Power transistors are key components of the power amplifiers in the transmitters of wireless communication systems [1Lai Y-L, Chang EY, Chang CY, et al. 5mm high-power-density dual-delta-doped power HEMT’s for 3 V L-band applications. IEEE Electron Device Lett 1996; 17(5): 229-31.
[http://dx.doi.org/10.1109/55.491838] -14Chang EY, Lai Y-L, Lee YS, Chen SH. GaAs/AlAs wet selective etch process for the gate recess of GaAs power metal-semiconductor field-effect transistors. J Electrochem Soc 2001; 148(1): G4-9.
[http://dx.doi.org/10.1149/1.1344555] ]. There is a great demand for high-performance power transistors which meet the requirements of advanced wireless communication systems [15Kim J, Kim D, Cho Y, et al. Supply modulator for envelope-tracking operation of dual-mode handset power amplifier In: Proceedings of European Microwave Conference (EuMC); 6-10 Oct 2013; Nuremberg, Germany; 2013; pp 1315-8. Available from: http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6686907&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D6686907-23Choi P, Goswami S, Radhakrishna U, et al. A 5.9-GHz fully integrated GaN frontend design with physics-based RF compact model. IEEE Trans Microw Theory Tech 2015; 63(4): 1163-73.
[http://dx.doi.org/10.1109/TMTT.2015.2405913] ]. Ion-implanted GaAs power metal-semiconductor field-effect transistors (MESFETs) have been studied as to their wireless communication applications [24Maeda M, Nishijima M, Takehara H, et al. A 3.5V, 1.3W GaAs power multichip IC for cellular phone. In: Proceedings of IEEE GaAs IC Symposium Digest; 10-13 Oct. 1993; San Jose, California, USA; 1993; pp. 53-6.
[http://dx.doi.org/10.1109/GAAS.1993.394501] -29Chang EY, Fuh C-S, Meng CC, Wang KB, Chen SH. A planar gate double beryllium implanted GaAs power MESFET for low voltage digital wireless communication application. IEEE Trans Electron Dev 2000; 47(6): 1134-8.
[http://dx.doi.org/10.1109/16.842954] ]. A GaAs power multi-chip integrated circuit using ion-implanted GaAs power MESFETs delivered an output power (P_out) of 1.3 W at 3.5 V for cellular phone communications from 890 to 950 MHz [24Maeda M, Nishijima M, Takehara H, et al. A 3.5V, 1.3W GaAs power multichip IC for cellular phone. In: Proceedings of IEEE GaAs IC Symposium Digest; 10-13 Oct. 1993; San Jose, California, USA; 1993; pp. 53-6.
[http://dx.doi.org/10.1109/GAAS.1993.394501] ]. High-efficiency GaAs power MESFETs with an asymmetrical lightly-doped drain structure gave 32.5-dB output power at 4.7 V for 950-MHz communications [25Yanagihara M, Ota Y, Nishii K, Ishikawa O, Tamura A. Highly efficient GaAs power MESFETs with n+-asymmetrical LDD structure. Electron Lett 1992; 28(7): 686-7.
[http://dx.doi.org/10.1049/el:19920433] ]. Conventional ion-implanted GaAs power MESFETs were developed for analog communication applications. Instead of analog wireless communications, digital wireless communications have dominated the wireless trend because of their higher capacity. An analog/digital dual-mode power module with ion-implanted GaAs power MESFETs showed a power-added efficiency (PAE) of 56% at 3.7 V for the North American digital cellular (NADC) communications [26Masato H, Maeda M, Fujimoto H, et al. Analogue/digital dual power module using ion-implanted GaAs MESFETs. In: Proceedings of IEEE MTT-S International Microwave Symposium Digest; 16-20 May 1995; Orlando, Florida, USA; 1995; 2: pp. 567-70.
[http://dx.doi.org/10.1109/MWSYM.1995.406023] ]. A 36-mm GaAs power MESFET using selective ion implantation and semi-insulating setback layer technologies delivered a P_out of 31.5 dBm at 5.8 V for digital communication applications [27Furukawa H, Tateoka K, Miyatsuji K, Sugimura A, Ueda D. A novel GaAs power MESFET with low distortion characteristics employing semi-insulating setback layer under the gate. IEEE Trans Electron Dev 1996; 43(2): 193-200.
[http://dx.doi.org/10.1109/16.481717] ]. Most ion-implanted GaAs power MESFETs have been implemented by means of selective ion implantation technology [24Maeda M, Nishijima M, Takehara H, et al. A 3.5V, 1.3W GaAs power multichip IC for cellular phone. In: Proceedings of IEEE GaAs IC Symposium Digest; 10-13 Oct. 1993; San Jose, California, USA; 1993; pp. 53-6.
[http://dx.doi.org/10.1109/GAAS.1993.394501] -27Furukawa H, Tateoka K, Miyatsuji K, Sugimura A, Ueda D. A novel GaAs power MESFET with low distortion characteristics employing semi-insulating setback layer under the gate. IEEE Trans Electron Dev 1996; 43(2): 193-200.
[http://dx.doi.org/10.1109/16.481717] ]. The alignment and process of the n⁺ source and drain selectively-implanted regions are critical for the characteristics of MESFETs. Lai and coworkers first reported on GaAs power MESFETs using direct ion implantation technology, with silicon (Si) and beryllium (Be) ion implantations, for low-voltage digital wireless communication applications [28Lai Y-L, Chang EY, Chang C-Y, et al. High-efficiency and low-distortion directly-ion-implanted GaAs power MESFET’s for digital personal handy-phone applications. IEEE Electron Dev Lett 1997; 18(9): 429-31.
[http://dx.doi.org/10.1109/55.622519] ]. A 2.2-V-operation GaAs power MESFET with a gate width of 2 mm demonstrated an adjacent channel power rejection (ACPR) of –58 dBc and a PAE of 57.2% at a P_out of 21.3 dBm. A planar-gate GaAs power MESFET with direct Si and Be ion implantations also showed good characteristics at 3.6 V [29Chang EY, Fuh C-S, Meng CC, Wang KB, Chen SH. A planar gate double beryllium implanted GaAs power MESFET for low voltage digital wireless communication application. IEEE Trans Electron Dev 2000; 47(6): 1134-8.
[http://dx.doi.org/10.1109/16.842954] ]. GaAs power MESFETs with direct ion implantations provide a cost-effective solution for digital wireless communications [30Feng M, Shen S-C, Caruth DC, Huang J-J. Device technologies for RF front-end circuits in next-generation wireless communications. Proc IEEE 2004; 92(2): 354-75.
[http://dx.doi.org/10.1109/JPROC.2003.821903] ].

