Study of self-heating in GaN/AlGaN Heterostructure FET using visible and ultraviolet micro-raman spectroscopy
Due to its large bandgap (3.4 eV), high breakdown field (3 ´ 106 V/cm), and large electron mobility (1500 cm2/V•s), gallium nitride (GaN) has been used to develop heterostructure field effect transistors (HFETs) for high power and microwave applications. These HFETs are produced using GaN capped with a thin AlGaN layer, thereby producing an offset in the band structure at their interface. The large piezoelectric and spontaneous polarization fields occurring in the AlGaN/GaN heterostructures generate a quasi-two-dimensional electron gas (2DEG) in the GaN without doping. Current flowing between the source and drain is primarily confined to the 2DEG region. Our self-consistent simulation for these devices show 2DEG current sheath is extremely thin (~ 10 nm) and has large electron density (~ 1019 e/cm3). For higher voltages, due to large current densities and localization of electric field in these devices results in significant local Joule heating. This self-heating can result in irreversible damage, such as deterioration of electrical contacts, causing the devices to fail. Thermal management is crucial for the reliability of these devices, motivating the need for accurate measurements and understanding of self-heating.
Temperature measurements based on micro-Raman spectroscopy are generally made using conventional visible excitation. Because GaN is transparent to visible light, one obtains only an average measure of temperature limited by the layer thickness or depth of field of the focusing optics. In contrast, Raman measurements using near band gap ultraviolet (UV) excitation sample a shallow, near-surface region. We find no reports of UV Raman studies of GaN HFETs, and few papers reporting the Raman temperature studies of these devices using Raman scattering in the visible. We used both visible and UV excitations to depth-profile the temperature in the GaN/AlGaN HFET. To understand experimentally measured temperature data from Raman spectroscopy, we calculated Joule heating theoretically by using self-consistent schrondinger-Poisson equation in association with Fermi statistic. The first time presence of hotspot of length ~0.1mm at the edge of the gate on drain side in 2DEG region was realized quantitively. The growth of hotspot with bias condition is the dominant source of heating in these devices. Temperature distribution in the device is calculated using thermal finite element simulation with calculated Joule heat density as an input for different bias conditions. Excellent agreements between theory and the experiment lead to better understanding of temperature distribution in HFET structure. The experimental and theoretical approach used in this study can be applied to study the temperature in different GaN based devices.