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Vertical design of cubic GaN-based high electron mobility transistors
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Image of FIG. 1.
FIG. 1.

(Color online) The cubic (left) and hexagonal (right) AlGaN/GaN heterostructures investigated in this work. The barrier thickness t bar, and thus the layer thicknesses t 1, t 2, and t 3 and the doping of the n-layer in the cubic structure have been varied. The hexagonal structure is undoped.

Image of FIG. 2.
FIG. 2.

(Color online) Simulated electron sheet densities as a function of the applied surface potential for cubic heterostructures with various doping levels (complete ionization). For each structure two curves are shown: The overall electron sheet density n S (dashed lines) and the integrated electron density in the GaN layer only, n S,GaN (solid lines). Also shown for comparison are results for an undoped hexagonal heterostructure. The upper x axis shows the gate voltage assuming a Ni/Au contact with a Schottky barrier of 1.4 eV.

Image of FIG. 3.
FIG. 3.

(Color online) Simulated conduction band edge E C and electron distribution in a cubic heterostructure doped with N D = 5 × 1019 cm−3 (complete ionization) biased in the saturation regime.

Image of FIG. 4.
FIG. 4.

(Color online) Conduction band edge, electron distribution and density of ionized donors in a cubic heterostructure with N D = 5 × 1019 cm−3 (incomplete ionization). The activation energy of the donors is 20 meV.

Image of FIG. 5.
FIG. 5.

(Color online) Electron sheet densities as a function of the applied surface potential of a cubic heterostructure with N D = 5 × 1019 cm−3. The two cases of complete and incomplete ionization are compared. n S,ch (solid lines) is the sheet density of channel electrons, which belong to the 2DEG at the heterojunction, including those penetrating into the AlGaN barrier.

Image of FIG. 6.
FIG. 6.

(Color online) Schematic illustration of the two potential components in Eq. (3): (a) ϕ0(x) and (b) ϕsc(x).

Image of FIG. 7.
FIG. 7.

(Color online) Critical and threshold surface potentials calculated from Eqs. (15) and (17), respectively, compared with those obtained from numerical Schrödinger-Poisson simulations, as a function of N D. The fitting parameters (δ*, ϕp, ϕch) for the analytical models are (3.06 nm, −75 mV, 0.97 mV) for incomplete ionization and (2.17 nm, 59 mV, 35 mV) for complete ionization. For the latter case we also considered a linear dependence of ϕp on N D (dash-dotted line), ϕp = a·N D + b, with the fitting parameters a = 1.28 × 10−21 Vcm3 and b = 6 mV.

Image of FIG. 8.
FIG. 8.

(Color online) Maximum usable effective gate voltage as a function of the critical surface potential for three combinations of t 2 and t 3. The variation of the critical surface potential is achieved by varying N D. Analytical and Schrödinger-Poisson results are compared. In the analytical models the linear fit of ϕp(N D), see Fig. 7, is used.

Image of FIG. 9.
FIG. 9.

(Color online) n S(E CS) curves for cubic and hexagonal heterostructures with three different barrier thicknesses obtained from Schrödinger-Poisson simulations.

Image of FIG. 10.
FIG. 10.

(Color online) Threshold voltage as a function of the barrier thickness for cubic and hexagonal heterostructures. For the cubic heterostructure, the parameters of the doping layer (t 2, t 3, N D) are the same as in Fig. 9. Analytical and Schrödinger-Poisson results are shown.


Generic image for table
Table I.

Material parameters used for the Schrödinger-Poisson simulations of the heterostructures shown in Fig. 1.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Vertical design of cubic GaN-based high electron mobility transistors