Analytical expressions are given for the difference between left-to-right a
nd right-to-left tunneling times in asymmetric single- and multiple-barrier
heterostructures. This tunneling time asymmetry is related to the phase di
fference of the reflection coefficients of the electron wavefunction for th
e two tunneling directions. Examples for single- and double-barrier heteros
tructures are given. The treatment in this paper can be used for designing
devices with asymmetric frequency characteristics with respect to the elect
ron tunneling direction.