*4.5. What Is the Best Approach?*

From what was described in the previous sections, it can be concluded that there is no universally best approach, and each one has its own pros and cons.

GaN-on-diamond is undoubtedly the more mature technology. Large-area deposition capability is a significant advantage and, despite being relatively complex, the fabrication process has been optimized and the main issues have, at least partially, been solved. Other companies, in addition to Element Six, have been involved in the process. In 2019 RFHIC reported a manufacturing procedure that allows the fabrication of 4" GaN-on-diamond HEMTs using a laser drilling process [102]. GaN-on-diamond HEMTs and RF power amplifiers can be currently purchased from Qorvo and Akash Systems, Inc. However, and despite the maturity and success of this technology, some room for improvement still exists (namely the decrease of the gate leakage current), and exciting improvements are expected in the coming years.

The bonding of GaN and diamond SCD substrates is also reaching a high level of maturity, and companies such as Mitsubishi Electric Corp. and Fujitsu Limited have reported the successful transfer of GaN and GaN-on-SiC HEMTS to diamond substrates. This anticipates a bright future for bonded GaN/diamond devices. On one side, these devices show a potential for decreasing the *TBR*GaN/diamond below the minimum achievable with GaN-on-diamond technology, since the 30 nm-thick SiN dielectric layer can be replaced by a 2–10 nm-thick Si-based layer. However, since the reported GaN HEMTs have been bonded to SCD substrates, this technique will have a significantly lower yield than the GaN-on-diamond approach and the full scaling up of the technology will be more challenging. If, however, future research deems the bonding of GaN HEMTs and large area PCD substrates feasible and reproducible this technique may compete with the GaN-on-diamond technology.

The epitaxy of GaN on diamond substrates, though feasible, may not bring any realistic advantage. On one side, the best quality GaN films have been grown on low-area SCD substrates. The ELO of GaN has made growth of GaN films with low dislocation density possible on PCD substrates, however functional HEMTs are yet to be demonstrated. On the other side, in the majority of approaches reported so far the nucleation and strain relief layers are part of the final HEMT material stack, and they will hinder the transport of heat to the diamond substrate.

Despite the low κ of the AlGaN barrier layer the capping of passivated HEMTs with a thin diamond film is expected to decrease the peak temperature of the devices between 8% and 20% in comparison with GaN-on-SiC and GaN-on-Si HEMTs, respectively. Despite being a relatively modest number—if compared with the improvement obtained with latest generation GaN-on-diamond devices—this approach is technologically simple, since the diamond can be deposited directly on the passivation layer, and allows for large area growth. A bright future is anticipated also for this approach, as proven by the work recently reported by Fujitsu researchers.

The comparative advantages and disadvantages of each approach are further summarized in Table 7.


**Table 7.**Current advantages and disadvantages of each approach.

 Severelimitation

#### **5. Conclusions**

The integration of diamond and GaN devices has been an active research topic for 20 years. The involvement of companies like Fujitsu and Mitsubishi, for instance, is representative of the impact that hybrid GaN/diamond electronic devices can have on some applications.

The integration of diamond and GaN has been achieved by different methods: the direct CVD of the diamond films on the back of GaN wafers, the bonding of HEMTs and diamond substrates, the direct epitaxy of the GaN layers on diamond substrates, and the diamond capping of passivated HEMTs. The technological advances, the room for improvement, and the advantages/disadvantages of each method have been presented and discussed.

Generally speaking, the fabrication of diamond-on-GaN wafers by direct diamond CVD on the back of the GaN wafers has been quite successful and commercial RF power amplifiers fabricated on GaN-on-diamond wafers are currently available for satellite communications. The bonding of GaN HEMTs and SCD substrates and the capping of GaN HEMTs have also been raising interest from companies such as Mitsubishi Electric Corp. and Fujitsu Limited. Recent advances in the epitaxial growth of GaN layers on PCD substrates anticipate interesting technological developments in a near future. Far from having reached the limits of the technology, it can be thus said that the integration of diamond and GaN will remain an active research topic in the years to come, involving academic and industrial players, with the ultimate goal of increasing the power density and reliability of GaN HEMTs.

**Author Contributions:** Conceptualization, J.C.M.; investigation, J.C.M.; writing—original draft preparation, J.C.M.; writing—review and editing, J.C.M., M.L. and C.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was co-funded by EU funds under the project UIDB/50008/2020-UIDP/50008/ 2020. Joana C. Mendes was hired by Instituto de Telecomunicações under the decree law Nr. 57/2016.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Appendix A. Performance of Diamond/GaN HEMT Transistors**


**Table A1.**Relevant electrical parameters of HEMTs fabricated on GaN-on-diamond wafers.

a *NF*—number of fingers; *L*G—gate length (µm); *W*G—gate width (µm); *L*SD—source-drain spacing (µm); *<sup>L</sup>*GD—gate-drain spacing (µm); *L*SG—source-gate spacing (µm); PW—pulse width; DC—duty cycle). b Fully packaged device.

#### **Table A2.** Relevant electrical parameters of HEMTs fabricated on GaN/diamond bonded wafers.



**Table A2.** *Cont.*

a

*NF*—number of fingers; *L*G—gate length (µm); *W*G—gate width (µm); *P*G—gate pitch (µm); PW—pulse width.




**Table A3.** *Cont.*

a

 *NF*—number of fingers;*L*G—gate length (µm);*W*G—gate width (µm);*L*SD—source-drain spacing (µm);*<sup>L</sup>*GD—gate-drain spacing (µm);*L*SG—source-gate spacing (µm).

**Table A4.** Relevant electrical parameters of HEMTs featuring capping diamond.


a *NF*—number of fingers; *L*G—gate length (µm); *W*G—gate width (µm); *L*SD—source-drain spacing (µm).
