Large Signal Characterization of GaN HEMT Transistor by Multi-Harmonic Source & Load Pull Tuner System

Shengjie Gao and Chan-Wang Park

University of Quebec in Rimouski, Rimouski, QC, G5L 3A1, Canada

Abstract In this paper, a GaN HEMT transistor is

characterized at 3.5 GHz by passive multi-harmonic source & load pull tuner system in large signal. In order to analyze the effect on PAE and output power from the source and load impedance at fundamental, 2nd and 3rd harmonic frequencies, multi-harmonic source and load pull tuner are used. By using this source & load pull tuner system, Crees GaN HEMT CGH40010 transistor is characterized at 3.5 GHz with considering the source and load impedances at 2nd and 3rd harmonic frequencies. The characterization result shows that maximum PAE could reach 74.21% with 39.92 dBm output power.

Index Terms Characterization, harmonic frequencies, power amplifier, source & load pull tuner.

I. INTRODUCTION

With the development of the modern telecommunication technology, the requirement for RF (Radio Frequency) PA (Power Amplifier) with respect to PAE (Power Added Efficiency) and output power is more stringent. In order to satisfy the increasing demand of the wireless communication technology, microwave transistor, as a key active device in PA, should be well characterized. Large signal characterization for the transistor is essential in order to estimate the output power and PAE in the non-linear domain. We can characterize the transistor under large signal excitation to have desired output power and PAE by using source and load pull tuner [1]. With this source & load pull techniques, the impedances both on the input and output of the transistor could be optimized to have target PAE and output power efficiently. With these optimum impedances, we can design PA.

For enhancing the efficiency of class F/inverse class F PA, 2nd and 3rd harmonic frequencies are usually to be considered [2]. Based on this need, the tuner manufacturing companies developed source/load pull tuner which could control multi harmonic frequencies [3] [4]. With these tuners, the source and load impedance at fundamental, 2nd and 3rd harmonic frequencies could be controlled independently [5].

In order to design a class F/inverse class F PA, the transistor should be characterized at fundamental frequency, 2nd and 3rd harmonic frequencies. In this work, a GaN HEMT transistor from Cree Inc. is characterized by multi-harmonic source & load pull tuner. In the next section, the setup of source & load pull tuner system is shown. In section III, Crees GaN HEMT CGH40010 transistor is characterized of at 3.5 GHz with

considering 2nd and 3rd harmonic frequencies by source & load pull tuner system. The characterization result is presented.

II. SETUP OF SOURCE & LOAD PULL TUNER SYSTEM

Figure 1 shows the setup for source & load pull tuner system. We use Rohde & Schwarz SMBV100A vector signal generator, Agilent E3633A DC power supply, N6705B DC power analyzer, Agilent MXA signal analyzer and Agilent N1912A P-series dual channel power meter in this system. These instruments are connected to a computer by GPIB cables. In order to control the source & load pull tuner and to obtain the measurement result from these instruments, corresponding software provided by the tuner manufacturing company is installed in the computer. The input block in the source & load tuner system consists of directional coupler, isolator and bias tee. The output block consists of bias tee, directional coupler and attenuator. [6] has analyzed the effect on the tunable region of the tuner at the DUT reference plane from these accessories in input and output block. Based on [6], the S11/S22 of the accessories such as directional coupler, isolator, bias tee and attenuator are chosen for output/input block at least less than -14 dB.

Fig. 1. Setup of source & load pull tuner system.

With this setup, the maximum tunable at the output of the transistor is 0.956 at 3.5 GHz, 0.946 at 7 GHz and 0.923 at 10.5 GHz. The maximum tunable at the input of the transistor is 0.984 at 3.5 GHz, 0.926 at 7 GHz and 0.936 at 10.5 GHz.

