Strain And Strain Gradient Tuning Of Negative Capacitance Field Effect Transistor

Negative capacitance field-effect transistors（NCTETs）,as one of the very promising device candidates for ultra-low power integrated circuits,differ from conventional metal-oxide-semiconductor-field-effect-transistors by incorporating a ferroelectric layer with negative capacitive behavior in the gate structure,which can break the sub-threshold swing Boltzmann limit through the gate voltage amplification effect.When the lattice constant or coefficient of thermal expansion of the ferroelectric film and the substrate are different,there will be large strain,and the strain will affect the properties of ferroelectric materials through electrostriction.There are also huge strain gradients in ferroelectric nanostructures such as nanotubes,which significantly affect the properties of ferroelectric materials through the flexoelectric effect.The changes in the properties of ferroelectric materials will have an impact on the negative capacitance effect of the ferroelectric layer in NCFET.Therefore,strain and strain gradient effects will inevitably have a significant impact on the electrical properties of NCFET.In order to investigate the effects of strain and strain gradient on the electrical properties of NCFETs,this paper establishes an analytical model of the surface potential,transfer characteristics,output characteristics,and subthreshold swing of NCTETs based on the Landau-Ginzburg-Devonshire theory,which takes into account the strain and flexoelectricity,and then the effects of strain and flexoelectricity on the electrical characteristics of NCTETs are analyzed based on the model.The main research contents and results of this paper are as follows:（1）An analytical model of the electrical properties of biaxially mismatched strain-regulated symmetric double-gate negative capacitance field-effect transistors（NC-DGFETs）was developed,and then the effects of ferroelectric layer thickness and biaxially mismatched strain on the electrical properties of two NC-DGFETs based on Pb Zr_0.5Ti_0.5O₃ and Cu In P₂S₆ materials were investigated comparatively based on this model,respectively.The results show that for the Pb Zr_0.5Ti_0.5O₃-based NC-DGFETs,the subthreshold swing and on-state current are improved when increasing their ferroelectric layer thickness or applying compressive strain,but the opposite effect occurs when applying tensile strain to them.For the Cu In P₂S₆-based NC-DGFETs,the performance improves when increasing the ferroelectric layer thickness or applying tensile strain,but the device is hysteretic under compressive strain.Comparing the two,it is found that the Cu In P₂S₆-based NC-DGFETs exhibits better performance than the Pb Zr_0.5Ti_0.5O₃-based NC-DGFETs at low gate voltages.（2）An analytical model of the electrical characteristics of long-channel gate-all-around negative capacitance field-effect transistors（NC-GAAFETs）considering the flexoelectricity is established,based on which the effects of ferroelectric film thickness,channel radius and flexoelectricity on the electrical performance of long-channel NC-GAAFETs are investigated separately.The results show that increasing the thickness of the ferroelectric layer and the channel radius can improve the performance of long-channel NC-GAAFETs,but the flexoelectricity has different effects on the performance:when the flexoelectric coefficient is negative,the electrical performance is enhanced at the same gate voltage relative to the device without the flexoelectricity effect,and when the flexoelectric coefficient is positive,the performance is weakened at the same gate voltage.The flexoelectricity is equivalent to an overall increase or decrease in gate voltage of the same magnitude and does not change the overall trend.（3）An analytical model of the electrical characteristics of short-channel gate-all-around negative-capacitance field-effect transistors（NC-GAAFETs）considering the flexoelectricity is developed.Based on this model,the electrical characteristics of conventional short-channel GAAFETs,short-channel NC-GAAFETs and the effect of flexoelectricity on the electrical properties of short-channel NC-GAAFETs are investigated separately.The results show that the subthreshold swing increases due to the decrease of gate length in short-channel NC-GAAFETs and GAAFETs,and that increasing the thickness of the ferroelectric layer and decreasing the channel radius can alleviate this problem.The impact of the flexoelectricity on the performance of short-channel NC-GAAFETs is different:the electrical performance at the same gate voltage is enhanced when the flexoelectric coefficient is negative relative to devices without the flexoelectricity,and the performance at the same gate voltage is diminished when the flexoelectric coefficient is positive.The flexoelectricity is equivalent to an overall increase or decrease in gate voltage of the same magnitude and does not change the overall trend.