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   4 Electrical properties of HEMT

   4.2 RF performance

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To study dynamic characteristics it is needed to demonstrate a simulation of an appropriated and recessed n-GaN/Al_xGa_1-xN/GaN HEMT which schematic cross section is shown on figure left. Aluminum content equal to 10% in Al_xGa_1-xN donor layer, the gate-source spacing equal to 0.35µm, and the gate length equal to 50nm was set. The cap-layer is n-doped with a concentration equal to 5×10²¹cm^-3, and the donor layer is n-doped with a concentration equal to 2×10²⁰cm^-3. All other layers are nonintentionally doped.

At high frequencies, the HEMTs are characterized by two important parameters: the cut-off frequency f_T for which the modulus of the current gain is equal to 1 (0dB), and the maximum oscillation frequency f_max for which the unilateral power gain is equal to 1 (0dB). We calculate these two frequencies at room temperature; results are given respectively in Figs. 4.4 a) and b). At room temperature, the simulated cut-off frequency f_T of about 331 GHz, and a maximum oscillation frequency f_max of about 1.1 THz which is a very promissing result for high frequency applications.⁽¹⁾

Fig. 4.4 Evolution of the a) current gain as a function of frequency b) power gain as a function of frequency⁽¹⁾

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