(Invited) Development of 4 Inch and 6 Inch GaN-on-Si for High-Voltage Operation
The off-state breakdown behavior is one of the most important factors to examine how good the materials and epitaxial structures are for high power applications. The off-state drain current is mainly come from the space charge injection of electrons into the GaN buffer , as well as the Schottky gate reverse leakage current. Suppression of the off-state drain leakage current by using high quality insulating buffer layers is very essential for HEMTs based on GaN-on-Si (GOS) materials to improve both the off-state breakdown voltage and the ON/OFF ratio in power switching applications. To acquire a highly-resistive blocking buffer layer, thicker GaN epitaxial layers employing super-lattice (SL) structures and intentional dopants , such as carbon or iron, are usually utilized.
AlGaN/GaN HEMT heterostructures were grown on (111) Si substrates by metal–organic chemical vapor deposition (MOCVD). The heterostructures consisted of a carbon doped GaN/AlN super-lattice and a carbon doped GaN buffer layer with a total thickness from 2.5 µm to 5 µm, followed by a 120-nm-thick undoped GaN channel layer, 18-21 nm of Al0.27Ga0.73N barrier, and 2-nm-thick GaN cap layer. The devices analyzed in this study were fabricated using our standard device fabrication technology for AlGaN/GaN HEMTs on Si. Ti/Al/Ni/Au multilayer was first deposited and annealed at 850°C in a nitrogen environment to form an Ohmic contact. Multiple doses and energies of nitrogen ion implantation were used for the device isolation and to maintain a planar geometry in the fabricated device to further reduce parasitic leakage current. Ni/Au based Schottky gate metallization was defined by using optical lithography followed by a standard lift-off process of the e-beam deposited metals. The gate dimension was 2 µm × 100 µm. The distances of gate-to-source (Lgs) was fixed at 2 µm while the gate-to-drain distance (Lgd) varied from 2 µm to 10 µm. The isolation characteristics were collected between two Ohmic metal pads with 5 µm spacing. A standard 250 µm × 150 µm Ohmic contact on the epitaxial side of the HEMT wafers and broad area Al contact on the back side of the substrates were also fabricated for vertical breakdown measurement.
This presentation will review IQE’s development of the AlGaN/GaN HEMTs grown by MOCVD on low cost and large size (111) Si substrates scaling in diameter from 4 inch to 6 inch. Typical DC characteristics will be shown and compared between different epitaxial structures. For high voltage operation, the two-terminal isolation blocking voltage/leakage, vertical breakdown voltage, as well as three-terminal off-state breakdown voltage and off-state drain leakage current for AlGaN/GaN HEMTs grown on silicon substrates with different buffer thickness and different carbon doping concentration will be discussed. As shown in Figure 1, isolation blocking voltage is demonstrated to increase up to about 1000V at 1µA/mm on 5µm-spacing isolation pads and leakage current to decrease to about 5 nA/mm with increasing carbon doping concentration and buffer thickness. With the improved insulating buffer, high off-state breakdown voltage of 1.2 kV and low leakage current of 5×10-8A/mm are achieved.
Figure 1. Isolation blocking voltage and leakage current measured between two isolation pads with 5µm spacing for a set of AlGaN/GaN HEMT structures grown on Si with different carbon doping concentration in buffer and different buffer thickness.
1. M. Micovic, P. Hashimoto, H. Ming, I. Milosavljevic, J. Duvall, P. J. Willadsen, W. S. Wong, A. M. Conway, A. Kurdoghlian, P. W. Deelman, Jeong-S Moon, A. Schmitz, and M. J. Delaney, 2004. IEDM Technical Digest. IEEE International, 807-810 (2004).