PACS 61.72.Ff, 61.72.Vv, 72.20.Fr, 78.55.Cr Free-standing GaN substrates are of great importance for short wavelength Laser Diodes (LDs) and high performance Light Emitting Diodes (LEDs) applications. One of the technical barriers for the applications is relatively high threading dislocation density (TDD) in GaN substrates known as non-radiative centers. In this paper, we analyzed the influence of TDD [5][6][7][8][9][10] for these device applications. But the devices have been fabricated using foreign substrates such as SiC and sapphire because GaN substrates have not been commercially produced. Nowadays, a freestanding GaN substrate with low threading dislocation density (TDD) is in large demand for lifetime improvement, process simplification and efficient current spreading in active region of LDs. Several methods have been tried to fabricate bulk GaN crystals such as a sublimation method [5], physical vapor transport(PVT) [6] and solution growth under ultrahigh nitrogen pressure [7], but since the volume of GaN crystal was not satisfying commercial requirements yet, hydride vapor phase epitaxy(HVPE) growing GaN thick film on sapphire or GaAs to obtain a large GaN substrate have been noticed as a promising method firstly launching the commercialisation [8][9][10]. However, there remains a problem even in current HVPE that uses foreign substrates initially. Such heteroepitaxy leads to relatively high TDD compared with other III-V substrate and bending due to the lattice mismatch and difference of thermal expansion coefficient between GaN and substrate. There have been a limited number of studies that evaluate the effect of such TDs on optical and electrical properties of III-V nitrides. J.S. Speck and S.J. Rosner reported that TD in III-V nitrides role optically non-radiative recombination center and electrically deep acceptor level for n-type material using cathodoluminescence (CL) and atomic force microscopy (AFM) results on GaN buffer layers [11]. In this paper, we will consider the influence of such TDs on Hall effects to verify the function of TDs as the trap or scattering centers disturbing the movement of n-carriers in free-standing GaN substrates.

Presented is a digital-RF transmitter using multi-bit ΔΣ modulators with noise-shaped segmentation and butterfly shufflers for shaping mismatches between digital-to-RF converter (DRFC) cells. The segmented butterfly shufflers prevent mismatch errors of DRFCs from aggravating the linearity and the LO leakage of the transmitter. A prototype based on a digitally-intensive top-down design is fabricated in 90 nm CMOS process. Measurement results for wideband-code division multiple access demonstrate an adjacent channel leakage ratio (ACLR) (5 MHz/10 MHz) of −49.6 dBc/−53.9 dBc and an Error Vector Magnitude of 1.89% with a main channel output power of −0.1 dBm centred at 1.95 GHz.