Fundamental Limitations of Wide-Bandgap Semiconductors for Light-Emitting Diodes

Fundamental limitations of wide-bandgap semiconductor devices are caused by systematic trends of the electron and hole effective mass, dopant ionization energy, and carrier drift mobility as the semiconductor's bandgap energy increases. We show that when transitioning from narrow-bandgap to wide-bandgap semiconductors the transport properties of charge carriers in pn junctions become increasingly asymmetric and characterized by poor p-type transport. As a result, the demonstration of viable devices based on bipolar carrier transport, such as pn junction diodes, bipolar transistors, light-emitting diodes (LEDs), and lasers, becomes increasingly difficult or even impossible as the bandgap energy increases. A systematic analysis of the efficiency droop in LEDs is conducted for room temperature and cryogenic temperature and for emission wavelengths ranging from the infrared, through the visible (red and blue), to the deep-ultraviolet part of the spectrum. We find that the efficiency droop generally increases with bandgap energy and at cryogenic temperatures. Both trends are consistent with increasingly asymmetric carrier-transport properties and increasingly weaker hole injection as the bandgap energy of LEDs increases, indicating that fundamental limitations of wide-bandgap semiconductor devices are being encountered.