In order to analyze the trade-off between high-frequency performance and threshold voltage achieved by the gate recess technique [42], results from two-dimensional hydrodynamic simulations supported by experimental data [44] are presented [393].
AlGaN/GaN DHEMT and EHEMT structures with T-gates of 250 nm length share the same layer specification and are processed on the same SiC wafer. The devices consist of GaN buffer, 22 nm thick AlGaN (AlGaNN for the EHEMT device) barrier layer, 3 nm thick GaN cap layer, and SiN passivation (Fig. 5.46). The cap and part of the barrier layer under the gate of the EHEMT are recessed by Cl plasma etching. A remaining AlGaN barrier thickness t 11 nm is assumed. The Ohmic contacts are assumed to reach the 2DEG in the channel.
The densities of the polarization charges at the channel/barrier interface and at the barrier/cap interface are determined by calibration against the experimental data to be 910 cm and 10 cm, respectively. Low Ohmic contact resistances of 0.2 mm are considered [44]. Self-heating effects are accounted with a substrate thermal contact resistance of R =5 K/W. This value lumps the thermal resistance of the nucleation layer and the substrate.
Fig. 5.47 compares the simulated transfer characteristics to experimental data. Both devices are simulated using the same set of models and model parameters, including the interface charge densities. A good agreement is obtained, both for the threshold voltage and the transconductance (Fig. 5.48).
The mismatch between the drain current at high gate voltages is due to the high gate leakage current in the real device, for which the simulation does not account. A possible explanation for the underestimation of the peak for the recessed device is an overestimation of the sheet resistance under the gate. Simulated output characteristics for both structures are compared to measurements in Fig. 5.49 and Fig. 5.50.
The RF simulations provide slightly higher cut-off frequency than the experiments for both structures (Fig. 5.51). Note, that both the measured and simulation data show an increase of and for the EHEMT structures.
Since the gate capacitance depends on the gate channel distance, several simulations are performed with variable recess depths, which means a variable barrier thickness t under the gate. As expected a shift in the threshold voltage is observed (Fig. 5.52) and increases with decreasing t (Fig. 5.53) due to the lack of charge control for thicker layers.
|
|
However, the simulated characteristics do not show any noticeable change (Fig. 5.54). Fig. 5.55 shows the gate source capacitance which increases with decreasing t . It compensates the increase in , thereby resulting in a constant . Thus, the major reason for the rise of and of EHEMTs in comparison to DHEMTs (Fig. 5.51) is the absence of barrier/cap negative interface charges under the gate. The exact depth of the recess has less influence on and , but has a significant impact on the threshold voltage and the transconductance.
|
|