The conditions for three-peak current–voltage(I–V)characteristics with two discrete resonant tunneling diodes (RTDs) connected in series are discussed and analyzed. With suitable design, one can obtain three-peakI–Vcharacteristics with two series-connected RTDs, and some equations are derived to help the design. The effect of parasitic resistance on theI–Vcharacteristics is investigated. A large-signal model of the RTD is developed by using the piecewise-linear approximation technique, and this model can be easily implemented in PSpice. The simulatedI–Vcurve of the series circuit is in good agreement with the measured results. The results in this paper can be applied to the design of multipeakI–Vcurve with less RTD structure in comparison with the traditional device that stacks the same RTDs to obtain a multipeakI–Vcurve. ©1997 American Institute of Physics.