Supercriticality and Transmission Resonances in the Dirac equation
  It is shown that a Dirac particle of mass $m$ and arbitrarily small momentum will tunnel without reflection through a potential barrier $V=U_c(x)$ of finite range provided that the potential well $V=-U_c(x)$ supports a bound state of energy $E=-m.$ This is called a supercritical potential well. 
N Dombey, P Kennedy and A Calogeracos