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Design and Investigation of the High Performance Doping-Less TFET with Ge/Si_{0.6}Ge_{0.4}/Si Heterojunction

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## Abstract

**:**

_{0.6}Ge

_{0.4}/Si heterojunction (H-DLTFET) is proposed in this paper. Compared to the conventional doping-less tunneling field effect transistor (DLTFET), the source and channel regions of H-DLTFET respectively use the germanium and Si

_{0.6}Ge

_{0.4}materials to get the steeper energy band, which can also increase the electric field of source/channel tunneling junction. Meanwhile, the double-gate process is used to improve the gate-to-channel control. In addition, the effects of Ge content, electrode work functions, and device structure parameters on the performance of H-DLTFET are researched in detail, and then the above optimal device structure parameters can be obtained. Compared to the DLTFET, the simulation results show that the maximum on-state current, trans-conductance, and output current of H-DLTFET are all increased by one order of magnitude, whereas the off-state current is reduced by two orders of magnitude, so the switching ratio increase by three orders of magnitude. At the same time, the cut-off frequency and gain bandwidth product of H-DLTFET increase from 1.75 GHz and 0.23 GHz to 23.6 GHz and 4.69 GHz, respectively. Therefore, the H-DLTFET is more suitable for the ultra-low power integrated circuits.

## 1. Introduction

_{0.6}Ge

_{0.4}/Si heterojunction (H-DLTFET) is proposed in this paper. Compared to the conventional DLTFET, the source and channel regions of the H-DLTFET respectively use the germanium and Si

_{0.6}Ge

_{0.4}materials. To make the energy band becomes steeper, source region use the narrow bandgap semiconductor material germanium, which can also effectively improve the electric field of source/channel tunneling junction. The electrons from the valence band of source region are easier to tunnel into the conduction band of channel region, which can increase the on-state current effectively [17]. The channel region uses Si

_{0.6}Ge

_{0.4}material to improve the channel carrier mobility. Meanwhile, the SS and frequency characteristics of H-DLTFET can be greatly improved. Besides, the dual gate process is used to improve the gate-to-channel control, which can effectively reduce the adverse effects of short channel effects [18]. Meanwhile, the stronger gate control can provide the higher saturation current density and lower off-state leakage current [19]. The main research contents of this paper are as follows: First, the simulation structure and models of DLTFET and H-DLTFET are introduced. Next, the input and output characteristics of DLTFET and H-DLTFET are analyzed. Then, the working mechanism of H-DLTFET is described. Afterwards, effects of Ge content, electrode work function, and structure parameters on the performance of H-DLTFET are researched systematically. Subsequently, the C-V and frequency characteristics of DLTFET and H-DLTFET are compared. Finally, the simulation results show that the H-DLTFET has the greater potential in the low-power ICs.

## 2. Device Structure and Models

_{0.6}Ge

_{0.4}materials are used in the source and channel regions, respectively. Then, the gate-to-channel control capability can be improved by enhancing the coupling between gate and back-gate. Finally, the H-DLTFET can obtain the higher on-state current by adjusting the length and height of effective channel tunneling region.

_{c}= 5 nm; gate oxide thickness T

_{ox}= 2 nm; the length of source region (L

_{s}), gap region (L

_{gap}), channel region (L

_{g}), and drain region (L

_{d}) are 10 nm, 5 nm, 20 nm, and 10 nm, respectively. Meanwhile, the doping concentration and type of L

_{s}, L

_{gap}, L

_{g}, and L

_{d}are 1 × 10

^{18}cm

^{−3}and n-type, respectively. Besides, the work functions of gate, source, drain, and back-gate electrodes are respectively 4 eV, 5.3 eV, 4.2 eV, and 5.1 eV.

## 3. Discussion of Simulation Results

#### 3.1. The Input Characteristics

^{−17}A/μm and 7.14 × 10

^{−19}A/μm, whereas the maximum on-state currents of DLTFET and H-DLTFET are 5.03 × 10

^{−6}A/μm and 4.71 × 10

^{−5}A/μm, respectively. The off-state current of H-DLTFET decreases by two orders of magnitude compared with the conventional DLTFET. And the on-state current and switching ratio are respectively increased by an order of magnitude and three orders of magnitude at the same operating voltage. The reason is that a large number of electron-hole pairs from source region can tunnel into the channel, and the tunneling current would increase exponentially with the electric field. However, the electrons of channel center are depleted and the on-state current is saturated when gate voltage continues to increase. Meanwhile, the average SS and point SS of H-DLTFET respectively decrease from 41.5 mV/Dec and 4 mV/Dec to 15.7 mV/Dec and 2.6 mV/Dec. It can be concluded that the H-DLTFET have the larger on-state current, higher switching ratio and smaller SS than that of DLTFET, so it has the greater potential in the ultra-low power ICs.