978-1-4673-4818-8/12/$31.00 2012 IEEE

III. PROCEDURE OF CHARACTERIZATION

In order to design a class F/inverse class F power amplifier with Crees GaN HEMT CGH40010 transistor, source & load pull tuner system is used to characterize this transistor at fundamental frequency 3.5 GHz and its 2nd and 3rd harmonic frequencies. During the procedure of CGH40010 transistor characterization, we choose the impedance which offers maximum PAE while output power is 40 dBm at each step.

A. IV curves

For class F/inverse class F operation, the transistor could be biased in class AB mode [2]. The IV curve measured by the source & load pull tuner system is shown in Figure 2. We choose -2.58V for VGS and 28V for VDS with 200mA drain quiescent current so that the transistor is biased in class AB mode. Thus, when the transistor is characterized by source & load pull tuner system, VDS and VGS are fixed to 28V and -2.58V, respectively.

Fig. 2. Measured IV curve for Crees GaN HEMT CGH40010 transistor by source/load pull tuner system.

B. Stability circle

For obtaining the stability circle of CGH40010 transistor at 3.5 GHz when the transistor is biased at VDS= 28V and IDS=200mA, the S-parameter of this transistor is measured by Agilent 8720ES VNA (Vector Network Analyzer). The setup for measuring the transistors S-parameter is shown in Figure 3. Bias tees are added to connect to the DC power supplies.

The S-parameter is measured at the measurement reference plane of VNA. In order to obtain the S-parameter at DUT reference plane, the bias tees should be de-embedded. With the de-embedded S-parameter of transistor, the stability circle at 3.5 GHz on both input and output of the transistor are outside of the Smith chart as shown in Figure 3, so the transistor is unconditionally stable.

Fig. 3. Source and load stability circle for CGH40010 transistor at 3.5 GHz.

C. Procedure of characterization of Crees GaN HEMT CGH40010 transistor

As a first step of characterization by source & load pull tuner system, load pull characterization at 3.5 GHz is performed when source impedance at fundamental frequency is fixed to 3.18-j13.30 Ohm (the source & load impedances at the 2nd and 3rd harmonic frequencies are fixed to 50 Ohm). The maximum PAE obtained in the load pull characterization is 62.41% with 40.47 dBm output power when the load impedance at 3.5 GHz is 0.603165.50 as shown in Figure 4 (a). The maximum output power is 40.73 dBm with 59.14% PAE when the impedance is 0.559173.30 as shown in Figure 4 (b). In order to obtain 40 dBm output and to have higher PAE, maximum PAE impedance point is chosen for the next step.

Then, as a second step, source pull characterization is done to find source impedance at fundamental frequency when load impedance at fundamental frequency is fixed to 0.603165.50. The source pull characterization result in Figure 5 shows that maximum PAE is 64.33% with 40.07 dBm output power when the source impedance at 3.5 GHz is 0.892-151.70. The maximum output power is 40.32 dBm with 64.09% PAE when the source impedance is 0.861-152.50. In order to have 40 dBm output power and higher

VDS= 28V

VGS= -2.58V

Drain quiescent current =200 mA

PAE, maximum PAE impedance point is chosen for further characterization. From the contour in 3D in Figure 4 and 5, we can also see that PAE is more sensitive to the impedance variation at 3.5 GHz than output power.

(a)

(b)

Fig. 4. Measured PAE (a) and output power contour (b) in 3D by load pull tuner at 3.5 GHz.

The next step is sweeping the load impedance at the 2nd and 3rd harmonic frequencies by source & load pull tuner system.

First, the load impedance at 2nd harmonic frequency is tuned when the source and load impedances at fundamental frequency are fixed to the values which are found in the previous steps (Source impedance at fundamental frequency: 0.892-151.70 and load impedance at fundamental frequency: 0.603165.50). The load impedance at 3rd harmonic frequency and source impedances at 2nd and 3rd harmonic frequencies are fixed to 50 Ohm. Figure 6 shows the measured PAE by load pull tuner at 7 GHz. PAE is increased to 68.48% with 40.06 dBm output power by tuning the load impedance at 2nd harmonic frequency to 0.938146.10. PAE is increased by 4.15% by tuning the load impedance at 2nd harmonic frequency.

