The comparison of the energy band diagram between the proposed device and the Si-DMDGTFET at thermal equilibrium (VGS = 0 V and VDS = 0 V) is depicted in Fig 3(a) and at ON state (VGS = 1 V and VDS = 1 V) is depicted in Fig 3(b). At thermal equilibrium, when no voltage is applied at the gate (VG = 0 V) , the aperture in between the valence energy band (VB) and the conduction energy band (CB) of the source and channel respectively, is too much, so the charge carriers are not able to tunnel between the source and the channel, thus the device remains OFF. When VG increases the valence band edge of the source moves downward towards the channel’s conduction band edge, thus, there is narrowing of the energy band between the source/channel junction and the tunneling of carrier occurs between the source/channel junction and thus current flows and the device turns ON. The electric field is highest at this source/channel junction. The tunneling width at the source/channel junction of the proposed device is less thus resulting in the improved ON-current.
Fig 4 depicts the carrier generation of the Si-DMDGTFET device and the proposed Si-VDMDGTFET device. In case of Si-VDMDGTFET device the concentration of the carrier occurs at the Si-epitaxial layer of source/gate interface. In the proposed device the tunneling pathway is orthogonal and is all beneath the gate. Due to this, the degradation of SS occurs because the diagonal tunneling path is minimized. Contrary, the tunneling in Si-DMDGTFET takes place only at the periphery of the gate which is at the interface of source and channel.
Fig 5 depicts the comparison of the transfer characteristics of the Si-DMDGTFET and Si-VDMDGTFET at the ON state.
Table 2 compiles the results of DC characteristic of Si-DMDGTFET and Si-VDMDGTFET devices.
Table 2 DC characteristics
Parameters
|
Si-DMDGTFET
|
Si-VDMDGTFET
|
ION (A/µm)
|
2.26 x 10-8
|
2.51 x 10-6
|
IOFF (A/µm)
|
1.22 x 10-16
|
1.81 x 10-17
|
IAmb (A/µm)
|
3.51 x 10-15
|
7.98 x 10-16
|
ION/ IOFF
|
1.89 x 108
|
1.39 x 1011
|
gm (S)
|
8.19 x 10-8
|
4.57 x 10-8
|
DIBL
|
0.009
|
0.002
|
SS (mV/dec)
|
62
|
48
|
The amplification capability is described by the transconductance (gm) of a device. The device whose transconductance is higher is preferred because the conversion of input voltage into output current is better. Transconductance is computed as
Fig 7 depicts that Si-VDMDGTFET has better responsiveness to convert the input voltage into drain current in comparison to the Si-DMDGTFET.
The effectiveness of Si-VDMDGTFET in the applications of RF is analyzed. When the device works at high frequencies, while determining the AC behavior the parasitic capacitances allied with the device have a vital job. The pathway between the input and the output is created by these parasitic capacitances whose outcome is signal distortion and circuit oscillation. The gate to drain capacitance (CGD) governs the total gate capacitance (CTotal) relatively than the gate to source capacitance (GGS), hence it is vital to minimize the CGD for improvement in RF performance.
Fig 8 depicts the CGD vs VGS at VDS = 0.5 V. The injection of carriers to gate from drain originates CGD. The CGD increases with increase in VGS because an inversion layer is formed from the drain region to source which reduces the energy barrier width at drain. Fig 9 depicts the CGS vs VGS at VDS = 0.5 V. The CGS reduces with the increase in VGS because at the source side the potential barrier increases leading to the reduction of the coupling at the source and gate electrode. Si-VDMDGTFET has higher CGS than the Si-DMDGTFET. The sum of CGD and CGS is the total capacitance (CTotal) that is depicted in Fig 10. Si-VDMDGTFET has improved electrical performances as its overall capacitance is less.
The different Figure of Merits (FOMs) such as transconductance (gm), cut of frequency (fT), Transconductance Frequency Product (TPF), transit time (τ) and Gain Bandwidth Product (GBP) have been studied that is vital in understanding the device performance regarding the RF applications.
The frequency at which the device’s short circuit current gain becomes unity is known as the cut-off frequency. It is computed as
fT results from the collective effect of CGD and reduction in gm owing to mobility degradation. The fT of Si-VDMDGTFET is 0.36 GHz which is considered appropriate for RF applications of high frequency. The Gain Bandwidth Product (GBP) of the device is formulated as
GBP is the trade-off between gain and bandwidth of the device. The GBD of the proposed Si-VDMDGTFET is better than Si-DMDGTFET because of the similar cause as that of fT.
Transit time (τ) is the time needed by the device to transport the charge carriers to drain from source. τ decreases as VGS increases because an inversion layer is considerably incremented and the charge carriers passes through the reduced layer of the inversion layer. τ is computed as
It is observed that Si-VDMDGTFET device has less transit time which implies that its switching speed is high.
Transconductance frequency product (TPF) is also a significant parameter for inspecting the functioning of the device at high frequency. It is computed as
Table 3 summarizes the result of AC characteristics.
Table 3 AC characteristics
Parameter
|
Si-DMDGTFET
|
Si-VDMDGTFET
|
CGD (F)
|
4.07 x 10-15
|
9.23 x 10-16
|
fT (Hz)
|
3.1 x 106
|
3.6 x 108
|
GBP
|
3.2 x 105
|
7.8 x 107
|
τ (sec)
|
5.13 x 10-9
|
4.42 x 10-11
|
TFP
|
1.12 x 107
|
6.6 x 108
|