**2.1 Typical architecture and power device for base station power amplifier**

In general, power amplifiers in mobile base stations are class AB amplifiers which offer both acceptable power efficiency and linearity. The operating point for the power device of this amplifier class is a compromise between those of highly efficient class B and highly linear class A. The conduction angles, output drain current waveforms, active load-lines and operating points of class A, AB, B, C, E and F amplifiers are depicted below in Fig. 2..

Fig. 2. Conduction angles, output drain current waveforms, active loadlines and operating points of class A, AB, B, C, E and F amplifiers. Vout, Iout, Vk, Vdd and Vbr are drain output voltage, drain current, knee voltage, drain voltage supply and drain breakdown voltage, respectively (source (Chalermwisutkul, 2007)).

Typically, lateral diffused metal oxide semiconductor (LDMOS) field effect transistors based on Silicon are used as power devices for base station power amplifiers. Silicon LDMOS is considered a mature power device technology for mobile base station amplifiers due to its high efficiency, high power density and high thermal conductivity. However, main reasons which make LDMOS standard device technology for base station amplifiers are its low cost and high reliability. Although it is known that the operating frequency of LDMOS devices is limited to a few GHz, progress in LDMOS technology is still ongoing and new LDMOS devices are continuously introduced into the market with higher operating frequency and other progresses in terms of power efficiency, linearity, etc. (Ma et al, 2005). Due to this fact, the dominance of LDMOS devices in low GHz high power applications has been ensured since the first devices came into the market. However, new challenges in power device technology keep emerging as modern wireless communications are required to cope not only with higher data rates at limited frequency resource, but also with energy saving issues. In other words, there are increasing demands in high power efficiency besides spectrum efficiency for the wireless communication infrastructure. In this regard, there are several cases where it is worth to look for alternative power device to overcome limitation of existing device technologies.

Despite of all advantages of LDMOS, the main drawback of this device is the bandwidth capability. Due to high output capacitance of LDMOS device, the Q factor tends to be high and the bandwidth is small. Also, the operating frequency limit hinders this device from being used in high frequency applications which are served with other device technology e.g. GaAs MESFET and HEMT. The research interest has been then attracted by widebandgap semiconductor materials for high frequency power devices. Silicon Carbide (SiC) is superior in thermal conductivity compared to other wide-bandgap semiconductors. However, the cost of SiC is relatively high. Moreover, this material is not appropriate for applications with very high operating frequencies. For Indium Phosphide (InP), another wide-bandgap compound semiconductor, the focus of research is on extremely high-speed digital applications where high power is not required.

The most prominent wide-bandgap semiconductor is Gallium Nitride (GaN). Comparing with Silicon device technology which is mainly driven by microprocessor and computer industries, GaN found its applications in screen industries enabled by GaN OLED (organic light emitting diode) technology and data storage industries utilizing blue laser produced by GaN laser diode to read out the data from a Blue-ray DiscTM. In automotive applications and power electronics, GaN devices are attractive due to high operating temperature and high breakdown field for switching power supply. For RF power amplifiers, GaN-based power devices offer extremely large bandwidth, high power density, high operating frequency and high output impedance. The advantages of GaN-based power devices for wireless communications will be discussed more thoroughly in the next section.
