AlGaN channel field effect transistors with graded heterostructure ohmic contacts

Abstract

Abstract:We report on ultra-wide bandgap (UWBG) Al0.75Ga0.25N channel metal-insulatorsemiconductor field-effect transistors (MISFET) with heterostructure engineered low-resistance ohmic contacts. The low intrinsic electron affinity of AlN (0.6 eV) leads to large Schottky barriers at metal-AlGaN interface, resulting in highly resistive ohmic contacts. In this work, we use reverse compositional graded n ++ AlGaN contact layer to achieve upward electron affinity grading, leading to a low specific contact resistance (ρsp) of 1.9x10 -6 Ω.cm 2 to n-Al0.75Ga0.25N channels (bandgap ~ 5.3 eV) with non-alloyed contacts. We also demonstrate UWBG Al0.75Ga0.25N channel MISFET device operation employing compositional graded n ++ ohmic contact layer and 20 nm atomic layer deposited Al2O3 as the gate-dielectric. Several studies have investigated alloyed ohmic contacts to n-AlGaN channels. While alloys using metals such as Titanium, Vanadium and Zirconium 10-15 were reported with low specific contact resistance values below 10 -5 Ω.cm 2 on n-AlGaN channels with Al alloy compositions up to 66%, they are challenging to reproduce reliably due to extremely high temperature processes and typically show non-uniformity in current-voltage characteristics. Alternate heterostructure engineering approaches are therefore needed to realize low contact resistance to large bandgap AlGaN.A low-resistance ohmic contact is formed by reducing the potential barrier between a metal and semiconductor. The ideal n-type ohmic contact would have a zero or low Schottky barrier height at the metal-semiconductor interface, which can be achieved by matching the semiconductor electron affinity 3 and metal work function. However, the intrinsic low electron affinity of AlN (0.6 eV) leads to larger metal-AlGaN Schottky barriers, resulting in a poor tunneling probability for electrons (probability~� . , where φb is the barrier height and W is the tunneling width), and therefore highly resistive ohmics. In this work, we take heterostructure engineering approach in which the Al alloy composition in the AlGaN channel is graded from wider bandgap to narrower bandgap under the ohmic contacts, hence grading up electron affinity and presenting a higher electron affinity at the metalsemiconductor interface (GaN electron affinity, χGaN=4.1 eV). AlGaN layers with compositional grading from GaN to AlGaN have been studied extensively, and shown to induce bulk three-dimensional electron distributions due to positive polarization (spontaneous+piezoelectric) charge [16][17][18][19] . The polarization-induced fixed charge, , = − • , where is the sum of spontaneous and piezoelectric polarization in AlGaN alloy. In case of layers with reverse Al compositional grading from wider to narrower bandgap AlGaN, a negative polarization charge is formed, causing a positive curvature in the energy band profile, and thereby creating a barrier to electron flow. To ensure that the conduction band stays flat, it is necessary to compensate for the negative polarization charge using donors. This...

Al0.85Ga0.15N/Al0.70Ga0.30N High Electron Mobility Transistors with Schottky Gates and Large On/Off Current Ratio over Temperature

“…32 However, no implementation of a selective area ohmic contact using regrowth 7,33 together with graded composition 32 in an AlGaN-channel HEMT has yet been reported.…”

Ultrawide‐Bandgap Semiconductors: Research Opportunities and Challenges

“…This approach has in fact achieved Ohmic contacts with specific contact resistivity in the low 10 −6 Ω cm 2 range to devices with Al 0.75 Ga 0.25 N channels. [338,339] For UWBG materials for which compositional grading is not possible, metals with a low Schottky barrier height to the semiconductor of interest should be explored, and methods to achieve high doping, as outlined above, should be investigated in parallel.…”

Review—Ultra-Wide-Bandgap AlGaN Power Electronic Devices

“…A recent report using group-III compositional grading in order to gradually reduce the bandgap from the channel composition to that of GaN at the contact interface appears to indicate a promising approach toward improving the ohmic contacts to AlGaN with 75% Al. 39 AlGaN-channel HEMTs are a promising power electronics technology, assuming better ohmic contacts can be incorporated in the future. As noted above, they may also be a promising RF technology, but the ohmic contact limitation precludes a full evaluation of the potential for use in RF applications.…”