We study high-field electrical breakdown and heat dissipation from carbon
nanotube (CNT) devices on SiO2 substrates. The thermal "footprint" of a CNT
caused by van der Waals interactions with the substrate is revealed through
molecular dynamics (MD) simulations. Experiments and modeling find the
CNT-substrate thermal coupling scales proportionally to CNT diameter and
inversely with SiO2 surface roughness (~d/{\Delta}). Comparison of diffuse
mismatch modeling (DMM) and data reveals the upper limit of thermal coupling
~0.4 W/K/m per unit length at room temperature, and ~0.7 W/K/m at 600 C for the
largest diameter (3-4 nm) CNTs. We also find semiconducting CNTs can break down
prematurely, and display more breakdown variability due to dynamic shifts in
threshold voltage, which metallic CNTs are immune to; this poses a fundamental
challenge for selective electrical breakdowns in CNT electronics