Hiroaki Ishizaka

Over 20-GHz cutoff frequency submicrometer-gate diamond MISFETs

Submicrometer-gate (0.2-0.5-/spl mu/m) diamond metal-insulator-semiconductor field-effect transistors (MISFETs) were fabricated on an H-terminated diamond surface. The maximum transconductance in dc mode reaches 165 mS/mm, while the average transconductance is 70 mS/mm in submicrometer-gate diamond MISFETs. The highest cutoff frequency of 23 GHz and the maximum frequency of oscillation of 25 GHz are realized in the 0.2-/spl mu/m-gate diamond MISFET. From the intrinsic transconductances or the cutoff frequencies, the saturation velocities are estimated to be 4/spl times/10/sup 6/ cm/s in the submicrometer-gate FETs. They are reduced by gate-drain capacitance and source resistance.

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.

Deep sub-micron gate diamond MISFETs

Abstract The basic characteristics of the short channel FETs have been investigated for the high-frequency performance of the diamond MISFETs. The deep sub-micron gate (0.23–0.5 μm) diamond MISFETs were fabricated on an H-terminated diamond surface. The short channel effect is well suppressed utilizing thin gate insulator. Accordingly, the transconductance is improved by reduction of gate length down to 0.2 μm. This tendency is observed in diamond MISFETs for the first time. Maximum transconductance reaches 71 mS/mm in DC mode and 51 mS/mm in AC mode. The f T of 40 GHz is expected in 0.2 μm gate MISFETs as a result.

Cryogenic operation of surface-channel diamond field-effect transistors

Abstract Cryogenic operation of field-effect transistors (FETs) fabricated on hydrogen-terminated (H-terminated) diamond surface conductive layers is investigated. 5-μm gate-length metal-insulator-semiconductor FETs (MISFETs) is fabricated using CaF2 film as a gate insulator. The MISFETs operate successfully even at 4.4 K. At low temperature, the contact between source/drain electrode and H-terminated diamond surface cannot maintain ohmic characteristics, because the thermal activation energy of the carriers is not high enough to overcome the barrier height at the interfaces between the source electrode and the H-terminated diamond. Estimated channel mobility increases from 63 cm2/V-s to 137 cm2/V-s and the maximum transconductance increases from 10.5 mS/mm to 14.5 mS/mm, as the temperature decreases from 300 K to 4.4 K, indicating reduced phonon scattering of the channel.

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.

Cryogenic operation of diamond surface-channel electronic devices

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 demonstrated the low-temperature (4.4 K) operation of the diamond FETs. Moreover, we also fabricated in-plane-gate FET structure and in-plane-gate single-electron transistor on diamond surface, and cryogenic operations are performed. DC characteristics of a 5 pm-gate Cu/CaF2ldiamond MISFET are shown in Fig. l. FET operates successfully at cryogenic temperature, even though the carrier freeze-out below the specific critical temperature was reported in diamond surface conductive layer [2]. In the DC characteristic at 4.4 K, drain current suppression occurs at V6s:0, which is related to the carrier fteeze out. However, in the FET structure, carrier can be induced by the sufficient field effect (field effect doping). In the low-temperature characteristics, we have to consider the drain threshold voltage which appears at Vns-0.3V. This is due to the energy barrier existing between the source/drain electrode and the surface conductive layer. At 300 K, caniers have enough thermal energy to overcome such barrier. However, at low temperatures, this small potential barrier is higher than thermal energy of carriers. This barrier is remarkably reduced by the longitudinal elechic field that results from the drain bias Vos. Diamond is a promising semiconductor material for the future electronics. Owing to its high break down field (107 V/cm), extremely high thermal conductivity (20 WcmK), high hote mobility (1800 r*t/Vs) 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. Up to now, the fabrication and the operation of MESFETs and MISFETs have been demonstrated using a surface conductive layer [1]. 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 for 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