Electrical and Electronic Devices, Circuits, and Materials. Группа авторов

Figure 2.10 3D - transconductance (g_m), cut-frequency (f_T) and applied gate drive voltage (V_GS) of homo DG -TFET.

      (2.4)

Figure 2.11 shows the variation of exiting capacitances of hetero and homo DG -TFET with applied gate voltage (VGS). The gate-gate capacitance is mainly composed of two capacitances between, gate-drain (Cgd), gate-source (Cgs) and gate-gate (Cgg). The gatesource capacitance (C_gs) is lower because of the presence of the tunnel effect, the gate-drain capacitance (C_gd) is a dominant capacitance due to the accumulation of the electrons of the source-channel and collected by the drain region.

Figure 2.11 The varibility of C-V characteristics with applied gate drive voltage (V_GS) and compersion of homo and hetero structure DG -TFET.

As an important circuit design parameter, the cut-off frequency (f_T) is used to evaluate the frequency characteristics of electronic devices. It can be obtained by the ratio of gm to Cgg, with following relation, Equation 2.5

(2.5)

      Figure 2.11 shows a variability study of available capacitance on DG -TFET with applied VGS. From Figure 2.11, it has been observed that, as the applied gate voltage increases (VGS), the cut-off frequency (f_T) increases to reach its maximum, then with increasing Cgg, it goes down, when the gate voltage (VGS) reaches 2.0 V the cut-off frequency (f_T) becomes constant. This is because the on-state current (I_ON) and its derivative, g_m value increase with the electronic B2B tunneling. The cut-off frequency (f_T) of hetero DG-TFET is much larger than that of homo (i.e. f_{T - hetero}~ 0.65 GHz > f_{T - homo}~ 0.55GHz); this is due to smaller Cgg of hetero DG - TFET and the larger of the gm value than homo DG -TFET. This observation is verified by obtained results, shown in Figure (2.8 & 2.11). From these figures, it has been observed that the obtained results predicts decreased gate capacitance with decreased applied gate voltage (VGS), as depicted in Figure 2.11, which gives the variation of the gate capacitance with VGS. It should be noted that the capacitances of TFET is bias-dependent. That is to say, the decrement rate of the gate capacitance with frequency is bias-dependent.

The gain bandwidth (GBW) product is an important design parameter in the analysis of RF characteristics, which can be calculated by the Equation 2.6. All the results of the simulation are summarized in Table 2.5. As shown in Figure 2.12 and Figure 2.13, there is a variation of gm and GWB versus applied gate voltage (V_GS) i.e., (gm, GWB vs. V_GS) and compression for homo and hetero DG -TFET. The variation of g_m, GWB with V_GS is similar to variation of g_m, f_T with V_GS from Figure 2.12 and Figure 2.13; clearly, it has been observed that hetero and homo structures transconductance, gain bandwidth product (g_m, GWB) increase rapidly as external applied gate voltage V_GS increases.

(2.6)

The maximum, GWB _hetero ~ 0.66 GHz and GWB _homo ~ 0.49 GHz. Another design parameter, the transconductance efficiency (g_m/I_DS) of for DG - TFET, shown in Figure 2.14, has been calculated. The factor g_m/I_DS plays a primary role in the behavior of the device; it shows the rate of amplification of the transistor. The g_m/I_DS of TFETs is dependent on several factors such as drain current (I_DS) and SS, barrier tunnel with (λ) i.e., applied gate voltage (V_GS). Figure 2.14 shows the g_m/I_DS dependence of applied gate voltage (V_GS). As shown in Figure 2.14, g_m/I_DS increases until it reaches a maximum value (around the threshold voltage). Figure 2.14 shows (g_m/I_DS) _hetero > (g_m/I_DS) _homo. The maximum value of g_m/I_DS ~ 285.84 V^-1 for hetero structure while ~ 241.01 V^-1 for homo respectively.

      Table 2.5 Lists of the computed RF parameters of the hetero and homo DG-TFET.

High-k gate dielectric HfO2 (≈ 25)	VDS = 0.5 V
	Cut-off frequency (fT)	GBW (GHz)
Hetero structure	~ 0.65 GHz	0.66 GHz
Homo structure	~ 0.55GHz	0.49 GHz

Figure 2.12 3D - transconductance (g_m), GBW and applied gate drive voltage (V_GS) of hetero DG -TFET.

Figure 2.13 3D - transconductance (g_m), GBW and applied gate drive voltage (V_GS) of homo DG -TFET.

Another important performance parameter for RF analysis is transit time (ι_d) estimated by Equation 2.7. The Equation 2.7 indicates that the delay time is inversely proportional to the cut-off frequency (f_T) i.e., the cut-off frequency increases as the transition time decreases. It is the time taken by the charge carriers (electrons) to cross the channel. Figure 2.15 shows the dependency of applied gate drive voltage (V_GS) vs. delay time (ι_d) and compression of delay time between