Modeling and simulation of multipeak resonant tunneling diode (RTD) devices and RTD-based multivalued memories

A novel equivalent circuit model for SPICE is developed to simulate the dc current-voltage (I-V) and the differential conductance-voltage (G-V) characteristics of multipeak resonant tunneling diodes (RTD's). For the study of the RTD dc and ac behavior in terms of the RTD equivalent circuit model, we mainly focus on (1) the RTD's I-V characteristic and RTD-based multivalued memory circuits, (2) the RTD maximum oscillation frequency f{dollar}sb{lcub}rm max{rcub}{dollar}, for the RTD dc and ac simulations, respectively, and (3) the stability and instability criteria of the RTD's I-V characteristics in the negative differential conductance (NDC) region. For dc analysis, based on the new proposed model, SPICE is used to simulate the I-V characteristics of two-peak, five-peak, and eight-peak RTD devices, and has shown good agreement with the measured experimental I-V data. The load lines and logic operation waveforms from (N + 1)-state RTD-based multivalued memory (MVM) circuits (e.g., three-state for two-peak RTD, six-state for five-peak RTD, and nine-state for eight-peak RTD) and (2N + 1)-state MVM circuits (e.g., five-state for two-peak RTD, eleven-state for five-peak RTD, and seventeen-state for eight-peak RTD) are also successfully simulated by utilizing this new multipeak RTD model. As for the ac analysis, the determination of ac maximum oscillation frequency f{dollar}sb{lcub}rm max{rcub}{dollar} is related to the estimation of the differential conductance G of the RTD. It is difficult to determine G from the experimental current-voltage (I-V) curve since this curve is highly distorted by spurious oscillation due to the circuit parasitics which will easily meet the instability conditions in RTD's NDC region. A new equivalent circuit derived by Brown et al. with the addition of a "quantum well inductance" in series with G of the RTD can yield better agreement with experimental results than theoretical predictions made without the inductance. The magnitude of the inductance is {dollar}tausb{lcub}rm N{rcub}{dollar}/G, where {dollar}rmtausb{lcub}N{rcub}{dollar} is the lifetime of the Nth quasibound state through which all of the conduction current is assumed to flow. We can use the RTD model to estimate the G-V curve and calculate the inductance of the RTD, to finally obtain the maximum oscillation frequency. The f{dollar}sb{lcub}rm max{rcub}{dollar} values estimated with and without inductance effects are discussed. As for the formulae in terms of the equivalent circuit parameters of the RTD and its resistive load for describing the stable and unstable conditions in the NDC region of dc I-V curve measurement, the derived results show that this oscillation phenomenon depends mainly on the inductance and is less influenced by the capacitance.