Characterizing Thermal Properties and Temperature-rise of High Frequency RF Transistors In-Operando at sub-50 nm Length Scales

B.M. Foley, T. Bates, P.E. Hopkins, J.T. Gaskins
Laser Thermal Inc.,
United States

Keywords: nanoscale, thermal transport, scanning probe microscopy, HEMT, transistor


Radio Frequency (RF) transmitters in radar and communication systems for military and commercial applications are operating below their theoretical potential due to thermal limitations. While wide bandgap semiconductor materials that are ubiquitous in these devices intrinsically possess relatively high thermal conductivities, extrinsic effects associated with device integration, material synthesis, and geometric factors (e.g., film thickness, interface density) result in increased thermal resistances. Due to this, operating output power densities in today’s state-of-the-art RF devices are thermally limited as the elevated temperatures produced by Joule heating during operation can be not efficiently dissipated. A key part of developing high electron mobility transistors (HEMTs) that provide maximum RF output power, paired with reliable and sustainable operation, is an accurate assessment of the thermal characteristics of the device itself and the materials they are made of on pertinent device length scales. The use of thermoreflectance techniques for both thermal property and device temperature characterization in-operando of everything from LEDs to HEMTs have primarily been done via time/frequency/steady-state thermoreflectance and transient thermal imaging (TTI), respectively. These techniques produce surface temperature information at a spatial resolution on the order of several hundred nanometers or larger and are often used to validate modeling efforts for thermal performance of a device under various RF/pulsed drive conditions. While this is adequate for micro/macro-scale knowledge of the temperature profile, these methods cannot provide thermal detail of what is happening within and/or across critical device layers/heterostructures on the < 100 nm scale, and certainly not with spatial resolution < 50 nm; for example, within the vicinity of the gate contact and 2DEG layer at the AlGaN/GaN interface in HEMTs. Given the complexity of the thermal problem due to the variety of material and structural design aspects to create an electrically optimal device (alloying, interfaces, geometries, etc.), information related to the local thermal properties and temperature distribution at the channel and/or epi-scale and smaller would help to understand (a) where the areas of largest thermal resistance are located and (b) exactly where the heat is generated and largest temperature spikes are located within the device structure. This information would help guide the electro-thermal co-design of the layers and features comprising the device and enable the successful suppression of localized thermal maxima and extreme temperature gradients within the HEMT structure. Here we present mapping of thermal properties and temperature distributions of HEMT devices in-operando at sub-50 nm lateral resolution. The measurements presented were conducted using a Nano-probe Thermoreflectance Microscope (NTM) which is a scanning probe-based platform developed at Laser Thermal Inc. GaN-based device heterostructures including both TLM structures and fully-integrated HEMT devices are characterized via NTM and spatial maps of thermal properties/resistances and temperature distributions during RF operation at X-band frequencies (8 – 12 GHz) are presented. This dual-capability to measure both thermal properties and performance of devices makes the NTM system an essential tool to support electro-thermal co-design, allowing device engineers to streamline their design loop and accelerate their delivery timeline of next-generation power, RF and opto-electronics.