A HfO 2 /n-InAlAs MOS-capacitor has the advantage of reducing the serious gate leakage current when it is adopted in InAs/AlSb HEMT instead of the conventional Schottky-gate. In this paper, three kinds of HfO 2 /n-InAlAs MOS-capacitor samples with different HfO 2 thickness values of 6, 8, and 10 nm are fabricated and used to investigate the interfacial and electrical characteristics. As the thickness is increased, the equivalent dielectric constant ε ox of HfO 2 layer is enhanced and the InAlAs-HfO 2 interface trap density D it is reduced, leading to an effective reduction of the leakage current. It is found that the HfO 2 thickness of 10 nm is a suitable value to satisfy the demands of most applications of a HfO 2 /n-InAlAs MOS-capacitor, with a sufficiently low leakage current compromised with the threshold voltage.
Impact ionization affects the radio-frequency (RF) behavior of high-electron-mobility transistors (HEMTs), which have narrow-bandgap semiconductor channels, and this necessitates complex parameter extraction procedures for HEMT modeling. In this paper, an enhanced small-signal equivalent circuit model is developed to investigate the impact ionization, and an improved method is presented in detail for direct extraction of intrinsic parameters using two-step measurements in low-frequency and high-frequency regimes. The practicability of the enhanced model and the proposed direct parameter extraction method are verified by comparing the simulated S-parameters with published experimental data from an InAs/AlSb HEMT operating over a wide frequency range. The results demonstrate that the enhanced model with optimal intrinsic parameter values that were obtained by the direct extraction approach can effectively characterize the effects of impact ionization on the RF performance of HEMTs.
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