An elaborate account on the Analog Performance of the devices has been described in this section. The Analog FoMs such as Drain Current (ID) with respect to Drain Voltage (VDS) and Gate Voltage (VGS), Conduction Band Energy Diagrams, Transconductance (gm), Intrinsic Gain (gmR0), Output Resistance (R0) and Early Voltage (Ve) have been studied by changing the dielectric constant of the gate oxide material.
Fig.2 depicts the Conduction Band Energy Diagram across the channel in both OFF state and ON state. From Fig.2(a), it is found that the barrier height is highest for the device in which the oxide with greater relative permittivity is present towards the source terminal and it is lowest for the device having higher dielectric material towards the drain terminal. This is because application of negative voltage at the gate terminal repels the electrons from the channel and potential drop along the channel gradually decreases from source terminal towards the drain terminal. Presence of high-K oxide material in the source side leads to greater repulsion of electrons as compared to device having low-K oxide at the source side. It is observed from Fig.2(b) that in the ON state, the devices having only HfO2 and only SiO2 as gate oxide material have undergone minimum and maximum Drain Induced Barrier Lowering respectively. For the devices having both oxide materials towards source and drain terminals, moderate lowering has occurred because the oxide capacitances offered by these materials together lie in between the capacitances offered by SiO2 and HfO2 individually and its effect is clearly evident from the plot of Drain Current against Drain Voltage at a constant Gate Voltage as shown in Fig.3 where the Drain Current is highest for SiO2, lowest for HfO2 and almost overlaps in case of the remaining two devices. The slope of the curves obtained from Fig.3 facilitates the determination of two significant device parameters namely Early Voltage and Output Resistance which are illustrated in Fig.4(a) and Fig.4(b) respectively. The Early Voltage increases considerably for SiO2 device as compared to HfO2 device because of the fact that the Drain Current in the former rises in a steeper manner whereas that in the latter is almost parallel to the horizontal axis as evident from Fig.3.
It can be deduced from Fig. 4(b) that Output Resistance, R0, and Drain Current are inversely proportional as a result of which the SiO2 device has the smallest value of R0 and HfO2 has the largest output resistance. Since the devices having SiO2 towards source terminal and HfO2 towards drain terminal and vice versa show nearly identical Drain Characteristics, their Early Voltages and Output Resistances are also approximately equal.
In Fig. 5(a), the variation of Drain Current with respect to Gate Voltage at a constant Drain Voltage has been presented for different gate oxide materials. The threshold voltage is least for SiO2 device whereas the rest of the devices have almost equal threshold voltage which implies that those turn on at the same value of Gate Voltage but the value of Drain Current escalates swiftly in case of HfO2-SiO2 device as compared to its fellow devices. Figure 5(b) further investigates the Transconductances (gm) of these devices i.e. the rate of change of Drain Current with respect to change in Gate Voltage and the HfO2-SiO2 device surpasses all other devices in terms of its responsively and sensitivity as it has the highest transconductance peak.
The product of Transconductance (gm) and Output Resistance (R0) or the Intrinsic Gain (gmR0) of this device diminishes considerably as reflected in Fig. 6 because it is entirely governed by the value of R0 thereby showing insignificant alteration with respect to gm. The device with HfO2 as the only oxide material has the highest Intrinsic Gain because of its exceptionally high Output Resistance of 10kΩ which when multiplied with its transconductance yields a large value. Hence it can be inferred that the HfO2 device can behave as an excellent amplifier with an average ability to respond to small changes at the input whereas the HfO2-SiO2 device, showing immense gate-sensitivity, possesses moderate amplifying capacity.
Further, detailed investigations of the analog performances of the device having HfO2 on source side and SiO2 on the drain side is carried out by varying the length of both sections while keeping the total channel length constant and presented herein.
The conduction band diagram of the three devices is illustrated in Fig. 7. In the ON state we observe that lowest barrier height is for lowest length of HfO2 i.e.50nm and SiO2 length 150nm, implying that highest current flows for this device. However, when oxide1 length is more, the device has least DIBL owing to maximum non-equilibrium potential barrier controlled by gate-bias in OFF state and it achieves improved gate control in ON state. Figure 8 illustrates the output characteristics (ID with respect to VDS) of the devices. As both the oxides are in parallel, the equivalent capacitance is result of addition of effect from both oxides. When the proportion of HfO2 length increases and SiO2 length decreases, it subsequently increases effective capacitance as HfO2 has higher dielectric and contributes more.
The drain current reduces in accordance to this and is least for HfO2 with 150nm length. The variation of Early Voltage (Ve) and Output Resistance (R0) is shown in Fig. 9. The curve having lowest slope in the output characteristics i.e. Figure 8 must have the highest negative value of Early voltage and largest value of Output Resistance.
The obtained results are in accordance to this variation, the highest value being for HfO2 length 150nm and SiO2 length 50nm. The Transfer characteristics (ID versus VGS) of the devices is depicted in Fig. 10(a). It is evident that the increase in resultant capacitance causes positive shift of the threshold voltage, the lowest being V=-3.5V for HfO2 length 50nm and SiO2 length 150nm, and the highest being V=-1.5V for HfO2 length 150nm and SiO2 length 50nm. The Transconductance (gm) graphs shown in Fig. 10(b) indicates that the device with HfO2 length 150nm and SiO2 length 50nm is most sensitive. The peak value of gm is 128mS/mm.
Intrinsic gain (gmR0) shown in Fig. 11 is significantly high for the device with HfO2 length 150nm, having a value of 2.9, due to the large value of output resistance as well as transconductance for that device.