(a)

(b)

Fig. 5. Measured PAE (a) and output power contour (b) in 3D by source pull tuner at 3.5 GHz.

Fig. 6. Measured PAE contour in 3D by load pull tuner at 7 GHz.

As a fourth step, load impedance at the 3rd harmonic frequency is tuned by load pull tuner. Source impedance at fundamental frequency is 0.892-151.70 and load impedance at fundamental frequency is 0.603165.50. Load

impedance at 2nd harmonic frequency is fixed to 0.938146.10 and source impedances at 2nd and 3rd harmonic frequencies are fixed to 50 Ohm. The measured PAE in Figure 7 shows that when the impedance at 3rd harmonic frequency is 0.91842.60, PAE reaches the maximum as a 69.42% with 40.01 dBm output power. By considering 3rd harmonic frequency at load, PAE is increased by 0.94%.

Fig. 7. Measured PAE contour in 3D by load pull tuner at 10.5 GHz.

After tuning the impedance at 3rd harmonic frequency, source impedance at 2nd harmonic frequency is tuned. Source and load impedance at fundamental frequency are fixed to 0.892-151.70 and 0.603165.50, respectively. Load impedances at 2nd and 3rd harmonic frequencies are fixed to 0.938146.10 and 0.91842.60, respectively. Source impedance at 3rd harmonic frequency is fixed to 50 Ohm. The measured PAE contour in Figure 8 shows that when source impedance at 2nd harmonic frequency is 0.900-38.50, the maximum PAE 73.08% can be obtained with 39.83 dBm output power. In this step, PAE is increased by 3.66%.

Fig. 8. Measured PAE contour in 3D by source pull tuner at 7 GHz.

At last, source impedance at 3rd harmonic frequency is tuned. By tuning the source impedance at 3rd harmonic frequency, the characterization result in Figure 9 shows that when source impedance at the 3rd harmonic frequency is 0.880-68.6, the maximum PAE 74.21% can be obtained with 39.92 dBm output power. In the last step, PAE is increased the by 1.13% by tuning the source impedance at 3rd harmonic frequency.

Fig. 9. Measured PAE contour in 3D by source pull tuner at 10.5 GHz.

IV. CONCLUSION

In this paper, a characterization procedure for GaN HEMT CGH40010 transistor at fundamental frequency 3.5 GHz, 2nd and 3rd harmonic frequencies by source & load pull tuner system is shown. The final result shows that maximum PAE 74.21% can be obtained when the output power is 39.92 dBm.

REFERENCES

[1] W. Liu, and C. Tsironis, Load pull characterization system for differential devices, 2003 ARFTG Microwave Measurement Symposium, pp. 201-204, 4-5 December 2003.

[2] D. Y.-T. Wu, and S. Boumaiza, 10W GaN inverse class F PA with input/output harmonic termination for high efficiency WiMAX transmitter, 2009 IEEE Wireless and Microwave Technology Conference, pp. 1-4, 20-21 April 2009.

[3] F. De Groote, O. Jardel, J.-P. Teyssier, T. Gasseling, J. Verspecht, V. Mallette, and C. Tsironis, On-wafer time domain load-pull optimization of transistor load cycle with the new multi-harmonic MPT tuner, 2007 ARFTG Microwave Measurement Conference, pp. 1-6, 8 June 2007.

[4] Maury Microwave, Multi-harmonic automated tuners, Maury Microwave Inc., Ontario, CA, 2012.

[5] Focus Microwave, iMPT-1818-TC, Focus Microwave Inc., Montreal, QC, Canada, 2007.

[6] S. Gao, Z. Wang, and C.-W. Park, Contour method to shift the tunable region of source/load pull tuners in power amplifier characterization, 2012 Asia-Pacific Microwave Conference, 4-7 December 2012.

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