The authors present a theoretical study of resonant tunnelling in the double-barrier structure under the application of a constant electric field, based upon an exact solution of the Schrodinger equation. By using the transfer matrix technique, the transmission coefficient of the structure is determined as a function of the applied voltage and the incident electron energy. The current-voltage characteristics in the case of bulk carriers tunnelling into a two-dimensional quantum well and, for the first time, in the case of two-dimensional electrons tunnelling into a one-dimensional quantum wire are also calculated. The influence of the symmetry of the structure is studied. It is concluded that a larger peak-to-valley ratio can be expected by mere modulation of the width of the barrier.