Some Heuristic Semiclassical Derivations of the Planck Length, the   Hawking Effect and the Unruh Effect
  The formulae for Planck length, Hawking temperature and Unruh-Davies temperature are derived by using only laws of classical physics together with the Heisenberg principle. Besides, it is shown how the Hawking relation can be deduced from the Unruh relation by means of the principle of equivalence; the deep link between Hawking effect and Unruh effect is in this way clarified. 
Fabio Scardigli