Integrated Electronics on Aluminum Nitride : Materials and Devices

Abstract

The ultra-wide bandgap (UWBG) semiconductor aluminum nitride (AlN) has conventionally been used in optoelectronics, and as piezoelectric layers in radio-frequency (RF) micro electro-mechanical systems (MEMS). AlN, as an electronics platform, is now positioned as a strong candidate to meet the demands of next-generation high-frequency communication. This is thanks to its unique capability of integrating RF active devices, such as transistors and passive RF devices, such as filters, antennas and waveguides, onto a single chip. This dissertation represents a significant step towards realizing the vision of integrated RF electronics on the AlN-platform. Conductive channels are realized on this otherwise electrical insulator, by employing careful polarization physics, heterostructure design and epitaxial growth, which are then used to fabricate record-performance complementary n- and p-channel transistors. First, the discovery of the long-missing undoped III-nitride 2D hole gas (2DHG) in GaN/AlN heterostructures is presented. The suppression of impurities from the substrate using carefully engineered blocking layers is found to be crucial in achieving repeatable, large-area growths of these 2DHGs. These 2DHGs overcome the limitations of acceptor doping of GaN to exhibit record high p-type conductivity. Combined with the best-in-class low-resistance Mg-InGaN ohmic contacts, these 2DHGs are then used to enable the first nitride p-channel transistors that break the GHz-speed barrier. Next, by adding a thin AlN layer on top of the GaN/AlN heterostructure, a parallel 2D electron gas (2DEG) is induced. Electroluminescence from this structure proves the presence of first-of-its-kind polarization-induced 2DEG-2DHG bilayer. The high-density 2DEG in the AlN/GaN/AlN heterostructure is then studied in detail for use as a channel in AlN high electron mobility transistor (HEMT)-based mm-wave power amplifiers (PAs). This 2DEG provides access to GaN conduction band electron states higher than previously possible in magnetotransport measurements, through which electron effective mass of 0.3.m_0 is extracted at densities of 2-3 x 10^13 /cm^2. The state-of-art scaled AlN HEMTs demonstrate output powers of ~2 W/mm at operating frequencies upto 94 GHz. A path to higher output powers is provided through a unique in-situ AlN passivation technique which drastically reduces the RF-DC dispersion from the surface states in AlN HEMTs. By scaling the GaN channel layer down to 3 nm, enhancement-mode operation is achieved for the first time in AlN metal-oxide-semiconductor (MOS)-HEMTs for use in complementary logic and RF circuits. The dissertation then details the integration of these active RF devices on the AlN-platform. Combined with the recent demonstrations of passive RF components like epitaxial-AlN bulk acoustic waveguide (BAW) filters and SiC substrate integrated waveguides (SIW) on the same materials as the active devices, this work should unlock new high-efficiency and low cost applications spaces for RF and complementary integrated UWBG electronics on the AlN-platform.