Improvements to epitaxial III-N field-effect transistor technology

Jori Lemettinen

Research output: ThesisDoctoral ThesisCollection of Articles


This dissertation aims to improve three-nitride (III-N) technology on two fronts, namely, to shift it to a higher level of integration, and to pave the way for future ultra-wide band gap (UWBG) aluminum nitride (AlN) based transistors. Gallium nitride (GaN) epitaxy on silicon-on-insulator (SOI) substrates was studied. These SOI substrates could offer better electrical isolation for power electronics and improved radio-frequency (RF) characteristics by reducing substrate conduction losses. Higher crystalline quality, lower strain and improved electrical characteristics were demonstrated compared to bulk silicon (Si). The buried oxide layer increased the vertical breakdown voltage by approximately 400 V. Synchrotron radiation X-ray topography analysis confirmed that the stress relief mechanism in GaN-on-SOI epitaxy is the formation of a dislocation network in the SOI device Si layer. P-channel GaN high electron mobility transistors (HEMTs) were monolithically fabricated on silicon together with the more common n-channel HEMTs. The fabricated devices show state-of-the-art performance when compared with other GaN/AlGaN p-FETs on sapphire substrates. The development of GaN-based complementary metal oxide semiconductor (CMOS) technology would tremendously increase the possible level of integration by enabling a monolithic gate driver logic. A front-end-of-line fabrication process was developed for an nitrogen-polar (N-polar) AlN-based transistor and an ion-implanted metal-polar AlN metal-semiconductor field-effect transistor (MESFET). UWBG AlN holds tremendous potential for high power applications due to its critical electric field being four times that of GaN and forty times that of Si. We demonstrated high-quality N-polar AlN by using a reduced substrate miscut combined with high-growth temperature, reduced growth rate, and high V/III-ratio. A resistive N-polar AlN buffer was developed. It was discovered that high temperature growth leads to unintentional Si incorporation. Therefore, a layer grown at reduced temperature was utilized to act as electrical insulation. The AlN buffer was used as a foundation for an N-polar AlGaN/AlN polarization-doped field-effect transistor (PolFET) on silicon carbide (SiC). This is the first demonstration of an N-polar AlN-based polarization doped (PolFET, HEMT) field-effect transistor (FET). The combination of the N-polar structure, an improved edge-based contact technology, and the large charge density of the polarization-generated three-dimensional (3D) electron gas allows the fabrication of devices with more than 120 mA/mm current density. This is the highest value reported for an AlGaN/AlN heterostructure thus far. In addition, ion-implanted metal-polar AlN MESFET was demonstrated. The off-state breakdown voltage was 2370 V showing the great potential of AlN high-power applications.
Translated title of the contributionParannuksia epitaksiseen III-N kanavatransistoriteknologiaan
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Sopanen, Markku, Supervising Professor
  • Suihkonen, Sami, Thesis Advisor
Print ISBNs978-952-64-0094-5
Electronic ISBNs978-952-64-0095-2
Publication statusPublished - 2020
MoE publication typeG5 Doctoral dissertation (article)


  • HEMT
  • PolFET
  • N-polar
  • AlN
  • GaN


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