InGaZnO/Graphene Cold Source Field Effect Transistors Enabling a Super-Steep Subthreshold Swing Below 60 mV/dec

Reducing the operating power has been crucial in future electronics. Practically, generative artificial intelligent (AI) systems that require extremely high energy, such as the ChatGPT and DALL-E, keep pushing the limits of the power supply over what is practical for humans to utilize. Reducing drive voltage (V_DD) with steeper subthreshold swing (SS) allows switching field effect transistors (FETs) to minimize power consumption while enabling faster switching speeds. However, it is theoretically restricted to realize sub-60 mV/dec SS at room temperature in a conventional silicon semiconductor transistor switching device due to even high carrier concentration generated by a thermal energy, known as Boltzmann’s tyranny. For a breakthrough to this performance limitation, the device structures with the new operation mechanism such as tunneling-, impact ionization-, and negative capacitance-FETs have been designed and demonstrated. Recently, a cold source FET (CSFET) is getting more attention due to its high on-current and reliable sub-60 mV/dec switching, which is based on the cold electron injection phenomenon triggered by low dimensional nanomaterials with super-exponentially decaying electron concentration at the channel/source interface. Although CSFETs based on single 1D carbon nanotube/2D graphene or 2D MoS₂/2D graphene achieved sub-60 mV/dec switching in recent works, their feasibility in large-scale electronic circuits remains challenging. Thus, we should explore practical strategies for producing an ultralow-powered switching device through large-scale fabrication.<br/>Herein, we first demonstrated the InGaZnO (IGZO)/graphene cold-source transistor with sub-60 mV/dec switching characteristics at a relatively low V_DDof 0.5 V. We utilized an amorphous IGZO as an n-type semiconducting channel and p-type graphene as a cold-source electrode. A unique energy band structure of graphene, which is a linear density of states as a function of energy, leads to super-exponentially decaying electron concentration under a vertical gating field to the IGZO/graphene interface, providing ultralow off-current and super-steep slope switching. The delicate gate overlap structure with the gate electrode and graphene cold source qualifies dynamic Schottky barrier modulation at a channel/source interface, allowing high on-current at the same time. Particularly, the use of high-k HfO₂ gate dielectric exhibited lower sub-60 mV/dec SS due to fast surface potential change by gate voltage, compared to that with an Al₂O₃ dielectric. Diverse material characterizations such as transmission electron microscopy, atomic force microscopy, and kelvin probe force microscopy validated an IGZO/graphene cold-source device configuration. Output and transfer characteristics were characterized to analyze switching performance, and electrical device parameters such as field effect mobility (μ_FE), on/off ratio (r_on/off), and SS were statistically summarized. Compared to the control device, the effective Schottky barrier was further modulated in the IGZO CSFET. SS values in an 8 × 8 IGZO CSFET transistor array revealed low variation, showing uniform switching parameters. The super-steep slope switching in the IGZO/graphene CSFET would pave the road toward next-generation ultralow-powered AI devices and electronic systems.