Note that for E >> V₀, T (E >> V₀) —> 1, (as the incoming electron does not feel the barrier), as shown in Fig 5.5.

The treatment above is for symmetric barrier and would correspond to tunneling with no bias across the system. However to observe I-V characteristics, it is necessary to put a bias which bends the band, creating a barrier which may be approximated as a asymmetric barrier as shown schematically in Fig 5.6, and then it is necessary to generalize the above treatment for the asymmetric barrier.