A computational analysis is performed on percolation transport and filament formation in amorphous Ge₂Sb₂Te₅ (a-GST) using 2D finite element multiphysics simulations with 2 nm out-of-plane depth using an electric field and temperature-dependent electronic transport model with carrier activation energies that vary locally around 0.3 eV and as a function of temperature. The snapback (threshold switching) behavior in the current–voltage (I–V) characteristics at ≈50 MV m⁻¹ electric field with 0.63 μA current for 300 K ambient temperature, where current collapses onto a single molten filament with ≈2 nm diameter, aligned with the electric field, and the device switches from a high-resistance state (10⁸ Ω) to a low-resistance state (10³ Ω) is observed. Further increase in voltage across the device leads to widening of the molten filament. Snapback current and electric field are strong functions of ambient temperature, ranging from ≈0.53 μA at 200 K to ≈16.93 μA at 800 K and ≈85 MV m⁻¹ at 150 K to 45 MV m⁻¹ at 350 K, respectively. Snapback electric field decreases exponentially with increasing device length, converging to ≈38 MV m⁻¹ for devices longer than 200 nm.