High-performance CVD Graphene Radio Frequency Devices And Circuits

Graphene,a gapless two-dimensional material with the merits of ultrahigh carrier mobility,high velocity,high thermal conduction,and good bendability,has been widely investigated for wide applications ranging from high-frequency circuits,gas sensors,flex-ible electronics and so on.In the field of radio frequency(RF)electronics,graphene RF devices and circuits have been studied by academia for more than 10 years.As of today,tremendous progress has been made in graphene RF transistors with cutoff frequency f _T of more than 400 GHz and maximum oscillation frequency f_max of up to 200 GHz.On the one hand,however,most results reported for graphene RF electronics are from individual or few devices and wafer-scale uniformity analysis is still lacking.On the other hand,the unsatisfactory power gain values in monolayer graphene(MLG)devices have become a bottleneck for high-gain graphene wireless communication circuits.In this thesis,we have systematically studied the uniformity of wafer-scale MLG RF devices and low-frequency noise and RF characteristics based on Bernal-stacked bilayer graphene(AB-BLG).The detailed research contains the following parts:1.Growth of wafer-scale MLG film and large-area AB-BLG domains:large-area graphene film was grown on the inner surface of copper pocket by chemical vapor deposi-tion(CVD)method.After growth,the graphene film was transferred onto a 4-in.high-resistivity Si substrate covered with an atomic layer deposited(ALD)HfSiO dielec-tric by a chemical etching transfer method.The average full width at half maximum(FWHM)of 2D peaks in wafer-scale graphene Raman spectrum is 35.57 cm^-1with a standard deviation of 2.33 cm^-1,and the corresponding coefficient of variation(CV)of the FWHM is only about 0.07.In addition,the proportion of multilayer graphene in the whole wafer is less than 6%,indicating that wafer-scale MLG film with high uniformity is ob-tained.Furthermore,AB-BLG was grown on the exterior of the copper pocket by an ex-ternal diffusion mechanism.After 120 min of growth,AB-BLG with multiple domains of around 100?m can be obtained.Larger AB-BLG domains more than 600?m can be ob-tained by further increasing the growth time.2.Dual-gate graphene devices fabrication and low-frequency noise characteristics of dual-gate AB-BLG devices:We developed two approaches to grow high-quality top-gate dielectric on graphene.The bandgap of more than 40 me V was obtained by a low-temperature measurement.Then we presented an experimental analysis of low-frequency noise in dual-gate graphene transistors based on CVD-grown AB-BLG.The results reveal an obvious M-shaped characteristic for the top-gate voltage dependent noise behavior at different fixed back-gate voltage,which can be well described by a charge-noise model.At 10 Hz,the minimal area normalized noise spectral density at room temperature reaches as low as 3×10^-10?m²·Hz^-1,which is among the best results reported for graphene devices.3.Graphene device physics and wafer-scale MLG RF devices:We firstly introduced basic RF measurement knowledge,including S-parameter definition,RF system calibra-tion,and device de-embedding process.Then we presented a detailed analysis on the physical characteristics of graphene RF devices and proposed a corresponding small-signal model.After the theoretical analysis,a comprehensive statistical analysis of the uniformity of DC and RF characteristics simultaneously for hundreds of transistors in a 4-in.HfSiO/Si wafer was carried out for the first time.For devices with the same gate length of 0.8?m,the CV for Dirac point voltage and peak transconductance is only 0.07and 0.03,respectively.For all devices,the peak transconductance is 398.7?S/?m with a standard deviation of 53.3?S/?m,resulting in a CV of about 0.13.In addition,the intrin-sic f _T*L_g is high up to about 20 GHz·?m,which is among the best value for the graphene RF transistors.More importantly,the CV for the f_max/f _T is as low as 0.11.These results show that high-performance wafer-scale graphene RF devices with high uniformity were obtained.4.AB-BLG RF transistors and circuits:We firstly focused on improving the DC and RF performance of individual AB-BLG devices by using different back-gate dielectrics and top-gate metal with different thickness.These AB-BLG devices show large output current density with excellent current saturation.Meanwhile,the AB-BLG device RF per-formance metrics including f _T,f_max,|Z₂₁/Z₁₁|and|S₂₁|² are significantly improved com-pared that of MLG devices.Both f _T and f_max at 4.3 K show a more than 20%increase compared to that of room temperature.Also,when the temperature reaches 500 K,the f _T*L_g is high up to 7.8 GHz·?m.Then we systematically studied the performance of AB-BLG mixers.For the active mixer,the conversion gain improves with the increase of device On/off ratio and a record conversion gain of-7 dB has been demonstrated,surpas-sing all previous results,and approaching that of commercial ?-? mixers.For the passive resistive mixer,both of the conversion loss(CL)and input third-order intercept point(IIP3)decrease when the device On/off ratio increases.The minimum CL is as low as 12.7 dB and the maximum IIP3 is high up to 23 dBm.