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costs and with an improved lithographic yield. HBTs are widely used in mobile handsets and also in MMIC circuits at frequencies up to X band.
2.4. Semiconductor Materials for High Frequency Power
Transistors
Semiconductor technology forms the basis of the modern information and communica- tion society. Practical applications of semiconductor materials have been found in the opto-electronics, power electronics, digital and analog circuits as well as in high fre- quency applications. Examples of semiconductor devices are diodes, logic gates, storage elements, power transistors for high frequency power amplification, etc. The choice of semiconductor materials is based on the physical properties of the materials which have to fulfill the requirements of the application e.g. operating temperature, frequency and bandwidth. In this section, semiconductor materials for RF and microwave applications will be discussed (see Table 2.3).
Material ⇒
Properties ⇓ Si GaAs InP SiC GaN
Bandgap (eV) 1.1 1.4 1.3 3.2 3.4 Saturation Velocity (cm/s) 1.0·10 7 2.1 ·10 7 2.3 ·10 7 2.0 ·10 7 2.7 ·10 7 Thermal Conductivity (W/cmK) 1.3 0.46 0.7 4.9 1.7 Breakdown Field (V/cm) 0.3·10 6 0.4 ·10 6 0.7 ·10 6 2.0 ·10 6 2.7 ·10 6 Electron Mobility (cm2/Vs) 1350 8500 5400 800 1500
Table 2.3.: Semiconductor materials for RF and microwave applications [28].
Silicon (Si)
Silicon is the most widespread semiconductor material used to fabricate transistors in integrated circuits (BJTs, MOSFETs). Due to the great availability of silicon, the cost of this material is low. However, the operating frequency of Si-based transistors is limited to few GHz, since the electron mobility in this material is relatively low. Silicon is thus, not suitable for applications operating at frequencies in higher GHz range. Silicon provides high quality oxide SiO2 for the processing of MOSFETs. Si-LDMOS-FETs
are the most frequently used devices for base station applications due to their low cost. However, due to the limitations of Si regarding frequency range, power level, operating
22 Chapter 2. Power Amplifiers for Mobile Communication Applications
temperature, etc., Si-LDMOS-FETs are expected to be replaced by devices based on other semiconductor materials for future base station applications.
Silicon Germanium (SiGe)
SiGe is an alloy of silicon and germanium and commonly used for producing HBTs. The use of heterojunction can improve the maximum operating frequency and reduce the base resistance which leads to higher gain compared to normal Si-BJTs. SiGe-HBTs are produced on conventional Si wafers and thus allow the use of Si processing tool sets. The cost of SiGe process is comparable to Si-CMOS process and lower than other heterojunction technologies e.g. GaAs. However, the efficiency of SiGe HBTs is lower than GaAs-HBTs. Another advantage of SiGe-devices is the integrability with CMOS logic for mixed-signal circuits.
Gallium Arsenide (GaAs)
Gallium Arsenide is the state-of-the-art semiconductor material for the fabrication of RF and microwave power transistors. With superior saturation velocity and electron mobility compared to Si, GaAs allows a higher frequency of operation from X-band to around 150 GHz. GaAs-based devices are considered as mature and very reliable for high frequency applications. Experience and understanding about the device process- ing are available to a great extent, so that the properties of these devices can be well described with device models. These models can be used in CAD-tools, which can ac- celerate the circuit development significantly. GaAs-transistors are available in several variations e.g. GaAs-MESFET (200 W at 2 GHz), HEMT, pHEMT (40 W at 2 GHz) and also AlGaAs/GaAs-HBT (mostly used in mobile phones and in MMICs with operating frequencies up to X-band). Since the power level and the operating frequency of high frequency circuits is steadily increasing, it is predictable, that RF power devices based on GaAs will soon come to their performance limitations. To satisfy the requirements of future RF and MW applications, numerous research programs have been initiated to develop novel semiconductor materials with superior properties. The most promising materials for high frequency, high power applications are silicon carbide (SiC) and Gal- lium Nitride (GaN). These semiconductor materials belong to the so-called wide-bandgap semiconductors, whose bandgap energy is relatively high (typically 3-6 eV).
Silicon Carbide (SiC)
The significant advantage of SiC is the high thermal conductivity. Thus, SiC-based devices can be used in applications with high operating temperatures. Due to its high temperature capability, SiC is also used as substrate for other semiconductor devices. SiC-based devices and devices on SiC substrate are normally used in RF circuits based on hybrid techniques but not in MMICs, since the material cost per mm2 of SiC wafer
2.4. Semiconductor Materials for High Frequency Power Transistors 23
is relatively high compared to other materials. The most widespread SiC-based devices are SiC-MESFETs.
Gallium Nitride (GaN)
In optoelectronics, GaN is a mature material for applications like blue laser diodes, high brightness LEDs and sensors utilizing its direct bandgap. With its high electron mobility and saturation velocity, GaN is also a proper material for high frequency applications. The relatively high thermal conductivity of this material allows GaN-based power devices to operate at high temperatures [6] and high power levels. Power densities of 12 W/mm2
and 8 W/mm2 have been reported for MOCVD and MBE grown GaN HEMTs, respec-
tively [7, 8]. The high power densities featured by GaN HEMTs (about ten times as high as GaAs HEMTs) are due to two main reasons. First, the high sheet carrier density at AlGaN/GaN interface (∼1x1013electrons/cm2), about five times higher compared to Al-
GaAs/GaAs interface, leads to high current capability. Second, due to its wide bandgap, GaN can endure internal electric fields about five times higher than Si or GaAs [9, 10] allowing the use of high operating voltages. Therefore, additional loss in DC conversion elements can be avoided. Moreover, GaN is very resistant against chemical and other external influences. Beside applications in wireless communication systems, high power and high temperature applications e.g. in automotive systems are also possible. On the architectural level, GaN-based devices provide high output impedance, which simplifies the matching to an external 50 Ω load. Widespread GaN-based power transistors are GaN-MESFETs and GaN HEMTs with the heterostructure AlGaN-GaN forming the 2DEG. GaN-based devices can be processed on the low cost Si- or Al2O3-substrates for
hybrid circuits and MMICs. For extremely high power, high temperature applications, SiC-substrates can be used. Figure 2.9 depicts the frequency and power ranges covered by various semiconductors. Due to its advantageous physical properties, GaN can cover large frequency and power ranges of RF and microwave applications and is expected to be the key technology for future high frequency, high power applications.
1 GHz 10 GHz 100 GHz Frequency RF Power 10 W 100 W InP GaAs SiGe Si SiC
GaN
24 Chapter 2. Power Amplifiers for Mobile Communication Applications