The variable threshold FET: Theory and experiment

Abstract

the loss of these charges occurs very slowly at lower biases (during interrogation) or at zero bias (during storage).The equations that describe this process are A FIELD-EFFECT T R A~S I S T O H with an electrically alterable VT, constitutes a nonvolatile, easily inte rated memory element,. This alterability can be attributed to cfarge transported into the gate insulator under the influence of a suitable bias. Recently this behavior has been put on a quantitative basis involving conduction mechanisms in a two layer model for the insulating gate*.3.4.5. The purpose of this paper is to show that the time rate of change of VT. both during charging (write-in) and discharging (storage), can be deduced from the conductivity properties of the two insulatingTiis information provides the necessary design criteria for memory la ers of the MI,I,S capacitor that 'forms the FET gate; Figure 1.applications. Qualitatively, the memory b e h a v k of the device may be understood as follows. When a voltage V , is applied across two insulating layers which have different field dependent conductivities, more current initially flows in the layer of higher conductivity than in the layer of low conductivity. This disparity of currents necessarily results in the accumulation of charge at the interface between the two insulators. The accumulated charge sets up a field opposing the field in the more highly conducting layer. Thus the net field causing conduction decrsases as the accumulation of charge continues. The system comes to equilibrium when when the particle currents in both layers are equal.When Va is removed, the internal fields set up by the stored charge kause the charge to leak off by the conduction mechanism appropriate to each insulating layer. The polarity of the stored charge is positive if the voltage V, applied to the metal electrode A4 of the M121,S capacitor is negative and the conductivity of I, is larger than that of 12; or if V , is positive, 2nd the conductivity of I2 is larger than that of I,. Conversely, the polarity of the stored charge is negative, if V, is negative and I, is more conductive than I , ; or if Va is positive and 1, is more conductive than I,.