In this section, DC characteristics are presented for NSFET. nanosheet thickness is varied in the range of 5 nm to 9 nm and width is varied from 10 nm to 50 nm to analyse the performance dependency on the geometry of NSFET. Section 3.1 discusses the various DC parameters and section 3.2 deals with different DC parameters for various gate length considerations of NSFET. Temperature and work function dependency on NSFET are shown in section 3.3.
3.1 Impact on DC performance of NSFET with geometric variations
The DC performance of NSFET is analysed by changing the nanosheet physical dimensions like width and thickness. The width is varied from 10 nm to 50 nm and thickness is varied from 5 nm to 9 nm. In this section, the key performance parameters like on current, off current, switching ratio, threshold voltage (Vth), DIBL, and SS are discussed.
The Contour plot of the potential distribution of NSFET is shown in Fig. 3(a). It can be observed that the potential distribution at the drain side is high compared to the source side. Also, it can be clearly seen that the drain potential impact on the channel is reduced because of the spacer. Fig. 3(b) depicts transfer characteristics in both linear and log scales for various widths of nanosheet at VDS = 0.7 V for constant nanosheet thickness of 5 nm.
Channel width controls the amount of flow of current through the NSFET. Each sheet width is varied in the range of 10-50 nm and thickness varied in the range of 5-9 nm to study its impact on DC metrics of NSFET a fixed gate length LG = 16 nm.
The on current is directly proportional to the effective width of the channel, which leads to an increment in the on current as the width of the sheet increases. From Fig. 4 (a), it can be seen that the maximum increment of 36% in on current is observed when the thickness is increased from 5 nm to 9 nm at nanosheet width of 10 nm. It is observed that for larger widths of nanosheet, higher ION is obtained due to the increment of effective width of nanosheet. On the other hand, in addition to the increment in on current, the increment in the off current also observed. As the width of the channel increases, the gate loses its control on the channel and leads to more leakages in the device. These leakages will increase the off current of the device. Moreover, the increment in off current is observed for higher thickness values of nanosheet because of reduction in potential barrier height, conduction band energy in the channel for off state condition [31].
For thinner nanosheets off state current can be reduced, however, the slight reduction in the on current is observed because of the mobility degradation due to enhancement in the perpendicular electric field [31]. These results are shown in Fig .4(b). The maximum increment of 20x in off current is observed for thickness ranging from 5 nm to 9 nm at nanosheet width of 50 nm.
Fig .5(a) shows ION/ IOFF and SS values for different widths of nanosheet. As the nanosheet width increases from 10 nm to 50 nm, the switching ratio followed decreasing manner because of the raise in off current. Moreover, as the width ranging from 10 nm to 50 nm, there is a decrement of 71% and 94% in switching ratio is observed for thickness values of 5 nm and 9 nm respectively. It can be concluded that the switching performance of the device gets deteriorated with the increment in geometrical values.
Sub-threshold swing (SS) is an important metric for device application in logic circuits. The SS is defined as the change in the gate voltage needed to get a decade change in drain current. The Sub-threshold swing (SS) can be calculated by using the formula (1)
$$SS={\left(\frac{\partial {log}_{10}{I}_{D} }{\partial {V}_{GS}}\right)}^{-1} \left(1\right)$$
From Fig .5(a), it can be seen that SS followed an increasing manner since the gate loses its control over the channel for higher widths. Furthermore, as the width ranging from 10 nm to 50 nm, there is an increment of 1.6% and 8.5% in SS is observed for thickness values of 5 nm and 9 nm respectively. Since a lower value of SS is preferred for good sub threshold performance, nanosheet having width and height of 10 nm and 5 nm respectively outperforms with SS of 62.5 mV/dec, which is near to ideal SS.
Fig .5(b) depicts DIBL and Vth for different widths of nanosheet. The threshold voltage is extracted by using the constant current method at 100× Weff /LG. Where Weff is given by n × (2× (T + W)), where T and W are the thickness and the width of the nanosheet respectively. The term n indicates the number of vertically stacked nanosheets [31–33]. As the thickness and width of the nanosheet are increasing, from Fig .5(b), it can be seen that the threshold voltage is reducing because of SCEs. Furthermore, there is a threshold voltage roll-off of 5.3% and 8% is observed for thickness values of 5 nm and 9 nm respectively when the nanosheet width is varied from 10 nm to 50 nm. It can be seen that the Vth roll-off is more for higher thickness values of nanosheet.
Drain induced barrier lowering (DIBL) is also one of the crucial sub-threshold performance metrics and it should be as low as possible for good performance of the device. The DIBL is computed by using the formula (2).
$$DIBL= \frac{{V}_{tlin}- {V}_{tsat}}{{V}_{Dsat}-{V}_{Dlin}} \left(2\right)$$
Here, Vtlin is the threshold voltage extracted at linear supply voltage, VDlin = 0.04 V and Vtsat is threshold voltage extracted at saturation supply voltage, VDsat = 0.7 V. It can be observed that, as the nanosheet dimensions are increasing, the DIBL gets increasing. Moreover, as the nanosheet width increased from 10 nm to 50 nm, there is an increment of 24% and 30% in DIBL observed for nanosheet thickness values of 5 nm and 9 nm respectively. The DIBL can be lowered by incorporating the nanosheets which are having smaller widths and thickness values. It can be concluded that DIBL is sensitive towards width and thickness variations.
