In this letter, a new L-shaped gate tunnel field-effect transistor (LG-TFET) is proposed and investigated by Silvaco Atlas simulation. The tunneling junction in the LG-TFET is perpendicular to the channel direction that facilitates the implementation of a relatively large tunneling junction area. The channel is U-shaped that makes the channel mainly distribute in the vertical direction, reducing the device area. The n+ pocket design is also introduced between the source and the intrinsic regions to improve the device characteristic. In addition, the gate and n+ pocket region overlap both in the vertical and the lateral directions resulting in an enhanced electric field, and the ON-state current of the LG-TFET is increased up to $\sim 50$ % compared with the previous L-shaped channel TFET. The minimum subthreshold swing of the LG-TFET is 38.5 mV/decade at 0.2 V gate-to-source voltage. By using the L-shaped gate, U-shaped channel, and the insertion of n+ pocket, the overall performance of the LG-TFET is optimized.

Tunnel Field-Effect Transistor With an L-Shaped Gate

A vertical dielectrically modulated tunnel field-effect transistor (V-DMTFET) as a label-free biosensor has been investigated in this paper for the first time and compared with lateral DMTFET (L-DMTFET) using underlap concept and gate work function engineering. To improve the performance of lateral biosensor (LB), a heavily doped front gate ${n}^{+}$ -pocket and gate-to-source overlap is introduced in the vertical biosensor (VB). The integrated effect of lateral tunneling as well as vertical tunneling in VB leads to enhanced ON-state current and decrease the subthreshold swing. To evaluate sensing ability of these devices, charged and charged neutral biomolecules are immobilized in nanogap cavity independently. A deep analysis has been performed to show the effect of variation in dielectric constant ( $k$ ), charge density ( $\rho $ ), ${x}$ -composition of Ge, % volume filling of ${t}_{\textsf {cavity}}$ , length and thickness of a ${n}^{+}$ -pocket and sensitivity of electrical parameters is also incorporated. Dual-pocket (front and back gate pocket) VB is studied and compared with the LB and VB in the tabular form. Noise characteristic of dielectrically modulated field-effect transistor, L-DMTFET, and V-DMTFET is also evaluated.

Performance Assessment of A Novel Vertical Dielectrically Modulated TFET-Based Biosensor

The source-pocket (p-n-p-n) tunnel field effect transistor (TFET) has a narrow and highly doped N
 +
 pocket layer between the source and channel to enhance the overall performance of the conventional p-i-n TFET. However, realizing this, N
 +
 pocket increases the fabrication complexity since either an epitaxial growth in vertical TFETs or an implantation in planar TFETs is required to create the N
 +
 pocket. In this letter, using the charge plasma concept, we propose a technique to realize an in-built N
 +
 pocket without the need for a separate implantation. We demonstrate using 2-D simulations that the proposed in-built N
 +
 pocket p-n-p-n TFET exhibits a higher I
 ON
 (~20 times) and a steeper subthreshold swing (25 mV/decade) as compared with the conventional p-i-n TFET. Our approach overcomes the difficulty of creating a narrow N
 +
 pocket doping and thus makes the p-n-p-n TFET more attractive in carrying on with the scaling trend.

In-Built N + Pocket p-n-p-n Tunnel Field-Effect Transistor

In this brief, we demonstrate using 2-D simulations that the use of a heterodielectric BOX (HDB) above a highly doped ground plane can control the tunneling width at the channel–drain interface and lead to a significant reduction in the ambipolar current in tunnel FETs (TFETs). The HDB consists of SiO2 under the source and the channel regions, and HfO2 under the drain region. When the thickness of the HDB is 25 nm and the ground plane is heavily doped, we show that the drain region at the channel–drain interface is effectively depleted. As a result, the tunneling width at the channel–drain interface increases leading to a complete suppression of ambipolar conduction in a TFET even when the gate voltage $V_{\textrm {GS}}= -0.8$ V.

Controlling the Drain Side Tunneling Width to Reduce Ambipolar Current in Tunnel FETs Using Heterodielectric BOX

To suppress the ambipolar behavior and improve RF performance in tunnel field-effect transistors (TFETs), a Z-shaped (ZS)-TFET is proposed. The proposed ZS-TFET is more scalable than other vertical band-to-band-based TFETs and provides higher ON-state current ( ${I} _{ {\mathrm{\scriptscriptstyle ON}}}$ ), larger ON/OFF current ratio ( ${I} _{ {\mathrm{\scriptscriptstyle ON}}}/{I} _{ {\mathrm{\scriptscriptstyle OFF}}}$ ) and lower subthreshold swing compared to conventional TFETs. These advantages stem from the tunneling junction in the ZS-TFET being perpendicular to the channel direction, which facilitates the formation of a relatively large tunneling junction area. The ZS body makes use of both vertical and horizontal fields while suppressing the lateral parasitic tunneling current. In addition, by using a ZS gate in the proposed device, the energy band diagram near the source is modulated to create an N+ source pocket which creates a downward band bending of the potential, similar to PNPN-like structures. Finally, the proposed structure significantly improves the analog/RF figure-of-merit.

A Novel PNPN-Like Z-Shaped Tunnel Field- Effect Transistor With Improved Ambipolar Behavior and RF Performance

In this paper, we have demonstrated that overlapping the gate on the drain can suppress the ambipolar conduction, which is an inherent property of a tunnel field effect transistor (TFET). Unlike in the conventional TFET where the gate controls the tunneling barrier width at both source-channel and channel-drain interfaces for different polarity of gate voltage, overlapping the gate on the drain limits the gate to control only the tunneling barrier width at the source-channel interface irrespective of the polarity of the gate voltage. As a result, the proposed overlapping gate-on-drain TFET exhibits suppressed ambipolar conduction even when the drain doping is as high as $1 \times 10^{19}$ cm $^{-3}$ .

Dawit Burusie Abdi

Papers

Controlling Ambipolar Current in Tunneling FETs Using Overlapping Gate-on-Drain

In-Built N + Pocket p-n-p-n Tunnel Field-Effect Transistor

Dielectric modulated overlapping gate-on-drain tunnel-FET as a label-free biosensor

Dopingless PNPN tunnel FET with improved performance: Design and analysis

PNPN tunnel FET with controllable drain side tunnel barrier width: Proposal and analysis