Research On Characteristics Of Enhancement-Mode Diamond Field Effect Transistors

The ultra-wide band gap semiconductor diamond material is very suitable for the preparation of power devices for its excellent characteristics.At present,most of the diamond devices working as normally-on devices,but for high power high frequency applications,enhancement-mode devices are usually more needed to simplify the design of drive circuits,reduce power losses and improve circuit reliability.To date,there are few reports on enhancement-mode diamond devices both at home and abroad.And the enhancement-mode devices already known have several problems such as a greater on-resistance and a lower source drain current.On the other hand,scaling down the device theoretically can increase the source drain current and the cut-off frequency of the device to optimize the device performance,but when the channel length reaches the sub-micron stage,the emergence of short channel effect will bring new challenges to device design and application.In view of the above problems,the research on diamond enhanced field effect devices is carried out in this paper,and the specific work is as follows:1.An enhancement-mode MESFET device with a threshold voltage of-0.4 V was fabricated on polycrystalline diamond.The MESFET with MoO₃ passivation layer shows a source-drain saturated current of-51.53 mA/mm,a maximum transconductance of 19.16mS/mm,which is 2 times the corresponding parameters of MESFET without passivation.And the MESFET with MoO₃ layer shows better temperature stability.The enhancement-mode hydrogen-terminated diamond?H-diamond?Schottky diode with MoO₃ passivation layer is analyzed.The enhancement-mode aluminum/H-diamond Schottky barrier height is 0.623 eV,and the effective barrier thickness is 9.7 nm.The capacitance dispersion is very small since the series resistance is low.These results indicate that the MoO₃ passivation layer optimizes the electrical characteristics and temperature stability of the device effectively.2.A high performance enhancement-mode Al₂O₃/H-diamond MOSFET is realized for the first time by combining low temperature annealing and partial oxygen channel.The enhancement-mode FET was achieved due to the surface characteristics of the partial C-O channel below the gate and the fixed positive charge in the dielectric layer.The electrical characteristics of devices with gate length of 2⁴0?m are analyzed.It is found that the threshold voltage of the device with a shorter gate length shifts forward.The device with a gate length of 2?m shows a threshold voltage of-0.53 V,a source-drain current of-51.6mA/mm,a maximum transconductance of 20 mS/mm,and a subthreshold swing of 92mV/dec.The C-V results show that the oxide capacitance C_ox is 0.497 F/cm²,and the positive fixed charge density in the oxide layer is about 2.67×10¹² cm^-2.The electrical performance of the device has reached the international advanced level.3.By analyzing the electrical characteristics of the enhancement-mode Al₂O₃-FATFET devices,the dielectric layer quality and the holes transport characteristics of the device are further studied.The gate leakage current analysis suggests that the dominant gate leakage mechanism changes from the TFE mechanism to the F-N tunneling mechanism when gate voltage increased.The hysteresis phenomenon of C-V indicates that there are positive mobile ions exist in the gate dielectric layer.Finally,the relationship between the effective mobility(?_eff)of the hole in channel and the effective vertical electric field F_eff is extracted.The low field mobility of 76 cm²/Vs was calculated by empirical model.4.The characteristics of the enhancement-mode H-diamond MOSFETs with sub-micron gate length are analyzed by TCAD simulation for the first time.The DC/AC characteristics of devices with gate length of 4?m⁵0 nm are simulated,and the variation of DC/AC parameters with gate length is summarized.The 50-nm device shows a saturated source-drain current of-589.8 mA/mm,a maximum transconductance of 171.7 mS/mm and a cut-off frequency of 67.82 GHz.It is found that for our simulation structure,the device characteristics begin to deviate significantly from the long-channel devices after the gate length decreases to 300 nm.There are three main reasons for this deviation.Firstly,the high electric field and the short gate length lead to the drain induction barrier lower effect?DIBL?,which leads to additional holes in the channel.Secondly,the series resistance voltage divider is becoming more and more significant.Thirdly,the high electric field degrades channel holes mobility.In summary,on the basis of the existing research on H-diamond FETs,high performance enhancement-mode diamond devices are realized by a more moderate process.And the electrical characteristics,temperature-dependent characteristics,carrier transport characteristics and interface characteristics of the device are analyzed.On the basis of experiments,a corresponding simulation model is established,and the effect of channel length on device performance is analyzed.The effect of short channel effect on devices is also analyzed,which lays a foundation for the preparation of short channel enhancement-mode diamond devices.