In this study, GaAs power MESFETs were fabricated by means of direct double Si ion implantations for digital wireless communication applications. The power characteristics of the MESFETs including P_out, PAE, gain, and ACPR were demonstrated.

The GaAs power MESFETs were fabricated with double ²⁹Si⁺-ion implantations into a semi-insulating GaAs wafer. Channel implantation was conducted by ²⁹Si⁺ ion implantation with an energy of 200 keV and a dosage of 3.0 × 10¹² cm^-2 to form a channel layer with high current capability. Surface implantation was implemented by ²⁹Si⁺ ion implantation with an energy of 50 keV and a dosage of 1.0 × 10¹² cm^-2 to form a low-resistivity ohmic layer.

The GaAs wafer was sputtered with silicon nitride film for the passivation process after double ²⁹Si⁺-ion implantations. The passivation process was used to prevent the decomposition of the GaAs material and the loss of arsenic atoms during the subsequent activation process with high-temperature annealing. The activation process was carried out by rapid thermal annealing (RTA) at 800°C. The sheet resistance of wafer after annealing was 512 Ω/□.

The Au/Ge/Ni/Au ohmic metal with a thickness of 0.4 μm was deposited by an electron gun system and alloyed by RTA at 330°C. The specific contact resistance of the ohmic contact was in the 10^-7-Ω∙cm² range. The Ti/Pt/Au gate metal with a gate length of 1 μm was formed after a gate recess process. The gate widths of the MESFETs included 150 μm and 15 mm. A silicon nitride passivation layer was deposited using a plasma-enhanced chemical vapor deposition (PECVD) system in order to protect the device from humidity and contamination. The fabricated power MESFET with a gate width of 15 mm is shown in Fig. (1). Once the front-side process was completed, the backside of the wafer was lapped to 4 mils and plated by gold metal in order to improve the thermal conductance.

Fig. (1) Fabricated GaAs power MESFET with a gate width of 15 mm.

The on-wafer measurements of the DC characteristics of the MESFETs, including the current-voltage and breakdown characteristics, were conducted in order to monitor and control the processes during the fabrication of the MESFETs. A MESFET with a gate width of 150 μm, rather than 15 mm, was used for the on-wafer DC measurement, because devices with a small gate width would not generate a current large enough to damage themselves. Fig. (2) shows the current-voltage characteristics of the MESFET with a gate width of 150 µm after the Si₃N₄ passivation. The saturation drain current of the device was 35.7 mA, corresponding to a current density of 238 mA/mm. Fig. (3) shows the gate-to-drain breakdown characteristics after the Si₃N₄ passivation. The gate-to-drain breakdown voltage was 17 V. The current-voltage characteristics of the 150-μm MESFET after the airbridge process are shown in Fig. (4). The saturation drain current of the MESFET was 35 mA, corresponding to a current density of 233 mA/mm. The saturation drain current characteristics after the Si₃N₄ passivation and airbridge processes were very close.

Fig. (2) Current-voltage characteristics of 150-µm MESFET after Si3N4 passivation.

