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A method is presented for contact modelling of heterojunction acoustic charge transport devices (HACT). The method is used to optimise the ohmic contact dimensions, especially contact depths. It uses a solution of the two-dimensional Poisson equation, the continuity equations for electrons and holes, the current densities, the displacement charges resulting from the acoustic wave as well surface state densities. This solution gives the potential and charge distribution in the charge injection region of HACT devices for different depths of ohmic contacts. The channel current densities are computed as a function of gate applied voltages. This technique is developed to calculate the I-V characteristics; then the most suitable choice of the ohmic contact depth taking into account these I-V characteristics is given. This method is applied to calculate the optimal depth of ohmic contacts in the case of a conventional HACT (n-HACT). This result will be very useful for HACT structure design.