Mechanical Analysis of Stretchable AlGaN/GaN High Electron Mobility Transistors
Authors: Tompkins Randy P. , Mahaboob Isra , Shahedipour-Sandvik Shadi , Lazarus Nathan 
Published: February 2020

Stretchable electronics are of interest for a variety of applications including wearable systems, soft robotics and medical diagnostics [1]. Creating devices and systems able to survive tens or hundreds of percent mechanical strain requires a shift to unconventional substrates such as silicone or polyurethane. Stretchable transistors based on silicon have been previously demonstrated based on thinning and lithographically patterning to relieve strain [1]. In this work, we investigate the possibility of using AlGaN/GaN high electron mobility transistors (HEMTs) in a similar stretchable system. One advantage of GaN based devices over Si devices is the higher critical field stemming from the wider bandgap of GaN compared to Si [2]. The higher critical field results in a lower specific on-resistance (Ron-sp) for a given breakdown voltage [3] shown in Figure 1 where AlGaN/GaN HEMTs are expected to have about a factor of 1000 lower Ron-sp for a given breakdown voltage. Thus the I2R power losses should be 1000 times less in GaN systems compared to Si systems for a given operating current. The lower Ron-spis of particular importance for wearable systems in close proximity to the skin where a temperature rise of just a few degrees can result in discomfort [4].
Semiconductors such as Si or GaN are rigid in nature and are not intrinsically stretchable. To render materials stretchable, device layers must be both sufficiently thin (~ 1 – 5 µm) and attached to stretchable substrates such as silicone. In addition, devices must have unique stretchable geometries such as a sinusoidal or horseshoe shape to relieve strain and prevent breakage. In this work, we have theoretically modeled the effects of device geometry on the mechanical properties of both GaN and Si using COMSOL finite element solver for the Hooke’s Law equation for continuous media using published values of the stiffness matrix and mass density of both Si [5] and GaN [6]. We examined straight, sinusoidal, horseshoe, rectangular and curve corner rectangular (CC-rectangular) device geometries for both materials with Sylgard 184 (Dow Corning) as a stretchable substrate [7]. Figure 2 shows our results with peak stress plotted as a function of peak to peak amplitude for multiple device geometries. Our results show that the peak stress for multiple device designs in GaN material is approximately 2.5 times larger than that of Si which can be attributed to the difference in Young’s Modulus and thus components of the stiffness tensor between the two materials. Because Si based devices have been rendered stretchable, our mechanical simulations suggest that GaN based devices such as the HEMT are also potential candidates for stretchable electronics.
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Figure 1