Advanced processing for scaled depletion and enhancement mode aluminum gallium nitride/gallium nitride HEMTs.

摘要

Demands for high-frequency signal amplification have been addressed over the past decades with high electron mobility transistors (HEMTs) based on III-V semi-conductors such as GaAs, which can operate in mm-wave frequency bands (30-300 GHz). The high dielectric breakdown strength, saturated electron velocity, and carrier mobility in AlGaN/GaN two-dimensional electron gas (2DEG) provides a platform for higher output power density for amplification over a similar frequency range compared to more mature material systems such as GaAs. Nitride semiconductors present unique challenges to the device designer in terms of their strong polarization fields and sensitivity of 2DEG to fluctuations in surface potential and chemistry. Aggressive geometry scaling is required in order to extend the frequency response of GaN-based HEMTs into the mm-wave regime, and short-channel effects (SCEs) thus far have resulted in diminishing returns as gate lengths (lg's) are reduced below ∼300 nm. As for other semiconductor materials, successful field-effect transistor scaling requires that not only lg be reduced for low gate capacitance, but that channel aspect ratio defined as l g/d, where d is the gate-channel distance, remain large. Satisfying this requirement enables vanishingly short gates to retain strong channel modulation efficiency. In order to exploit the fast frequency response of the scaled intrinsic device we must also maintain low parasitic impedances through which the device is accessed.;n+-GaN-capped HEMT structures are used in this work to exploit surface potential screening of the cap layer and its expected advantage in terms of DC-RF current dispersion. To overcome processing challenges associated with aggressive scaling of GaN-based HEMTs we have developed technology enabling us to construct gates shorter than 30 nm and to precisely remove GaN and AlGaN from the gate area of (GaN/)AlGaN/GaN HEMTs to reduce d. The optical transparency of GaN has made consistent sub-100-nm gate definition challenging for electron beam lithography tools that employ optical height detection, and our transparent substrate electron beam focusing strategy eliminates such difficulty. Recessed gate GaN-based HEMTs have historically been plagued with high gate leakage and DC-RF current dispersion due to ion damage incurred during recess etching, but the N2/Cl2/O2 inductively-coupled plasma etch process described herein can provide very high GaN over AlGaN etch selectivity and does not require energetic ion bombardment. We verify this process using diodes on GaN/AlGaN/GaN and show that leakage currents are reduced by Schottky recessing when compared to non-recessed diodes, suggesting a low damage process.;We begin HEMT fabrication work by comparing two AlGaN/GaN epitaxial structures in terms of channel confinement and surface trapping effects by preparing and testing 220 nm baseline devices on said structures. Record sub-threshold characteristics for submicron GaN HEMTs including sub-VT slope of 67 mV/dec and on/off drain current ratio greater than 1010 are achieved, suggesting good potential scalability of HEMTs on this structure which employs acceptor compensation doping in the buffer layer. Next we demonstrate first generation GaN/AlGaN/GaN HEMTs with dielectrically-defined recessed 260 and 125 nm gates that exhibit reasonable DC performance, but low ftlg product of 10 GHz-mum due primarily to large fringing capacitance associated with long gate caps and high (0:87 O-mm) contact resistance. Electric field plating in these devices enables good X-band power performance -- e.g. 4.8 W/mm output power with PAE of 51%. Finally, we present systematic scaling of GaN/AlGaN/GaN HEMTs with process improvements such as recessed ohmic contacts providing uniform contact resistance below 0.5 O-mm. Variations of the gate recess process are used for controlled (∼1 nm/min) etching through the GaN cap and into the AlGaN barrier to fabricate HEMTs with dielectrically-defined gate lengths ranging from 29 to 1680 nm with barrier thicknesses of 29 (non recessed), 19, and 11 nm. We demonstrate enhancement-mode HEMTs with transconductance as high as 340 mS/mm and maximum drain current greater than 0.9 A/mm, as well as depletion-mode HEMTs with maximum drain current above 1 A/mm, small signal ftlg products exceeding 15 GHz-mum for lg down to 200 nm, and reasonable X-band output power of 3.3 W/mm with 46% PAE. With this we claim that by using our low-damage gate recess process we have effectively mitigated short channel effects into this lg regime.