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AlGaN Back Barrier Aids HEMT Scaling

Forming double-heterostructure FETs with an AlGaN barrier improves electron confinement, leading to a higher drain bias.

RESEARCHERS at the Institute of Electronics, Microelectronics and Nanotechnology in Villeneuve d’Ascq, France, claim to have built the first ultra-short nitride HEMTs with an AlGaN back barrier. “The uniqueness of this work comes from highly scaled, ultrathin barrier AlN/GaN devices suitable for high frequency GaN-onsilicon operation and high-voltage operation," says lead author Farid Medjdoub.

The AlGaN back-barrier HEMTs, which have a 0.2 μm gate length and a 1 μm separation between the gate and drain, operate at cut-off frequencies in excess of 50 GHz, deliver twice as much current as standard nitride HEMTs and produce a high blocking voltage.

Superiority over standard HEMTs stems from better electron confinement, which allows the transistor to be biased at a higher voltage. Medjdoub and his co-workers fabricated their HEMTs on 4-inch, high-resistivity silicon (111) substrates. MOCVD was used to form a standard HEMT, which acted as a control, and the slightly more complex double heterostructure FET that had an Al0.08Ga0.92N buffer acting as a back barrier.

According to Medjdoub, it is no more difficult to grow a highquality AlGaN buffer than one made of GaN. “In term of growth, the challenge is definitely the achievement of high AlN quality [for the top barrier]. This provides huge polarization and results in extremely high current density," says Medjdoub. He and his coworkers primarily attribute the high-quality of their 6 nm AlN barrier to the in-situ growth of a 3 nm-thick SiN cap that prevents strain relaxation. Ohmic contacts have been formed directly on top of the AlN barrier by etching the SiN layer and depositing a Ti/Al/Ni/Au stack. Nitrogen implantation isolated the devices, and deposition of a 50 nm-thick SiN film by plasma-enhanced CVD, followed by e-beam lithography, defined a 2 μm gate length.

Plasma etching removes SiN under the gate, and electron-beam lithography adds a Ni/Au gate to create transistors with a 50 μm device width and gate-source and gate-drain spacings of 0.3 μm and 1 μm, respectively. These are passivated with a 100 nm-thick film of SiN. Evidence for the improvement in electron confinement with the addition of an AlGaN back-barrier is provided by plots of the drain current verses source-drain voltage (see Figure). The addition of this ternary back barrier enables operation of the device with a drain voltage of 50 V or more.

Introducing the AlGaN barrier also led to an increase in cut-off frequency from 44 GHz to 52 GHz, and a hike in the maximum oscillation frequency from 68 GHz to 91 GHz.The French team has noticed a thermal limitation with the 8 percent aluminium, AlGaN back barrier. “We plan to reduce the aluminium content to less than 5 percent in order to significantly improve the thermal management while maintaining excellent electron confinement," says Medjdoub. “Then, we expect to achieve, for the first time, GaN-on-silicon high power in the millimetre range."

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