Room temperature negative differential resistance in a GaN-based Tunneling Hot Electron Transistor

Transistor Z. C. Yang, D. N. Nath, and S. Rajan Department of Electrical and Computer Engineering, The Ohio State University Columbus, OH, 43210 Phone: (614)-717-8245 | E-mail: yangzh@ece.osu.edu In this work, we use ballistic quantum transport in a III-nitride to realize room temperature negative differential resistance (NDR) in a GaN-based Tunneling Hot Electron Transistor. The results showed reproducible double-sweep characteristics, with peak-to-valley ratio (PVCR) of 7.2 and peak current density (PCD) about 143 A/cm. This is the first report of repeatable room temperature negative differential resistance in a III-nitride device. GaN-based materials have attracted considerable interests because of several advantages such as larger band gap, larger conduction band offset, better thermal stability, and higher breakdown field. In the reported GaN-based RTDs, reproducibility has been poor and the reverse-sweep NDR is usually not observed. The results are highly influenced by dislocation density, interface roughness and particularly polarization effect. In this work, we show that repeatable NDR can be measured at room temperature in a GaN-based tunneling hot electron transistor[1]. In a tunneling hot electron transistor, electrons are injected from the emitter across the base. The collector barrier forms a high pass filter for the injected hot electrons. When the injected electrons have lower energy than the collector barrier, the flow into the base. When the injected energy exceeds the collector barrier, they flow ballistically into the collector, inducing NDR in the base current. We have done detailed Monte Carlo simulations of transport in this device, and find that ballistic transport across thin base layers (< 7 nm) can enable coherent quantum phenomena to be observed. The investigated epitaxial structure consists of a 3.8 nm Al0.6Ga0.4N emitter tunneling barrier, a 22 nm n+ GaN base transit layer (Si doping on the order of 10 cm), and a collector barrier of 5.3/7.9/9 nm AlxGa1-xN (graded from 30% to 15%)/Al0.3Ga0.7N/AlxGa1-xN (graded from 50% to 32%), as confirmed by XRD ω-2θ scan. The structure was grown in Ga-rich conditions by plasma-assisted MBE on free-standing GaN substrates (St. Gobain, n-type doped, TDD ~5x10 cm). Devices have an emitter contact of Ti/Au/Ni (20/50/30 nm), and two base contacts of Al/Ni/Au/Ni (20/20/50/30 nm). The active emitter area is about 12 μm, and mesa area around 100 μm. The investigated structure shows low base-collector leakage, moderate emitter-base injection current, ohmic base-base characteristic. High composition tunneling barrier is applied to narrow the electron energy distribution and reduce percolative transport in low composition barrier, which is important for the observation of NDR. The devices were measured by sweeping emitter bias with both base and collector grounded. At baseemitter bias from 1.5 to 2.8 V, the I-V characteristics shows PVCR of 7.2, and PCD of 143 A/cm at 1.5 V. The repeatability is demonstrated by multiple forward and reverse sweeps, with different voltage steps. The voltage at which NDR emerges is consistent among different devices, and is close to the collector barrier height, which is also expected from the energy distribution. The good repeatability and double-sweep characteristics indicate the potential of such device in high frequency applications We acknowledge funding from ONR N00014-11-1-0721 DATE MURI (Program manager: Dr. Paul Maki).