Zhen Tan

Novel Field-Effect Schottky Barrier Transistors Based on Graphene-MoS2 Heterojunctions

Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio. However, the carrier mobility in backgate MoS2 FET is rather low (typically 0.5–20 cm2/V·s). Here, we report a novel field-effect Schottky barrier transistors (FESBT) based on graphene-MoS2 heterojunction (GMH), where the characteristics of high mobility from graphene and high on-off ratio from MoS2 are properly balanced in the novel transistors. Large modulation on the device current (on/off ratio of 105) is achieved by adjusting the backgate (through 300 nm SiO2) voltage to modulate the graphene-MoS2 Schottky barrier. Moreover, the field effective mobility of the FESBT is up to 58.7 cm2/V·s. Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature. This provides an opportunity to overcome the limitation of 60 mV/decade for conventional CMOS devices. The FESBT implemented with a high on-off ratio, a relatively high mobility and a low subthreshold promises low-voltage and low-power applications for future electronics.

A Two-Dimensional Analytical Model for Tunnel Field Effect Transistor and Its Applications

In this paper, a two-dimensional analytical model for the tunnel field effect transistor (TFET) on the silicon-on-insulator substrate is proposed. The accurate electrostatic potential and electric field of the device are obtained by solving the Poisson equation with appropriate boundary conditions. The accuracy of the proposed analytical model is verified by comparing with numerical simulation. It is shown that the electrical behavior of the TFET is more properly described by defining the zero vertical electric field at the channel/buried oxide interface. Furthermore, this analytical model is extended to implement in the hetero-material-gate (HMG) TFET. The physical principle of the HMG TFET can also be depicted, and electrical properties are characterized using this model.

Effects of Sulfur Passivation on GaSb Metal–Oxide–Semiconductor Capacitors with Neutralized and Unneutralized (NH4)2S Solutions of Varied Concentrations

The effects of sulfur passivation on HfO2/GaSb MOS capacitors (MOSCAPs) with neutralized and unneutralized (NH4)2S solutions of varied concentrations were investigated. Treatment with neutralized (NH4)2S aqueous solutions reduced the interface trap density (Dit) by ~23%, improving the effects of sulfur passivation and producing a smoother interface compared with that obtained by treatment with unneutralized (NH4)2S aqueous solutions. The improved performance of GaSb MOSCAPs is attributed to solution neutralization rather than the change in concentration because the distributions of Dit were similar for samples treated with (NH4)2S solutions of different concentrations.

A small-signal generator based on a multi-layer graphene/molybdenum disulfide heterojunction

In this work, we fabricate a heterojunction small-signal generator (HSSG) based on a graphene-molybdenum disulfide (MoS2) heterojunction. The HSSG is fundamentally different from any analog device developed previously. The HSSG is composed of two quasi-2D heterojunctions and has three terminals named injector (I), recombinator (R), and generator (G). MoS2 serves as I and G, and graphene works as R in the HSSG. The scale coefficient (β = IG/IR) of the HSSG is 1.14 × 10−4 (VIG, IR = 0.2 V) to 1.95 × 10−4 (VIG, IR = 1 V). The current generated from G could be as low as pA scale, which reveals the good performance of the HSSG.

Mobility enhancement of strained GaSb p-channel metal–oxide–semiconductor field-effect transistors with biaxial compressive strain*

Various biaxial compressive strained GaSb p-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) are experimentally and theoretically investigated. The biaxial compressive strained GaSb MOSFETs show a high peak mobility of 638 cm2/Vs, which is 3.86 times of the extracted mobility of the fabricated GaSb MOSFETs without strain. Meanwhile, first principles calculations show that the hole effective mass of GaSb depends on the biaxial compressive strain. The biaxial compressive strain brings a remarkable enhancement of the hole mobility caused by a significant reduction in the hole effective mass due to the modulation of the valence bands.

Heteromaterial-gate line tunnel field-effect transistor based on Si/Ge heterojunction*

A Si/Ge heterojunction line tunnel field-effect transistor (LTFET) with a symmetric heteromaterial gate is proposed. Compared to single-material-gate LTFETs, the heteromaterial gate LTFET shows an off-state leakage current that is three orders of magnitude lower, and steeper subthreshold characteristics, without degradation in the on-state current. We reveal that these improvements are due to the induced local potential barrier, which arises from the energy-band profile modulation effect. Based on this novel structure, the impacts of the physical parameters of the gap region between the pocket and the drain, including the work-function mismatch between the pocket gate and the gap gate, the type of dopant, and the doping concentration, on the device performance are investigated. Simulation and theoretical calculation results indicate that the gap gate material and n-type doping level in the gap region should be optimized simultaneously to make this region fully depleted for further suppression of the off-state leakage current.

Compressively strained GaSb P-channel MOSFETs with high hole mobility

III-V compound semiconductors have stirred a significant interest over the recent years, due to their advantageous carrier transport properties [1,2]. Particularly, the high hole mobility makes GaSb very attractive for the p-channel MOSFET application [3]. Although many techniques have been developed to improve the performance of GaSb MOSFETs [4], effects of strain engineering on GaSb MOSFETs have not been investigated yet. In this work, we demonstrated GaSb p-channel MOSFETs with various compressive strains and a peak hole mobility of 638 cm2/Vs is achieved.

Effects of Ozone pre-deposition treatment on GaSb MOS capacitors

GaSb metal-oxide-semiconductor capacitors (MOSCAPs) with Ozone pre-deposition treatment at various temperatures are studied. It is found that Ozone treatment can improve the characteristics of High-k/GaSb MOSCAPs. The Interface Trap Density (Dit) is reduced by 50% after Ozone pre-deposition treatment at 200°C, and gate leakage current is reduced by around 70% after Ozone treatment at 100°C.

Fabrication of strained Si 0.55 Ge 0.45 channel PMOSFETs directly on a Si substrate

Biaxial strained Si0.55Ge0.45 channel pMOSFETs were fabricated using a process compatible with traditional bulk Si CMOS devices. The thermal budget was carefully controlled in the device fabrication process to avoid strain relaxation. Channel materials were formed by pseudomorphic growth of 7nm strained Si0.55Ge0.45 directly on a Si substrate followed by a growth of a 4 nm Si cap. At room temperature, the strained Si0.55Ge0.45/Si p-MOSFETs showed a significant mobility enhancement of ~77% over Si control devices in a low vertical electric field. Moreover, the drain current of the long channel devices was increased by ~47% with a gate overdrive of 2.5V. It was also observed that the enhancement of effective hole mobility degraded as the vertical electric field increased.