We experimentally demonstrate transistor operation in a vertical p+-MoS2/n-HfS2 van der Waals (vdW) heterostructure configuration for the first time. The HfS2/MoS2 heterojunction transistor exhibits an ON/OFF ratio of 104 and a maximum drain current of 20 nA. These values are comparable with the corresponding reported values for vdW heterojunction TFETs. Moreover, we study the effect of atmospheric exposure on the subthreshold slope (SS) of the HfS2/MoS2 transistor. Unpassivated and passivated devices are compared in terms of their SS values and IDS-VGS hysteresis. While the unpassivated HfS2/MoS2 heterojunction transistor exhibits a minimum SS value of 2000 mV/dec, the same device passivated with a 20-nm-thick HfO2 film exhibits a significantly lower SS value of 700 mV/dec. HfO2 passivation protects the device from contamination caused by atmospheric moisture and oxygen and also reduces the effect of surface traps. We believe that our findings will contribute to the practical realization of HfS2-based vdW heterojunction TFETs.

In this paper, we report a reduction in the access resistance of InP/InGaAs composite-channel metal-oxide-semiconductor field-effect-transistors (MOSFETs) with a back-source electrode. The source region has two electrodes. The source electrode on the surface side is connected to the channel through a doped layer and supplies the electrons. The back-source electrode is constructed under the channel layer and is insulated from the doped layer in order to avoid current leakage. The function of the back-source electrode is to increase the carrier concentration in the channel layer of the source region. In the simulation, the electron density in the channel layer is almost doubled by the effect of the back-source voltage. The fabricated III-V MOSFET has a channel length of 6 µm. A 6% increase in the maximum drain current density (Id) and a 6.8% increase in the transconductance (gm) (Vd = 2 V) are observed. The increase in the carrier density in the channel is estimated to be 20% when the applied voltage of the back-source electrode is 6 V.

The performance of devices based on two dimensional (2D) materials is significantly affected upon prolonged exposure to atmosphere. We analyzed time based environmental degradation of electrical properties of HfS2 field effect transistors. Atmospheric entities like oxygen and moisture adversely affect the device surface and reduction in drain current is observed over period of 48 hours. Two corrective measures, namely, PMMA passivation and vacuum annealing, have been studied to address the diminution of current by contaminants. PMMA passivation prevents the device from environment and reduces the effect of Coulomb scattering. Improvement in current characteristics signifies the importance of dielectric passivation for 2D materials. On the other hand, vacuum annealing is useful in removing contaminants from the affected surface. In order to figure out optimum process conditions, properties have been studied at various annealing temperatures. The improvement in drain current level was observed upon vacuum annealing within optimum range of annealing temperature.