An equivalent circuit is developed for a single well resonant tunneling diode (RTD). Based on this equivalent circuit, the current-voltage (I-V) characteristics of vertically integrated resonant tunneling diodes (VID) are analyzed, assuming each RTD is quantum mechanically isolated from the others. By using a piecewise linear technique, the I-V curve of the multipeaked VID is divided into several regions, and the model of each region is developed and simplified individually. By incorporating the switch model of SPICE circuit simulator, the individual models are combined to form a complete VID model so that the VID model can be used with the SPICE circuit simulator. The simulated result of a four-bit VID-based A/D converter using this model is shown.