**3. GaN-based power amplifiers**

As mentioned in section 2.1, GaN is a promising semiconductor material for high power and high frequency power transistors which are used as power devices in mobile base station power amplifiers. The advantages of GaN originate from physical properties of this widebandgap semiconductor. Table 1 shows physical properties of various semiconductor materials including GaN.


Table 1. Physical properties of semiconductor materials for RF and microwave applications.

Fig. 8. Principle and block diagram of digital predistortion for linearization of power

In comparison to other techniques, digital predistortion offers higher efficiency and greater flexibility at low cost and represents a mature linearization technique for mobile base stations. Due to the mentioned flexibility and simple architecture, digital predistortion has gained it popularities in the power amplifier design community. In most of the cases where no extremely large bandwidth is required, high efficiency amplifiers e.g. Doherty and switched-mode amplifiers are combined with digital predistortion to

As mentioned in section 2.1, GaN is a promising semiconductor material for high power and high frequency power transistors which are used as power devices in mobile base station power amplifiers. The advantages of GaN originate from physical properties of this widebandgap semiconductor. Table 1 shows physical properties of various semiconductor

Material/Properties Si GaAs InP SiC GaN Bandgap (eV) 1.1 1.4 1.3 3.2 3.4

(\*107 cm/s) 1.0 2.1 2.3 2.0 2.7

(\*106 V/cm) 0.3 0.4 0.7 2.0 2.7

(cm2/Vs) 1350 8500 5400 800 1500

Table 1. Physical properties of semiconductor materials for RF and microwave applications.

1.3 0.46 0.7 4.9 1.7

amplifiers.

improve the linearity.

materials including GaN.

**3. GaN-based power amplifiers** 

Saturation Velocity

Thermal Conductivity (W/cmK)

Breakdown Field

Electron Mobility

From table 1, advantages of GaN compared to other semiconductor materials for RF and microwave applications are obvious. GaN offers very high saturation velocity leading to high operating frequency up to 100 GHz or higher. High breakdown field allows GaN-based devices to operate with high supply voltage which is advantageous for the off state of switched mode amplifiers and for obtaining high output power with high output impedance. Due to higher supply voltage, efficiency is also improved due to the reduction of the need for voltage conversion. For extreme operating environment e.g. for automotive applications, GaN offers wide bandgap and high thermal conductivity leading to the capability to operate at high temperature.

The most prominent GaN-based device for RF and microwave applications is GaN-based high electron mobility transistor (GaN HEMT). This kind of device offers extremely high operating frequency due to high electron mobility in the so-called 2DEG channel (Smorchkova, 2001). Moreover, one of the most impressive features of this device is the extremely high power density meaning that the device's size can be much smaller compared to other device technology for the same output power. With size reduction, output impedance becomes larger and parasitic capacitances smaller leading to large bandwidth and uncomplicated matching to 50 Ohm. It was also mentioned in the literature that GaN HEMT can offer better noise performance than that of MESFET's (Mishra et al, 2007).

For wireless communication infrastructure, GaN HEMT has proven itself to be an attractive alternative power device besides LDMOS FET for base station power amplifiers. For WCDMA base station, a GaN HEMT-based transmitter with output power higher than 200 W and supply voltage of 50 V was published in 2004 (Kikkawa et al, 2004). Reliability--one of the biggest concerns regarding GaN HEMT compared to LDMOS--was also presented in that work. However, at this point, it is not possible to foresee when GaN HEMT's will take the place of LDMOS FET's in base station power amplifiers. Even if the frequency of operation is limited to a few GHz for LDMOS, this device technology is continuously developed regarding power, reliability, linearity, etc.. Moreover, LDMOS is considered a cost-effective and mature power device technology with a large LDMOS amplifier designer community. Consequently, knowhow and design experience for this device is available to a great extent. Regarding this consideration, GaN HEMT will find its importance first in applications where large bandwidth is required or high power is desirable at high frequency. Besides reliability, charge carrier trapping in GaN HEMT has been a big issue for device technology improvement. Numerous investigations have been done regarding trapping effects of GaN devices. Charge carrier traps can cause dependency of the pulsemeasured I-V characteristic on the quiescent point. This is a phenomenon of the so-called electrical memory effect (Chalermwisutkul, 2008). Other phenomena of memory effects are gate lag and drain lag in time domain where the drain current reaches its final value after some delay as the bias voltages are abruptly changed. In frequency domain, dispersion of output impedance is the consequence of electrical memory effect leading to dynamic nonlinearity with a large bandwidth of spectral regrowth (Fischer, 2004). Improvement of GaN device technology regarding charge carrier trapping and reliability has been reported occasionally e.g. SiN passivation or use of the field plate for traps reduction (Mishra, 2007). In this section, results from the works regarding GaN device modeling and GaN power amplifier design in which the author has been involved will be presented.