3.2 Impact on DC performance of NSFET with scaling.
In this section, the impact of scaling on the DC performance of NSFET is analysed for sub -7 nm technology nodes. The gate length is downscaled from 20 nm to 5 nm and the results are depicted in Fig. 6(a). The transfer characteristics are obtained by fixing the thickness to 5 nm and width to 10 nm for the nanosheet.
From Fig. 6(b), it is observed that as the channel length decreases, the on current is increased because the distance between source and drain is decreased. However, the OFF current also increased as the channel length decreases due to the SCEs.
Figure 7(a) depicts the switching ratio and SS as a function of the gate length. It can be seen that as the channel length is decreasing, the switching ratio has deteriorated. However, even at LG of 5 nm, the switching ratio of 106 is maintained which ensures that NSFET is a suitable candidate for good logic applications and continued scaling [34]. Moreover, as the gate length decreasing, the SS is increased and there is an increment of 31% is observed as the gate length is scaled from 20 nm to 5 nm. The increment in SS is due to an increment in leakage currents. DIBL and Vth are shown in Fig .7(b). DIBL is another parameter that affects severely with scaling. As the gate length decreases, the impact of drain potential is more on the channel and leads to the increment of DIBL. From Figure 7(b), it can be seen that the DIBL is increased and a growth rate of 339% is observed as the gate length ranging from 20 nm to 5 nm. The threshold voltage variations as a function of gate length are depicted in Figure 7(b). The deterioration in threshold voltage is observed as the gate length is increasing due to the threshold voltage roll-off. As the gate length decreased from 20 nm to 10 nm, a reduction of 1.9% in threshold voltage is observed, whereas from 10 nm to 5 nm, a reduction of 7% is observed. It is obvious that for sub -10 nm, the threshold voltage roll-off is more due to increased SCEs.
3.3 Impact of temperature and work function variations on NSFET
Nanoscale transistors are used in a wide variety of fields like communication, automobiles, medical equipment, analog and digital integrated circuits, sensing applications and power electronics. As per the requirement, the nano transistors are used at different temperatures and are crucial to analyze the performance of transistors at various ranges of temperatures [35].
Temperature dependency on drain current as a function of gate voltage is plotted in Fig. 8(a). The temperature is varied in the range of -43oC to 127oC. It is very much required to bias these circuits so that the performance or V-I characteristics are insensitive or independent towards temperature variations. IC designers are very much interested to know this inflection point of temperature. This biasing point is called the temperature compensation point (TCP) [36]. From Fig. 8(a), the TCP is observed at VGS = 0.55 V and ID = 3.48×10−6 A. From Fig. 8(b), it is clear that as the temperature increases, increment in off currents is observed because of the dominance of impurity scattering. Moreover, a slight decrement in on current is observed with raise in temperature due to the mobility reduction.
ION/IOFF and SS variations as a function of temperature are presented in Fig. 9(a). It is observed that there is deterioration in the switching ratio as the temperature increases because of the significant improvement in off current. Furthermore, the value of SS is less at low temperatures and ensures faster operation of the device [37]. Fig. 9(b) depicts DIBL and Vth variations with respect to temperature and is observed that as temperature increases, the rise in DIBL is observed because of the decrement in threshold voltage. Furthermore, threshold voltage deteriorated with the rise in temperature due to scattering phenomena [38]. A Threshold voltage roll-off of 9.7% is observed as temperature increased from -43oC to 127oC.
Work function is one of the important process parameter which plays a detrimental role in turning on and off of a device. Higher gate work function ensures that the device is fully depleted rapidly and that the device performs better in the off state [39].
Work function impact on drain current as a function of gate voltage is shown in Fig. 10(a). The work function of the device is varied from 4.3 eV to 4.7 eV. From Fig. 10(b) it can be inferred that for higher work function of the device, along with the deterioration in on current, significant reduction in off current is observed. An Increment in work function lowers the tunnelling between gate and channel and also between gate and drain/source [40], which helps to reduce the leakage current.
Switching ratio variations with respect to work function are shown in Fig. 11 (a). The switching ratio improved significantly with the rise in work function due to a huge reduction in off current. With the increment of work function, the SS gets decremented and ensures good sub threshold performance. Fig. 11(b) depicts the DIBL and threshold voltage variations as a function of the work function. DIBL is decreasing with the rise in work function and lesser DIBL indicates that electrostatic integrity of the gate is more on the device and less sensitive to drain voltage variations. A minimum DIBL of 30.30mV/V is obtained for work function of 4.7 eV and there is a decrement of 28.6% in DIBL is observed when the work function varied from 4.3 eV to 4.7 eV. However, there is a huge decrement in threshold voltage with the fall in work function of the device. There is a decrement of 3.9x in threshold voltage as the work function is decreased from 4.7 eV to 4.3 eV, and leads to more SCEs. Moreover, the study tells that the devices with more work function will have high threshold voltages and makes the device sluggish.