Hirotada Taniuchi

High-frequency performance of diamond field-effect transistor

The microwave performance of a diamond metal-semiconductor field-effect transistor (MESFET) is reported for the first time. MESFETs with a gate length of 2-3 /spl mu/m and a source-gate spacing of 0.1 /spl mu/m were fabricated on the hydrogen-terminated surface of an undoped diamond film grown by microwave plasma chemical vapor deposition (CVD) utilizing a self-aligned gate fabrication process. A maximum transconductance of 70 mS/mm was obtained on a 2 /spl mu/m gate MESFET at V/sub GS/=-1.5 V and V/sub DS/=-5 V,for which a cutoff frequency f/sub T/ and a maximum oscillating frequency f/sub max/ of 2.2 GHz and 7 GHz were obtained, respectively.

Potential applications of surface channel diamond field-effect transistors

Abstract In order to realize high frequency and high power diamond devices, diamond FETs on the hydrogen-terminated diamond surface conductive layer have been fabricated. The fabricated diamond MESFETs show high breakdown voltage and output capability of 1 W mm −1 . High transconductance diamond MESFET utilizing a self-aligned gate FET fabrication process has been operated in high frequency for the first time. In the 2 μm gate MESFETs, the obtained cut off frequency f T and maximum frequency of oscillation f max are 2.2 and 7 GHz, respectively. It is expected that the diamond MESFET with 0.5 μm gate length fabricated by self-aligned gate process shows 8 GHz of f T and 30 GHz of f max .

Cu/CaF2/Diamond Metal-Insulator-Semiconductor Field-Effect Transistor Utilizing Self-Aligned Gate Fabrication Process

High-performance metal-insulator-semiconductor field-effect transistors (MISFET) on hydrogen-terminated homoepitaxial diamond films are demonstrated. The gate insulator is evaporated CaF2 which does not cause interface states. This is the first study of a CaF2/diamond MISFET fabricated by a self-aligned gate fabrication process by which the gate length and the source gate spacing are effectively reduced. The maximum transconductance is 86 mS/mm, which is the highest value in diamond MISFETs at present.

High performance diamond MISFETs using CaF2 gate insulator

Abstract A cut-off frequency of 15 GHz and a maximum frequency of oscillation of 20 GHz are realized in a 0.4-μm gate diamond metal–insulator–semiconductor field-effect transistor (MISFET). The cut-off frequency is the highest value for diamond FETs ever reported. The RF characteristics of the MISFETs are higher than those of metal–semiconductor FETs at the same gate lengths. The CaF2 gate insulator improves the carrier mobility according to the Hall measurement system. The mobility increases in the surface conductive layer result in high RF performance. The source–gate passivation of CaF2 results in the high DC transconductance because of the reduction of series resistances. A cut-off frequency of more than 30 GHz is expected with the gate minimization and the CaF2 passivation of source–gate and gate–drain spacings.

RF Performance of High Transconductance and High-Channel-Mobility Surface-Channel Polycrystalline Diamond Metal-Insulator-Semiconductor Field-Effect Transistors

The RF device potential of surface-channel polycrystalline diamond metal-insulator-semiconductor field-effect transistors (MISFETs) is demonstrated for the first time. Utilizing a self-aligned gate field-effect transistor (FET) fabrication process, effective transconductance of 70 mS/mm is realized at 0.7 µm gate length. This FET also shows high fT and fmax of 2.7 and 3.8 GHz, respectively. However, the breakdown voltage and fmax/fT ratio are lower than those for the homoepitaxial layer because of the parasitic capacitance at the grain boundaries in the drain region. Because of the fluctuation of channel mobility, the fluctuation of gm and fT is observed. In order to realize high-power operation at high frequency, the fabrication of the FET on a single grain to reduce the parasitic capacitance is required.

DC and RF characteristics of 0.7-μm-gate-length diamond metal–insulator–semiconductor field effect transistor

Abstract A 0.7-μm-gate-length metal–insulator–semiconductor field effect transistor (MISFET) was fabricated on a hydrogen-terminated diamond surface conductive layer. The maximum transconductance of 100 mS/mm was obtained by DC measurement. The cut-off frequency of 11 GHz and the maximum frequency of oscillation of 18 GHz were achieved for the fabricated MISFET biased at VGS=0 V and VDS=−12 V. These are the highest values for diamond MISFETs ever reported. In the MISFET, high-frequency small-signal equivalent circuit analysis is carried out at VGS=0 V and VDS=−3, −5, −8, −10 and −12 V. The analysis indicates that the reduction of parasitic resistance between the source and gate is necessary for realizing higher output power.

RF performance of diamond MISFETs

A cutoff frequency (f/sub T/) of 11 GHz is realized in the hydrogen-terminated surface channel diamond metal-insulator-semiconductor field-effect transistor (MISFET) with 0.7 /spl mu/m gate length. This value is five times higher than that of 2 /spl mu/m gate metal-semiconductor (MES) FETs and the maximum value in diamond FETs at present. Utilizing CaF/sub 2/ as an insulator in the MIS structure, the gate-source capacitance is reduced to half that of the diamond MESFET because of the gate insulator capacitance being in series to the surface-channel capacitance. This FET also exhibits the highest f/sub max/ of 18 GHz and 15 dB of power gain at 2 GHz. The high-frequency equivalent circuits of diamond MISFET are deduced from the S-parameters obtained from RF measurement.

Microwave performance of diamond field-effect transistors

The microwave performance of diamond metal semiconductor field-effect transistors (MESFET) and metal insulator semiconductor field-effect transistors (MISFET) fabricated on hydrogen-terminated diamond surface is investigated. A cut-off frequency of 2.2 GHz is obtained on a 2 µm Cu gate MESFET with a transconductance of 70 mS/mm. A cut-off frequency of 11 GHz is obtained on a 0.7 µm gate MISFET with a transconductance of 40 mS/mm. Despite the lower transconductance, the cut-off frequency of MISFET is higher than that of MESFET due to not only gate minimization but also increased carrier mobility due to the use of CaF2 as the gate insulator. High-frequency equivalent circuits are derived from S-parameters for MISFET with various gate lengths. Reduction of gate-source parasitic resistance and capacitance in MISFET by the improvement of device structure yield high frequency performance.

High frequency application of high transconductance surface-channel diamond field-effect transistors

High frequency operations of diamond field-effect transistors (FETs) on the hydrogen-terminated surface channel are realized for the first time. The cut-off frequency (f/sub T/) and maximum oscillation frequency (f/sub max/) of surface-channel diamond metal-semiconductor (MES)FET with 2 /spl mu/m gate length are 2.2 and 7 GHz respectively. Due to the effect of gate insulator insertion, the source-gate capacitance (C/sub GS/) of surface-channel diamond (MIS) FET is reduced as half as that of diamond MESFETs. The 1 /spl mu/m gate MISFET shows higher f/sub T/ of 4.8 GHz and f/sub max/ of 11 GHz in spite of comparatively low transconductance. An f/sub T/ of more than 20 GHz is expected at 0.5 /spl mu/m gate MISFET, because transconductance of a 90 mS/mm diamond MISFET with 1 /spl mu/m gate length has been already demonstrated.