Abstract
1. Introduction
Due to their wide band gap (5.47 eV), high breakdown electric fields (~10 MV/cm), large thermal conductivity (22 W/(cm·K)) and large carrier saturation velocity (~1 × 107 cm/s), hydrogen-terminated diamond field-effect transistors (FETs) are promising materials for high-power and high-frequency devices[
In this work, a comparative study was performed on the DC and RF performance of hydrogen-terminated polycrystalline and single crystal diamond FETs considering the influence of defect concentration of the diamond substrates. A self-aligned fabrication process was used to fabricate the diamond FETs. Ohmic contact metal was Au and gate dielectric was self-oxidized alumina.
2. Experiments
Three diamond samples were used to fabricate diamond FETs, as shown in Table 1. For the polycrystalline diamond (samples of I-PC, and II-PC), hydrogen termination was formed by the microwave plasma chemical vapor deposition (MPCVD) treatment technique in H2 plasma. For the single crystal (001) diamond sample (III-SC), the hydrogen-termination was formed by homoepitaxial growth process as stated in Ref. [7]. Micro-Raman scattering experiments with laser line of 532.2 nm and powder X-ray diffraction (XRD) were performed at RT. The self-aligned fabrication process of the diamond FETs can be found in our previous study[
3. Results and discussion
Fig. 1 shows the Raman spectra and XRD pattern of the three diamond samples. It can be seen that the background of the sample I-PC is very small. But for the samples II-PC and III-SC, the background line shows a significant upward movement, which is proven to be due to the increase of defect and impurity content[
Figure 1.(Color online) (a) Raman spectra of the I-PC, II-PC, and III-SC diamond samples. (b) XRD pattern of I-PC, and II-PC diamond samples.
The gate length and source–drain space of the three diamond samples were shown in Table 1[
Pulsed I–V characteristics for the III-SC sample at different quiescent bias points were measured, as shown in Fig. 2. The measured results for the I-PC and II-PC samples have been shown in our previous work[
Figure 2.(Color online) Pulsed
The small signal S-parameters of the diamond FETs were measured between 0.1–30 GHz[
Figure 3.(Color online) Relationship of cutoff frequency
The component parameters of the three diamond samples extracted from the small single parameters are shown in Table 2. The three samples show comparable cut-off frequency fT, but fmax values show big difference. The extrinsic fmax for transistors can be expressed as[
It can be seen that the parasitic resistance has strong influence on the extrinsic fmax for transistors. The fmax value of sample II-PC is the lowest. This is due to its rectangular gate structure, which makes the gate resistance Rg large, as shown in Table 2.
Fig. 4 shows the RF power output characteristic measured at 2 GHz under a continuous-wave signal (A-class) for the I-PC diamond FET. As shown in the figure, the maximum gain is 18.3 dB and the power added efficiency (PAE) is 22.9%. The maximum output power density (Pout) reaches 877 mW/mm at 2 GHz for our diamond FET. It is the best reported output power density for diamond FETs measured at 2 GHz[
Figure 4.(Color online) Large signal performance of I-PC diamond FET at 2 GHz power sweep (A-class).
where Ids-max is the maximum drain current density, Vwork is the drain voltage for the measurement of Pout, and Vknee is the knee voltage. The large-signal power gain result shows that the device exhibits a large compression even at class-A operation. The possible reasons are that the drain current density (323 mA/mm) is small, and the knee voltage (~6 V) is high for the H-terminated diamond FETs. The sheet resistance of the H-terminated diamond is high (~kΩ/□), and the parasitic resistance is high (poor Ohmic contact). Table 3 shows the compare of measured output power density and calculated output power density for the three diamond samples (I-PC, II-PC, and III-PC). The drain voltage values for the measurements of the three samples are –25, –24, and –25 V, respectively. It can be seen that for all three samples, the measured output power densities are lower than the calculated output power densities, which should be due to the trapping effects. The knee voltage will increase at continuous drain voltage and the drain current will degrade. Both of them would cause a decrease in output power. The II-PC sample shows the largest degrade in output power density. This is consistent with the pulsed I–V measurement. This sample shows the largest maximum drain current degeneration induced by drain-lag effect. These results indicate that defects and N impurities in the diamond act as traps in the carrier transport and have great influence on the output power characteristics of diamond FETs.
4. Conclusion
In summary, three kinds of diamond FETs were fabricated on polycrystalline and single crystal hydrogen-terminated diamond with different defect levels and impurity contents. Direct current and radio frequency performances analysis show that the frequency of devices depends mainly on the parasitic parameters, which are closely related to the device structure. Meanwhile, the output power density is greatly influence by the defect and impurity level of the samples. The defects and impurities in the diamond act as traps in the carrier transport. The trapping effects induce the knee voltage increase and the drain current degrade at continuous drain voltage. Diamond with high crystal quality and low impurity level is in great demand for microwave power devices.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 51702296), and Excellent Youth Foundation of Hebei Scientific Committee (Grant No. F2019516002).
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