We report GaN high-electron-mobility transistors (HEMTs) with a thick (7.7 µm) GaN buffer on a Czochralski low resistivity Si (LRS) substrate. The GaN HEMTs exhibit high performance for both radio-frequency (RF) amplification and power conversion. The thick GaN buffer was grown by means of vacancy engineering, delivering a low dislocation density of ∼1.6 × 10⁸ cm⁻², contributing to suppressed RF signal coupling to the lossy Si substrate and a high vertical voltage blocking capability. For RF performance, GaN HEMTs with a 650 nm gate exhibit an f_T/f_MAX value of 25.1/32.3 GHz and a maximum output power P_OUT of 2.2 W/mm at 4 GHz with a drain voltage V_DS of 20 V, which is comparable with the performance of RF GaN HEMTs on a high-resistivity silicon substrate without the existence of the field plate. For power performance, the vertical breakdown voltage of the wafer is 1160 V, and the three-terminal lateral breakdown voltage is 885 V in a GaN HEMT with a gate-to-drain distance of 8 µm. The thick GaN layer on the LRS substrate scheme thus provides a compelling platform for monolithic integration of high-performance RF devices and high-voltage power devices.

The electroluminescence (EL) properties of InGaN/AlGaN ultraviolet light-emitting multiple quantum wells (MQWs) with identical average In content but different In gradients (In content increases linearly, along the growth direction) are investigated numerically. It is found that the luminescence efficiency is improved, and the EL spectral peak wavelength becomes longer for the MQW sample with a larger In gradient. Since the influence of In gradient is different for the conduction and valence bands in InGaN layers, the distribution of electrons and holes in QWs may be changed, leading to a redshift of EL spectra. In particular, when the In gradient increases, the overlap integral of electron-hole wavefunction in InGaN QWs increases, resulting in a higher radiative recombination rate and an enhanced EL intensity.

To dominate the illumination market, applications of high-power, group III-nitride light-emitting diodes (LEDs) with lower cost and higher efficiency at high injection current density must prevail. In this chapter, three possible origins of efficiency droop (including electron leakage, poor hole injection, and delocalization of carriers) in III-nitride LEDs are systematically summarized. To seek a more comprehensive understanding of the efficiency droop, experimental results based on commercialized LEDs are obtained to explain the physical mechanisms. Proposals for droop mitigation, such as (1) improving hole injection, and (2) increasing effective optical volume or reducing carrier density in the active region, are introduced. Finally, a simple expression for the effects of V-shaped pits on the droop is demonstrated.