(a) Schematic view of the heterojunction-based ternary inverter. (b) Transfer characteristics of the BP FET and lateral heterojunction FET at small drain bias of 0.2 V (left) and large drain bias of 2 V (right), showing overlapped regions increasing at larger drain biases. (c) Inverter formed by a BP FET and lateral heterojunction FET showing binary logic under a small driving voltage and ternary logic under a large driving voltage; V_ddvaries from 0.2 V to 2 V and the inflection point is∼1 V. Inset: another ternary inverter biased at 4 V with large mid-logic window. (d) Voltage gain for both switching states for the ternary inverter at different Vdd. Inset: typical voltage gain dependence of V_in.

（NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2017.208）

Background

Two-dimensional (2D) materials offer a unique opportunity for the integration of heterogeneous systems due to the weak van der Waals interactions between individual layers, without dangling bonds on the surfaces. It is therefore possible to easily assemble different 2D materials into functional devices without the restraint of lattice mismatch and the need for the sophisticated and sometimes impractical growth procedures required for fabrication of conventional heterojunctions. BP is an unintentionally p-doped narrow-bandgap semiconductor, and MoS₂is a large bandgap n-type semiconductor with reasonably high electron mobility. Moreover, the electron affinity difference for these materials is only 0.1 eV. The combination of the two materials opens the possibility of tuning the energy band offset.

What we discover?

In lateral heterojunction FET (HJFET) structure, in which the two electrodes are separately located on a BP flake and MoS₂sheet away from the overlapped junction region (left part of figure a). Under forward bias, the energy band of the BP side will be pulled down. When V_dis small, the barrier for electrons in MoS₂is always very small close to Ohmic contact, and the barrier for holes in BP is large. When V_dincreases, the valence band of BP will be pulled down beyond the valence band position of MoS₂(at about V_d= 1 V). Then the barrier for holes decreases to zero and the BP channel will dominate current transport, leading to much larger parallel regions of this heterojunction and the BP channel (Figure b). As a result, the heterojunction based inverter shows binary logic under small driving voltage but ternary logic under large V_dd, as shown in Figure c. For example, at V_dd= 0.2 V, the output voltage shows a high logic value of 1 before -0.5 V and a low logic of 0 at V_in> 0 V. At V_dd= 2 V, in addition to ‘logic 1’ at V_in< -0.5 V and ‘logic 0’ at V_in> 0.5 V, a new middle logic value of V_out∼1.1 V appears in the range of -0.4 V < V_in< 0 V. At logic 1, the BP2 transistor in the pull-up network will provide a low resistive path to the supply voltage. At logic 0, the resistance of the heterojunction in the pull-down network is small enough that the logic low levels are almost equal to GND. Moreover, these FETs show fast switching in gate modulation, leading to a high voltage gain in the inverters, thanks to the large specific capacitance of the high-κ dielectric HfSiO used. The gain increases with V_dd, and the switching gains of the first and second logic states can reach up 12 and 8 at V_dd= 2 V, as shown in Figure d.

Why is this important?

We have designed and fabricated various types of multi-value inverters, in which the output logic state and window of the mid-logic can be controlled by specific pairs of channel length and, most importantly, by the electric field, which shifts the band-structure alignment across the heterojunction. To the best of our knowledge, this is the first tunable ternary inverter.

Why did they need WHMFC?

Wuhan National High Magnetic Field Center provides great technical support during low temperature electrical measurements.

Who did the research?

Mingqiang Huang¹, Shengman Li¹, Zhenfeng Zhang¹, Xiong Xiong¹, Xuefei Li^1,2and Yanqing Wu^1,2*

¹Wuhan National High Magnetic Field Center and school of physics, Huazhong University of Science & Technology, 1037 Luoyu road, Wuhan 430074, People’s Republic of China.

²School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China.