundamental active elements of modern electronics, and there are two major families with distinct operating principles. The metal-oxide-semiconductor field-effect transistor (MOSFET) controls a relatively large current between two of its terminals via the voltage applied to a third terminal—the gate—with virtually no current flowing into that gate; in its most useful mode as an amplifier, the controlled current increases much more rapidly than the excess voltage above the threshold required to activate the transistor.

The continuous reduction in the physical size of these transistors—now measuring just a few nanometers in the most advanced chips—has historically driven the entire semiconductor industry, although unwanted effects begin to emerge at these scales, including the very current-density limitation predicted by the local Ohm's law model.

The bipolar junction transistor (BJT)—the other major family—controls an output current using a much smaller input current, with an amplification ratio characteristic of each specific transistor; while generally more linear than the MOSFET, it does consume some current at its control terminal, unlike the MOSFET.

Beyond conventional silicon, wide-bandgap semiconductors—such as gallium nitride or silicon carbide—withstand much higher voltages, temperatures, and operating frequencies than traditional silicon. They reflect the relationship between bandgap width and material behavior established by the earlier model and currently serve as the key technology behind modern power electronics, including electric vehicles, fast chargers, and renewable energy systems.

Thus, this module—which begins by distinguishing between the two transistor families based on which terminal controls the other—concludes by demonstrating that the limit on further miniaturization for either family is not a matter of arbitrary engineering choices, but rather a direct consequence of the current-density physics established here as a fundamental principle.

ID:4054