Quantum tunneling is a quantum mechanical phenomenon where a particle penetrates a potential energy barrier, even if the particle's total energy is less than the barrier's peak energy. Classically, a particle lacking sufficient energy to overcome a barrier would be reflected; however, quantum mechanics predicts a non-zero probability of finding the particle on the other side of the barrier. This probability of transmission decreases exponentially as the width or height of the barrier increases. The effect is foundational to understanding various physical processes, including radioactive decay, the operation of scanning tunneling microscopes, and the function of certain semiconductor devices.