Two-dimensional (2D) transition metal dichalcogenides (TMDs)-based van der Waals (vdW) PN junctions have been used for heterojunction diodes, which basically utilize out-of-plane current across the junction interface. In fact, the same vdW PN junction structure can be utilized for another important device application, junction field effect transistors (JFETs), where in-plane current is possible along with 2D–2D heterojunction interface. Moreover, the 2D TMD-based JFET can use both p- and n-channel for low voltage operation, which might be its unique feature. Here we report vdW JFETs as an in-plane current device with heterojunction between semiconducting p- and n-TMDs. Since this vdW JFET would have low-density traps at the vdW interface unlike 2D TMD-based metal insulator semiconductor field effect transistors (MISFETs), little hysteresis of 0.0–0.1 V and best subthreshold swing of ~100 mV/dec were achieved. Easy saturation was observed either from n-channel or p-channel JFET as another advantage over 2D MISFETs, exhibiting early pinch-off at ~1 V. Operational gate voltage for threshold was near 0 V and our highest mobility reaches to ~>500 cm2/V·s for n-channel JFET with MoS2 channel. For 1 V JFET operation, our best ON/OFF current ratio was observed to be ~104.

We report the fabrication of hybrid PN junction diode and complementary (CMOS) inverters, where 2D p-type MoTe and n-type thin film InGaZnO (IGZO) are coupled for each device process. IGZO thin film was initially patterned by conventional photolithography either for n-type material in a PN diode or for n-channel of top-gate field-effect transistors (FET) in CMOS inverter. The hybrid PN junction diode shows a good ideality factor of 1.57 and quite a high ON/OFF rectification ratio of ∼3 × 10. Under photons, our hybrid PN diode appeared somewhat stable only responding to high-energy photons of blue and ultraviolet. Our 2D nanosheet-oxide film hybrid CMOS inverter exhibits voltage gains as high as ∼40 at 5 V, low power consumption less than around a few nW at 1 V, and ∼200 μs switching dynamics.

The two-dimensional transition-metal dichalcogenide semiconductor MoS has received extensive attention for decades because of its outstanding electrical and mechanical properties for next-generation devices. One weakness of MoS, however, is that it shows only n-type conduction, revealing its limitations for homogeneous PN diodes and complementary inverters. Here, we introduce a charge-transfer method to modify the conduction property of MoS from n- to p-type. We initially deposited an n-type InGaZnO (IGZO) film on top of the MoS flake so that electron charges might be transferred from MoS to IGZO during air ambient annealing. As a result, electron charges were depleted in MoS. Such charge depletion lowered the MoS Fermi level, which makes hole conduction favorable in MoS when optimum source/drain electrodes with a high work function are selected. Our IGZO-supported MoS flake field effect transistors (FETs) clearly display channel-type conversion from n- to p-channel in this way. Under short- and long-annealing conditions, n- and p-channel MoS FETs are achieved, respectively, and a low-voltage complementary inverter is demonstrated using both channels in a single MoS flake.

By inserting hydroxyl-group free organic dielectric between hydrophilic oxide dielectric and 2D TMD channel, highly stable 2D FETs are achieved. This concept was successfully extended to a practical device application such as stable 1 V operation of 2D MoTe2 FET.

Polycrystalline cadmium telluride (CdTe) X-ray photodetector with advanced performance was fabricated in a Schottky diode form by direct thermal deposition (evaporation) on pixelized complementary metal oxide semiconductor (CMOS) readout panel. Our CdTe X-ray detector shows such a variety of benefits as relatively low process temperature, low cost, low operation voltage less than 40 V, and higher sensitivity and spatial resolution than those of commercial a-Se detectors. CdTe has cubic Zinc Blende structure and maintains p-type conduction after growth in general. For low voltage operation, we succeeded in Cl doping at all stage of CdTe film deposition, and as a result, hole concentration of p-type CdTe was reduced to ~1012 cm−3 from ~1015 cm−3, and such concentration reduction could enable our Schottky diode with Ti electrode to operate at a reverse bias of less than 40 V. Our CdTe Schottky diode/CMOS pixel array as a direct conversion type imager demonstrates much higher resolution X-ray imaging in 7 × 9 cm2 large scale than that of CsI/CMOS array, an indirect conversion imager. To our limited knowledge, our results on polycrystalline CdTe Schottky diode/CMOS array would be very novel as a first demonstration of active pixel sensor system equipped with directly deposited large scale X-ray detector.