Radiation-Induced DC Parametric Degradation of Enhancement Mode GaN-on-(111)Silicon High-Electron-Mobility Transistors
Authors: Lu Yizhou , Christou Aristos 
Published: February 2020

AlGaN/GaN high-electron-mobility transistors (HEMTs) for applications in radiation environments has required the exact determination of their radiation hardness. Although bulk GaN is the preferred substrate for the epitaxial growth of GaN and AlGaN, (111) silicon is a possible substrate for HEMT devices. The stability of electrical parameters of enhancement-mode (E-mode) GaN HEMTs under gamma irradiation is an essential characterization for reliability. The present work reports the transfer characteristics degradation of E-mode GaN HEMTs under various total dose irradiation from 5krad up to 60Mrads. This represents the first comprehensive total dose study ranging from a few krads up the tens of Mrads. The measured device-to-device variations within groups of devices are presented. The evolutions of critical DC parameters with accumulated total dose is found to be significant at low doses and saturates at high dose levels. The degradation mechanism is very similar to the reported radiation-induced shallow electron traps that trap and release electrons in the near surface GaN and AlGaN layers.
Due to the unique properties including high breakdown voltage, high electron mobility and high thermal stability, wide-bandgap semiconductor power electronic devices, especially Gallium Nitride (GaN) high-electron-mobility transistors (HEMTs), have shown great promise in space, defense and nuclear applications, which often involve radiation-hardened electronics that require high radiation tolerance to fluxes of neutrons or gamma ray of devices. To turn the promising potential of GaN HEMTs into reality, investigations on the influence of irradiation on the device performance and reliability are essential. Many studies have investigated the radiation hardness of group III–nitride compound semiconductor devices under various irradiation types, such as electrons, protons, neutrons, and X-rays and at various total irradiation doses. They generally showed higher radiation tolerance compared with Si because of high displacement energy of GaN, demonstrating the favorability of GaN devices in hard radiation environments.
Many studies have investigated the influence of 60Co gamma radiation on the device electrical characteristics. However, they were all focusing on depletion mode (D-mode) devices. Enhancement mode (E-mode) GaN-on-Si transistors have been commercially available for years. Their major advantages over D-mode devices are elimination of negative voltage supply and the enabling of direct-coupled FET logic (DCFL) for digital circuits. They also improve the efficiency of commercial DC-DC converters in various power levels and topologies. A previous work on gamma radiation of E-mode GaN-on-Si HEMTs only reported cumulative total doses of up to 1 Mrads (Si). The present work provides a complete coverage of low-to-high irradiation doses from 5krads up to 60Mrads (Si), for commercially available E-mode GaN HEMTs and the devices are characterized to gauge variations in the critical DC parameters. To the best of the author’s knowledge, the present work is the first to investigate the gamma radiation tolerance of E-mode GaN HEMTs covering such a wide range of total doses. This facilitates the observation of the complete degradation behavior of DC parameters in hard radiation applications of the E-mode GaN HEMTs.
The devices were supplied in the form of 0.9mm square passivated die with round solder bumps for contacts. They have a rated continuous drain-to-source voltage VDS of 60V, and a rated continuous drain current ID of 1A. These GaN-on-Si devices were fabricated on Si wafers considering process compatibility and cost. An aluminum nitride (AlN) thin layer was grown on the Si wafer to be a seed layer for the AlGaN/GaN heterostructure. Subsequently, the AlGaN/GaN heterostructure was grown on the AlN layer, with a thin AlGaN layer grown above the highly resistive GaN serving as a strained interface between the GaN and AlGaN crystal layers, allowing the creation of a two-dimensional electron gas (2DEG) filled with abundant free electrons with ultra-high mobility. The gate electrode was processed subsequently forming a depletion region under the gate. As an E-mode transistor, the device is turned on by applying a positive bias to the gate.
All the irradiations were conducted at room temperature by 60Co gamma-rays with a flux of 318.5Rads (Si)/s, at the dry-cell panoramic gamma irradiator in the University of Maryland Radiation Facilities. The entire irradiation procedure consists of two parts. The first part is higher-dose irradiation with total doses of up to 60Mrads based on two stages. At the first stage, the devices were irradiated at 10Mrads, while at the second stage, the devices were irradiated for another 50Mrads, with three of them taken out after 25Mrads during the second stage. The second part is lower-dose irradiations with total doses of up to 2Mrads. The temperature throughout the irradiation process was monitored to be below 30°C.