Device used to study energy-filtered cold electron transport at room temperature: (a) The device structure. (b) Energy diagram for energy-filtered cold electron transport. The quantum well is formed in the conduction band of the Cr203 layer through band bending and a quantum well state serves as an energy filter.

Altering the thermal characteristics of semiconductors can prolong battery life.

Excessive heat dissipation (or power consumption) of modem integrated circuits is an undesirable effect that imposes substantial limitations on the performance of many electronic devices. For example, the level of heat dissipation /power consumption of smart phones, tablets, and laptops is such that it prohibits a continuous and prolonged operation of these devices, requiring frequent recharging. Large power consumption of electronic devices requires large energy storage in batteries, increasing the battery weights that soldiers carry in their missions or the weights of remote controlled equipment such as unmanned aerial vehicles (UAVs). Therefore, technology that enables electronic devices to operate with extremely small energy consumption promises a broad range of commercial, military and space applications.

The root cause of heat dissipation of current metal-oxide-semiconductor field effect transistors (MOSFETs) is the thermal excitation of electrons that obeys thermodynamics, i.e., the Fenni-Dirac energy distribution of electrons. The thermally excited electrons at the tail of the Fenni-Dirac distribution can overcome the energy barrier set in the OFF state of the MOSFETs. This causes substantial OFF state leakage currents even after the gate voltage is reduced below the threshold voltage, resulting in large heat dissipation or energy consumption for integrated circuits. The challenge for this large heat dissipation is that its root cause is an intrinsic phenomenon of thermodynamics (Fermi-Dirac distribution) that cannot be directly manipulated.

Previous studies have demonstrated that it is possible to indirectly suppress electron thermal excitations by utilizing discrete energy levels present in quantum dots (QDs). Here the electrons are made to pass through the QD energy level and this discrete level serves as an energy filter, allowing only those electrons whose energies match the discrete QD level to pass through. It has been experimentally demonstrated that this energy filtering can lower the effective temperature of electrons. Until now, the energy filtering has been demonstrated only when the entire system is cooled to very low temperatures, typically below 1 Kelvin. For practical applications, however, the energy filtering and effective suppression of electron thermal excitations will need to function at room temperature.

This project aimed to investigate a new method that can effectively suppress electron thermal excitations at room temperature and to fabricate device structures in which energy-suppressed cold electrons are transported through device components at room temperature. An important feature of this approach is that the quantum states for the energy filtering are formed in a quantum well of a very thin (-2 nm) layer, so that their energy level spacing is made to be much larger than the room temperature thermal energy, enabling the electron energy filtering and cold-electron transport to function even at room temperature. Fabrication of device structures that enable cold-electron transport at room temperature is demonstrated. A comprehensive microscopic model of the cold electron transport is provided along with numerical calculations. Application of the energy-filtered cold electron transport to single-electron transistors is demonstrated. Device architecture for a large-scale fabrication of energy- filtered tunnel transistors for energy-efficient electronics is presented. Process and material developments for this transistor architecture are presented.

This work was done by Seong Jin Koh of The University of Texas at Arlington for the Office of Naval Research. NRL-0069


This Brief includes a Technical Support Package (TSP).
Energy-Filtered Tunnel Transistor: A New Device Concept Toward Extremely Low Energy Consumption Electronics

(reference NRL-0069) is currently available for download from the TSP library.

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This article first appeared in the February, 2017 issue of Aerospace & Defense Technology Magazine.

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