All of the measured AlGaN/GaN devices share some basic properties like identical substrate, layer sequence, and material quality. However, there are differences in the general geometry and AlGaN layer composition in the particular structures. As an illustration of the device surface geometry a scanning electron microscopy (SEM) image is shown in Fig. 5.1. The top metalization is the gate, the middle one is the source, and the bottom one is the drain. The active transistor area is the shallow channel between the source and drain metalization, with the thin gate electrode running along. In this figure two transistors are shown.
For good control of the sheet carrier concentration in the two-dimensional electron gas (2DEG), the alloy composition and the abruptness of the AlGaN/GaN interface has to be determined. Various methods such as high resolution X-ray diffraction, transmission electron microscopy, and elastic recoil detection have been used [358,366,367]. A good estimate of the effective channel thickness of the conducting region is required for the simulator. A nominal value for the thickness of the 2DEG region has been found in the literature to be in the order of 23 nm, see e.g. , depending on the Al mole fraction in the AlGaN layer. However, the effective thickness of the conducting region may be wider than the 2DEG, albeit with a lower density. For the purpose of calibrating the simulator to produce the same current density as in the measured devices, various effective thicknesses of the defect-free conducting GaN layer were analyzed. A value of 50 nm was used in all simulations throughout this chapter. Self-heating effects are accounted for by using a properly adapted ambient temperature. The barrier height of the Schottky contact to GaN was experimentally determined to be 1.0 eV at room temperature in agreement with experiments by other groups .
Devices from three different HEMT generations are measured and simulated: first a device with field-plate structure (Device A), next a device with shield-plate structure (Device B), and last a state of the art device with T-gate only (Device C) . The layer properties are summarized in Table 5.2 and the geometry is shown in Fig. 5.2.
Device A has gate length , field-plate extension length =0.6 m, and gate width 100 m. The Al composition in the AlGaN supply layer is 30%. The latter is -doped in order to provide additional carriers and to improve access resistance. Contact resistances of 4 mm are assumed.
Device B is a device featuring a source shield-plate. The gate is T-shaped. The Al composition in the barrier layer is 30% with a doping, too.
The last device has a T-shaped gate with and a gate width =250 m (taken as 1100 m in the simulations). The Al composition in the supply layer is 22%, contact resistance is 0.2 mm.
Using the same setup the three generations of AlGaN/GaN based HEMTs are simulated and the results are compared to experimental data. In the following the results are discussed.
AC simulations are performed to compare the calculated and experimental figures of merit e.g. cut-off and maximum frequency. Fig. 5.9 shows the measured and simulated cut-off frequency (again at =7 V). In order to account for the parasitics introduced by the measurement equipment, the intrinsic parameters obtained in the simulation are transformed using a standard two-port pad parasitic equivalent circuit. Both models provide a very good agreement with the experiment.
Fig. 5.10 compares the measured and simulated (using Model B) extrinsic S-parameters at V and V. An excellent agreement is achieved for all parameters in the frequency range 100 MHz26 GHz.
The electron transport in the channel under the gate is studied at the same bias point. As the electric field reaches its maximum under the drain side of the gate , the peak of the electron temperature is also found there (the gate edge is at m in Fig. 5.11). Consequently, in the same region a pronounced velocity overshoot is observed. Interestingly, temperature and velocity profiles obtained using both models do not differ significantly.