_{m}) is an important characterization parameter of frequency characteristics, which can determine the intrinsic gain of semiconductor devices. Figure 2b shows the g

_{m}value of DLTFET and H-DLTFET as a function of gate voltage. The g

_{m}can be calculated by the following Equation (1) [22]:

_{m}value mainly depends on the output leakage current, and the faster the increase rate of output leakage current is, the larger the g

_{m}is. The g

_{m}values of DLTFET and H-DLTFET increase with the gate voltage increases. This is because the barrier width of tunneling junction decreases, the tunneling electrons increase, and the output leakage current increases gradually. It can be seen from Figure 2b that the g

_{m}values of DLTFET and H-DLTFET are respectively 2.07 × 10

^{−5}S/μm and 1.89 × 10

^{−4}S/μm. Due to the higher on-state current, the maximum g

_{m}value of H-DLTFET can be increased by an order of magnitude.

#### 3.2. The Output Characteristics

^{−6}A/μm and 4.71 × 10

^{−5}A/μm. The output current of H-DLTFET can be increased by an order of magnitude compared with the DLTFET, so the H-DLTFET has the higher saturation output current.

#### 3.3. The Operating Mechanism of H-DLTFET

#### 3.4. Effect of the Device Parameters on the Performance of H-DLTFET

_{ox}) increases. At the same time, the electric field at the interface between the gate oxide dielectric and channel is weakened. The on-state current increases from 4.4 × 10

^{−8}A/μm to 3.1 × 10

^{−5}A/μm, and the switching ratio is almost increased by three orders of magnitude when T

_{ox}decrease from 5 nm to 2 nm. Figure 7b shows the energy band of the source-channel tunneling junction at the different T

_{ox}. As the T

_{ox}decreases, the energy band becomes steeper. Both the electron BTBT probability and the tunneling area are increased, and the gate-to-channel control capability increases, so the on-state current can be effectively improved. In a word, the optimal T

_{ox}is 2 nm.

_{1-x}Ge

_{x}material increases. This is because the germanium material can provide the smaller electron tunneling quality [28]. The number of tunneling electrons increase, and the band gap becomes narrower when the germanium content increases, which can improve the e-BTBT probability and SS characteristics of H-DLTFET. As shown in Figure 8b, the energy band width becomes narrower, and the energy band becomes steeper with the germanium content increases, which result in the higher e-BTBT probability. The H-DLTFET can obtain the relatively smaller off-state current and the larger on-state current when the optimal Ge content is 0.4. Therefore, the channel of H-DLTFET uses the Si

_{0.6}Ge

_{0.4}material.

#### 3.5. The C-V Characteristics

_{gg}) is an important indicator for evaluating the frequency characteristics [30]. Figure 10a,b respectively show the C-V characteristics of DLTFET and H-DLTFET at the operating frequency f = 1 MHz. The C

_{gg}mainly includes the gate-drain capacitance (C

_{gd}), gate-source capacitance (C

_{gs}) and gate-back gate capacitance (C

_{gbg}) [31]. It can be seen from Figure 10 that the C

_{gd}increases exponentially with the gate voltage increases. The reason is that the tunneling barrier width decreases when the gate voltage increases, and the inversion layer extending from drain region to source region can be formed. The C

_{gs}is mainly composed of parasitic capacitance, and the order of magnitude is small relative to the C

_{gd}under the inversion layer, which is weakly affected by the bias voltage. Therefore, the C

_{gg}is mainly determined by the C

_{gd}at the high gate voltage. Almost all electrons of H-DLTFET can be collected by the drain region immediately, and the channel electron concentration is lower, which would result in the smaller C

_{gg}and C

_{gd}. It can be seen from Figure 10 that the C

_{gg}and C

_{gd}of H-DLTFET respectively decrease from 1.98 fF/μm and 1.68 fF/μm to 1.28 fF/μm and 0.77 fF/μm compared with the conventional DLTFET, so the H-DLTFET has the better frequency characteristics.

