Technology and characterization of GaN-based heterostructure field effect transistors (HFETs)
Heterostructure field effect transistors (HFETs) on the basis of Group III nitrides are increasingly gaining importance for the application as RF power amplifier devices. A common feature of the layer structure of such transistors is a GaN buffer which is usually deposited on non-native substrates, e.g. sapphire, silicon or SiC. A subsequent ternary barrier layer, such as AlGaN or AlInN, allows the realization of a so-called two-dimensional electron gas (2DEG) at the GaN interface. Even without further doping and only due to polarisation induced carriers, an approximately triangular quantum well in the GaN buffer can be generated. This structure is characterized by a high sheet carrier concentration as well as by a high mobility. This fact in combination with the outstanding physical properties of GaN (high band gap and high breakdown field) is the precondition for the devices in the above mentioned field of application. In order to realize such HFETs, a baseline process was developed and established at the ITHE. In one part of this thesis, the development and the physical basis of each process step are described. Furthermore, the interactions between different process parameters and the physical properties of the resulting devices were identified and interpreted. During this process, the mesa isolation was refined, the metal semiconductor contacts were improved, i.e. the ohmic contact resistance as well the gate leackage current were reduced, and the whole process chain was optimized in terms of reproducibility and reliability. Although every single process step is important, the surface passivation plays an especially essential role in the device performance. Without a passivation layer, surface-related dispersion effects cannot be minimized or even eliminated. Therefore, one focus of this thesis was the development of such a passivation process. Based on a comparison between the literature on surface passivation and data obtained in experiments, a layer with good passivating properties could be realized. As a result, a deeper understanding of the physical process of the realization of such a surface passivation could be gained. Furthermore, the influence of a passivation ealier in the process cycle was investigated. Based on this baseline process, AlGaN/GaN as well as AlInN/GaN HFETs were produced, investigated and compared with regard to their DC and RF characteristics. The influence of the aluminium concentration in the AlGaN barrier layer on the electrical properties of SiN passivated transistors was examined. Al0.25Ga0.75N/GaN HFETs on silicon substrates with a gate length of 1µm and a gate width of 100 µm exhibited a power density and a power added efficiency of 2 W/mm and 26%, respectively, at a frequency of 2.14 GHz. The gate leackage currents, which were increased after depositing the passivation layer, could be reduced by using a thin insulating layer inserted between the gate metal and the barrier layer. These so called metal-insulator heterostructure field effet transistors (MISHFETs) were evaluated in terms of their physical properties. The deposition of the passivation layer as early as possible offers further potential for the improvement of the device performance. Nearly lattice-matched AlInN/GaN HFETs were successfully processed. Due to higher polarization charges compared to their AlGaN/GaN counterparts, these devices are characterized by high current densities, even at thin barrier layers. HFETs with a nearly lattice-matched 10 nm thick barrier layer exhibited sheet carrier concentrations of 1.7x1013cm-2, leading to current densities of more than 1 A/mm. An important advantage of a thin barrier layer is the positive influence on the frequency behavior. A current gain cut-off frequency and a maximum power gain cut-off frequency of 14 GHz and 32 GHz, respectively, at a gate length of 1 µm could be achieved. A SiN-based surface passivation did not prove as effective in reducing the dispersion effects as demonstrated for AlGaN-based transistors. As a consequence, there is still need for further development of an appropriate passivation process for transistors based on this materialsystem.
info:eu-repo/classification/ddc/620, Heterostruktur-Bauelement, Nitride, HEMT, Ingenieurwissenschaften, HFET, GaN, AlInN, AlGaN
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