Building blocks of vertical GaN-based devices

摘要

Gallium nitride (GaN) is a promising candidate to substitute silicon in high-voltage and high-power applications. Due to its large bandgap, GaN exhibits a thousand times higher breakdown voltages than silicon for comparable on-resistance. Furthermore, heterostructure field effect transistors (HFET) based on the two-dimensional electron gas (2DEG) induced at the interface of AlGaN and GaN allow high current densities and switching speeds. Up to now, mostly lateral GaN-based HFET have been investigated. An enhancement in breakdown voltage is mainly achieved by increasing the gate-drain separation. This increase in lateral dimensions raises the chip size and thus costs. Another approach to yield higher breakdown voltages is the transition to vertical devices. Here, the drain contact is located at the backside of the structure. Hence, a separation of the gate and drain electrode through the total device thickness is achieved.In this thesis, vertical HFET relying on the current aperture vertical transistor (CAVET) concept are investigated. This device applies a 2DEG as channel and uses a thick n-doped drift region to block high voltages. A current blocking layer (CBL) with an aperture underneath the gate is introduced into the structure to guide the current from the top-side source to the bottom drain. Typically, the CBL can be realized with a buried insulating or p-doped GaN layer.The thesis comprises a comprehensive simulation of growth- and process-related parameters. Hereby, design rules for doping concentrations of each layer and lateral and vertical dimensions of the device are deduced in order to target an optimum trade-off between on-resistance and breakdown voltage.The fabrication of a CAVET encompasses several building blocks which were investigated separately before they were combined in a fully working device. Particularly, the drift region was optimized in order to obtain a high breakdown voltage and low on-resistance. The CBL was realized with an Mg-doped p-GaN layer. This yielded a sufficient blocking voltage and temperature stability above 1050 °C. To generate the 2DEG on top of the CBL, a regrowth of the AlGaN/GaN heterostructure is necessary. However, the management of the regrowth interface is one of the main challenges of the CAVET. Therefore, regrowth on various templates was performed to gain fundamental understanding of its characteristics. Even MOCVD regrowth on as-grown GaN templates already showed significant impact on the electrical characteristics of test devices. The regrowth on an etched surface, as needed in a complete CAVET process, led to a severe leakage current at the regrowth interface. Furthermore, for the regrowth on the CBL, only a low-temperature process, preferably with MBE, was found advantageous to suppress Mg-diffusion. Another challenge in processing a CAVET is the regrowth on textured and masked templates. Hereby, different routes to planarize etched trenches were investigated.Based on the results of each of the building blocks and the simulations, CAVET were processed. A CAVET with p-GaN CBL and MOCVD regrowth of the AlGaN/GaN heterostructure showed a high on-resistance, attributed to a large aperture resistance caused by Mg out-diffusion. However, also excellent breakdown voltages of 140 V were obtained. In conjunction with the thickness of the drift region of 1 µm, this results in a breakdown field of 3 MV/cm, close to the theoretical maximum.For the second device, low-temperature MBE regrowth was performed on the p-GaN CBL. For this CAVET, high on-currents were achieved and an on-resistance as low as 2 mΩcm2 was realized. However, high leakage currents prevented a large breakdown voltage.In summary, the successful realization of vertical GaN-based HFET has been shown. So far, the presented devices either showed large on-resistance and excellent breakdown voltage or good on-state characteristics and high leakage currents. From the insights gained by the investigation of the single building blocks and the simulations, further improvement of vertical devices on the way to high-power applications is expected.