#### 3.6. The Frequency Characteristics

_{gg}, and the specific calculation formula is shown in Equation (2) [32].

_{m}value increase with the electronic BTBT efficiency increases. However, the cut-off frequency has no change or even decreases when the gate voltage continues to increase to the high gate voltage. This is due to the increase of C

_{gg}and the decrease of g

_{m}, which is caused by the mobility degradation. In addition, the cut-off frequency of H-DLTFET is much larger than that of DLTFET, which can be explained by the smaller C

_{gg}of H-DLTFET. The larger the g

_{m}value is, the higher the cut-off frequency is.

_{m}, the gain bandwidth product initially increases as the gate voltage increases. However, the gain bandwidth product decreases when the gate voltage is greater than 0.8 V. This is because the common impact between the mobility degradation and the parasitic capacitance. In addition, the overall trend of gain bandwidth product as a function of gate voltage is consistent with the cut-off frequency. Compared to the conventional DLTFET, the cut-off frequency and gain bandwidth product of H-DLTFET respectively increase from 1.75 GHz and 0.23 GHz to 23.6 GHz and 4.69 GHz. Therefore, the H-DLTFET has the better frequency characteristics.

## 4. Conclusions

_{0.6}Ge

_{0.4}material, the electric field of source/channel tunneling junction increases and the energy band becomes steeper. At the same time, the dual gate process can improve the gate-to-channel control capability. In addition, the effects of germanium content, electrode work function, and device structure parameters on the performance of H-DLTFET are analyzed systematically, and then the above optimal parameters can be obtained to optimize the overall performance of H-DLTFET. Compared to the conventional DLTFET, the simulation results show that the on-state current and switching ratio of H-DLTFET can respectively increase by one order of magnitude and three orders of magnitude, and the off-state current is reduced by two orders of magnitude at the same operating voltage. At the same time, both the maximum g

_{m}and output current are increased by an order of magnitude. The average SS and point SS of H-DLTFET respectively decrease from 41.5 mV/Dec and 4 mV/Dec to 15.7 mV/Dec and 2.6 mV/Dec. And the C

_{gg}and C

_{gd}of H-DLTFET are also decreased from 1.98 fF/μm and 1.68 fF/μm to 1.28 fF/μm and 0.77 fF/μm, respectively. Meanwhile, the cut-off frequency and gain bandwidth product of H-DLTFET respectively increase from 1.75 GHz and 0.23 GHz to 23.6 GHz and 4.69 GHz. Therefore, the H-DLTFET is more suitable for the ultra-low power integrated circuits.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 4.**(

**a**) e-BTBT rate; (

**b**) h-BTBT rate; (

**c**) Electric field; and (

**d**) Potential distribution of the H-DLTFET.

**Figure 5.**On-state energy band of DLTFET and H-DLTFET (

**a**) from source to drain; (

**b**) from channel top to channel bottom.

**Figure 6.**Effect of device parameters on the transfer characteristics of H-DLTFET (

**a**) channel thickness Hc, (

**b**) channel length Lg.

**Figure 9.**Effect of work function (

**a**) back gate Ψbg; (

**b**) drain Ψd; (

**c**) gate Ψg on the transfer characteristics of H-DLTFET.

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## Share and Cite

**MDPI and ACS Style**

Han, T.; Liu, H.; Chen, S.; Wang, S.; Li, W.
Design and Investigation of the High Performance Doping-Less TFET with Ge/Si_{0.6}Ge_{0.4}/Si Heterojunction. *Micromachines* **2019**, *10*, 424.
https://doi.org/10.3390/mi10060424

**AMA Style**

Han T, Liu H, Chen S, Wang S, Li W.
Design and Investigation of the High Performance Doping-Less TFET with Ge/Si_{0.6}Ge_{0.4}/Si Heterojunction. *Micromachines*. 2019; 10(6):424.
https://doi.org/10.3390/mi10060424

**Chicago/Turabian Style**

Han, Tao, Hongxia Liu, Shupeng Chen, Shulong Wang, and Wei Li.
2019. "Design and Investigation of the High Performance Doping-Less TFET with Ge/Si_{0.6}Ge_{0.4}/Si Heterojunction" *Micromachines* 10, no. 6: 424.
https://doi.org/10.3390/mi10060424