Fig. (3) Gate-to-drain breakdown characteristics after Si3N4 passivation.

The gate-to-drain breakdown characteristics after the airbridge process are shown in Fig. (5). The gate-to-drain breakdown voltage was 17 V. The gate-to-drain breakdown characteristics after the Si₃N₄ passivation and airbridge processes were the same.

Fig. (4) Current-voltage characteristics of 150-µm MESFET after airbridge process.

The power characteristics of the MESFET with a gate width of 15 mm were measured by means of a load-pull power tuning system. The impedances of the input and output tuners of the system were tuned to obtain the optimum source and load matching for maximum P_out and PAE. The measurement frequency was 1.88 GHz. The MESFET was biased at operation voltages of 3.4 V and 4.7 V. When the MESFET was biased at a drain voltage of 3.4 V, it was tested under a class-AB condition with a quiescent drain current of 600 mA.

When the MESFET was biased at a drain voltage of 4.7 V, it was expected that it would have better power performance than the one biased at 3.4 V because of the higher voltage supply. A more stringent condition, a deep class-B condition with a quiescent drain current of 50 mA, was adopted at 4.7 V in order to evaluate the extreme power capability of the MESFET as far as the linearity was concerned.

Fig. (5) Gate-to-drain breakdown characteristics after airbridge process.

Fig. (6) depicts the P_out and PAE as functions of the input power for the MESFET biased at 3.4 V and 4.7 V. The device operating at 3.4 V demonstrated a maximum P_out of 31.1 dBm and a maximum PAE of 58.0%. The P_out at the 1-dB gain compression point (P_1dB) and the associated PAE were 29.2 dBm and 45.0%, respectively. The MESFET operating at a drain voltage of 4.7 V achieved a maximum P_out of 33.1 dBm and a maximum PAE of 55.9%. At a P_1dB of 31.8 dBm, the MESFET operating at 4.7 V exhibited a PAE of 47.1%. It was found that the MESFET operating at 4.7 V displayed better maximum P_out and P_1dB, even under a deep class-B condition, than one operating at 3.4 V under a class-AB condition.

Fig. (6) Pout and PAE as a function of input power for 15-mm MESFET biased at 3.4 V and 4.7 V.

Fig. (7) shows the power gain as a function of the input power for the MESFET biased at 3.4 V and 4.7 V. The linear gain characteristics of the MESFET operating at 3.4 V and 4.7 V were 11.3 dB and 11.4 dB, respectively. The linear gain characteristics of the MESFET at 3.4 V under the class-AB condition were comparable to those at 4.7 V under the deep class-B condition. The GaAs power MESFET using double ion implantation technology demonstrated high P_out and PAE. In addition, the high P_1dB associated with high efficiency indicated high linearity and low distortion for digital wireless communication applications.

Fig. (7) Power gain as a function of input power for 15-mm MESFET biased at 3.4 V and 4.7 V.

The 15-mm MESFET was tested for digital wireless applications. The device was measured using modulated IS-95 code-division multiple access (CDMA) standard signals. The signal source was the Anritsu-MG3670C digital modulation signal generator. The output power spectrum of the device was measured by the HP-8595E spectrum analyzer. The data were taken when the device was biased at a drain voltage of 4.7 V under a deep class-B condition. The quiescent drain current of the device was 50 mA. Fig. (8) shows the CDMA output power spectrum of the MESFET. At a P_out of 28 dBm, the device exhibited an ACPR of –31.2 dBc at +1.25 MHz apart from the 1.88-GHz center frequency, and an ACPR of –45.7 dBc at +2.25 MHz. The power MESFET achieved excellent ACPR characteristics at an extremely low bias current for CDMA wireless communication applications.

In this study, the GaAs power MESFETs were fabricated using double ²⁹Si⁺-ion implantations, including channel implantation with an implantation energy of 200 keV and an implantation dosage of 3.0 × 10¹² cm^-2, and surface implantation with an implantation energy of 50 keV and an implantation dosage of 1.0 × 10¹² cm^-2. On the basis of the study results, more implantation energy and dosage conditions can be set as parameters for further study of the characteristics of GaAs power MESFETs with respect to various modern wireless communication applications.

Fig. (8) CDMA output power spectrum of 15-mm MESFET.

The GaAs power MESFETs were fabricated by means of double Si ion implantation technology for wireless communications. The power performance of the MESFET for digital wireless communication applications was studied. The MESFET operating at 3.4 V and 1.88 GHz achieved a P_out of 31.1 dBm, a PAE of 58.0%, and a linear gain of 11.3 dB. At a deep class-B condition, the MESFET biased at 50 mA and 4.7 V exhibited an ACPR of –31.2 dBc at +1.25 MHz apart from the 1.88-GHz center frequency, while the P_out was 28 dBm. The power MESFET using direct double Si ion implantations, with the advantages of high yields and low costs, demonstrated excellent power characteristics for digital wireless communication applications.