In last two decades, III-N high-electron mobility transistors (HEMTs) have gained a large interest due to outstanding prospect for high frequency and power applications. This was also due to a specific phenomenon of polarization doping, when a large-density two-dimensional electron gas (2DEG) is formed in the quantum well (QW) at a III-N barrier/channel interface by compensating the residing polarization charge. This mechanism is completely different from more conventional modulation doping in e.g. GaAs-based HEMTs, when 2DEG formation is driven by a barrier/channel conduction band discontinuity (DEC) and the barrier impurity doping. For polarization doped III-N MOST HEMTs we suggest that i) ionized surface donors behave like a fixed charge and are distinguishable from trapping states, ii) open channel drain current is independent on the surface donor density, iii) if Nd,surf > PQW/q than 2DEG is populated by electrons transferred from surface donors, and iv) if Nd,surf < PQW/q than at channel opening 2DEG is populated both by surface donors (if not completely eliminated) and injecting source contact.
Dependence of (a) maximum drain current (IDSmax) and (b) threshold voltage (Vth) on the oxide thickness (tox) of Ni/Al2O3/AlGaN/GaN MOS heterostructures with and without post-deposition annealing (PDA) of Al2O3 gate dielectric grown by MOCVD. The slope of the latter is used for the surface donor density determination (Nd,s). (c) and (d) shows C-V characteristics and extracted maximum 2DEG density (n2DEG) for structures corresponding to those shown in (a) and (b).
Gucmann, M. Ťapajna, O. Pohorelec, Š. Haščík, K. Hušeková, and J. Kuzmík: Creation of two-dimesional electron gas and role of surface donors in III-N metal-oxide-semiconductor high-electron mobility transistors. Phys. Status Solidi A 215 (2018) 1800090.
Ťapajna, M., Drobny, J., Gucmann, F., Hušeková, K., Hashizume, T., and Kuzmík, J.: Impact of oxide/barrier fixed charge on threshold voltage instabilities in AlGaN/GaN metal-oxide-semiconductor heterostructures. In: Inter. Workshop on Nitride Semicond. (IWN 2018) Kanazawa 2018.