High Speed Tunneling Transistor based on Metallic Materials

Abstract

There has been increasing demand for electronic devices with ultra-high working frequency or speed to process big data quickly, especially in the fields of communications, the military, and aerospace as well as a high performance control process unit (CPU). Although parallel computing or computational method have been studied to process big data, it is limited by Amdahl’s law which describes speed saturation even with large number of processors. Therefore, improvement of fundamental components such as diode and transis-tor can be a basic solution. The p-n junction and Schottky barrier are widely used for switching behavior in diode and transistor structure, so far. The switching speed can be controlled easily by the channel length be-tween source and drain in conventional metal-oxide semiconductor field-effect transistor (MOSFET). The channel should be as short as possible to drive high switching speed, while the electrons cannot be controlled well by gate bias due to short channel effect in the p-n junction and Schottky barrier based MOSFET. Elec-tron mobility was considered instead of physical dimension. Although high electron mobility transistor (HEMT), based on III-V group materials, is expected to increase the speed, its theoretical frequency limit is less than 1THz, even with several nanometer channel length. Therefore, conventional switching mechanisms should be substituted to new mechanism which can drive high speed without electron retardation. Tunneling mechanism in non-semiconductive materials was considered in metal-insulator-metal (MIM) structure. Alt-hough MIM tunneling structure can provide ultra-high working speeds (>THz), the very low contrast ratio between forward and reverse currents results in poor working efficiency. Improving efficiency in MIM tunnel devices is highly motivated for high-speed efficient switching elements.

Improving efficiency in various MIM tunnel structures is the main work of this dissertation. Although the work function differences and multi-stack insulators in MIM structure are widely used to increase effi-ciency, these are limited by low work function differences in metallic materials and increase of uncertainty factors, respectively. In here, we suggested asymmetrically geometric design to obtain different tunneling probabilities in MIM structure. 50% ratio of shifted tunneling distance could be obtained by small angle of pointed shape. By theoretical investigation and simulation, geometric design can induces much shorter or longer tunneling distance, which is effective approach for enhancing contrast ratio. In the vertical structure, over 70% ratio of shifted tunneling distance could be theoretically expected by electrical field simulation (5nm Al2O3 and 20° angle of pointed shape). Resulting from comparison of 20° and 90° angle of pointed shape shows large different tunneling probabilities of angle dependence (0.306% and 0.014%, respectively). Therefore, geometric effect is promising method to increase working efficiency.

The quality of insulator materials is one of the key parameters to enhance geometric effect. The vacu-um which is closed to ideal insulation was used to investigate only geometric effect in MIM transistor struc-ture. Laterally moved tunneling electrons can be controlled by bottom a gate electrode, proved by quite good contrast ratio (12.7) and controlling threshold voltage of FN tunneling (5.25V) under vacuum state (3 × 10-6 torr). Even though these parameters related to efficiency are better than that of conventional metal-insulator-metal diode structure, the threshold voltage should be within 5V for compatible to conventional complemen-tary metal-oxide semiconductor (CMOS). Lateral MIM structure, by using sputtered SiO2 instead of vacuum to decrease threshold voltage and working voltage, shows poor contrast ratio and a little gate effect. While the vacuum guarantees quality of insulation by vacuum pressure, the quality of insulator materials is defined by defect or pinhole, which are normally ranked by deposition methods. Furthermore, selection of insulation materials was also important for improving efficiency. Although the band gap of Al2O3 is larger than that of HfO2, the working efficiency was significantly improved by blocking reverse current well. In general, large band gap insulator materials are inappropriate for a tunneling device, due to the low tunneling current. How-ever, in our approach, since the issue of low tunneling probability is compensated by the sharp tip structure, the larger band gap insulator produced better working efficiency with the appropriate current density.

Directionality of electron movement was considered in geometric designed MIM transistor structure for high-speed and improving efficiency. It is relevant to technical ways to define the channel length. The gap between source (geometric designed electrode) and drain is important parameter to operate high speed in MIM transistor structure. While small gap is not reliable in lateral structure, defined by even electron beam lithography, reliable gap in vertical structure was achieved by deposition system controlled with Å level. Alt-hough the electrons are not effectively controlled in vertical orient structure, applying a floating electrode in vertical MIM transistor can control the tunneling current with little leakage current. Low threshold voltage (0.75V) and considerable efficiency (37 contrast ratio) can be achieved by bias control of floating electrode. It shows reliable I-V characteristics under high temperature environment up to 398K, proved that tunneling phenomenon is dominant. By estimating cut-off frequency (0.82 THz), vertical MIM tunneling transistor with floating electrode can be expected for THz applications in various fields, such as communication devic-es, high speed electrical switches, and high performance control process units (CPUs), or other new concept devices. ⓒ 2017 DGIST