Low-temperature operation of diamond surface-channel field-effect transistors
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Low-temperature operation of diamond surface-channel field-effect transistors
Minoru Tachiki, Hiroaki Ishizaka, Tokishige Banno, Toshikatsu Sakai, Kwang-Soup Song, Hitoshi Umezawa and Hiroshi Kawarada School of Science & Engineering, Waseda University, Tokyo, Japan. CREST, Japan Science and Technology Corporation (JST), Japan. E-mail: [email protected] ABSTRACT Cryogenic operation of the diamond surface-channel field-effect transistors (FETs) is investigated. Metal-insulator-semiconductor FETs (MISFETs) are fabricated using CaF2 as a gate insulator. MISFETs operate successfully even at 4.4 K. At low temperature, field-effect enhances the drain current, even if the surface holes become almost frozen-out. Channel mobility increases as temperature decreases to 4.4 K, which indicates the reduced phonon scattering. INTRODUCTION Diamond is a promising semiconductor material for the future electronics. Owing to its high breakdown field (107 Vcm-1), extremely high thermal conductivity (20 Wcm-1K-1), high hole mobility (1800 cm2V-1s-1) and low dielectric constant (5.7), diamond is expected as a candidate for high power, high-frequency devices. However, room temperature device operation is still problematic in impurity-doped (boron for p-type, phosphorus or sulfur for n-type) diamond due to their deep activation energy. In that sense, hydrogen-terminated diamond is attractive for electrical applications because it induces p-type surface conduction even if the diamond is not intentionally doped. Hydrogen termination of diamond also stabilizes the surface structure and can reduce surface state density [1]. This pinning-free surface is quite favorable when the diamond surface is used for the metal-semiconductor (MES) or metal-insulator-semiconductor (MIS) field-effect transistor (FET) applications. Up to now, the fabrication and the operation of MESFETs, MISFETs, single electron transistors (SETs) and ion sensitive FETs (ISFETs) have been demonstrated using a surface conductive layer. High transconductance FETs (110 mS/mm for 1 µm-gate MESFET [2], 86 mS/mm for 1.1 µm-gate MISFET [3]) and High-frequency FETs (cutoff frequency of ~10 GHz) [4] have been realized. On the other hand, cryogenic operation of the semiconductor devices is an interesting issue not only for studying the physical properties of semiconductor materials and devices, but also practical aspects. Expected advantages of low temperature operation of electronic systems are higher device performance because of increased carrier mobility and saturation velocity, lower power dissipation because of the sharper turn-on characteristics of FETs, reduced thermally activated degradations of the device performance and so on. To investigate the carrier behavior of the surface conductive layer at low temperature, and to elucidate the mechanism of the surface conductive layer, we demonstrate the low-temperature (~ 4.4 K) operation of the diamond MISFETs for the first time.
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