The Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT) has been considered primary technology for realizing solid state high power, high-frequency power amplifiers. The amplifiers based on GaN Technology are being widely used in various application such that Radar, electronic warfare, communication links etc. While GaAs HEMT is already available in the market and widely used as Power amplifiers, TWT VED is still used in high power applications. The primary focus of GaN HEMT has been high power, the focus is now shifting to further advantages such as high efficiency and low energy consumption and GaN is making its way into green technology. GaN HEMT utilizes high-density two-dimensional electron gas (2DEG) accumulated in the boundary layer between GaN and AlGaN through their piezoelectric effect and natural polarization effect. This makes it possible to realize low on-state resistance(Ron) combined with a high breakdown voltage.
 |
| The basic configuration of GaN HEMT (Image: Fujitsu) |
Advantages of a High Electron Mobility Transistor (HEMT)
- A high gain-bandwidth product, due to high electron velocity in large electric fields
- High transconductance due to small gate-to channel separation
Structure of GaN HEMT
GaN HEMT is manufactured by growing a single-crystal GaN film on different substrates such as silicon carbide (SiC), sapphire (α-Al2O3) and silicon, SiC. Threshold Voltage Vth and mobility parameters of a transistor depends on temperature. As the temperature decreases device performance improves and vice versa. It makes thermal profile management an imperative task. Thermal conductivities of Si, SiC and sapphire are shown in the figure below.
 |
Thermal conductivities of Si, SiC, Sapphire, W and Mo( Image courtesy: Development of High-Reliability GaN HEMT- Fumikazu YAMAKI)
|
Despite being expansive, thermal and mechanical properties of SiC makes it possible for it to be used in high power applications. Accelerated life test data for GaN HEMTs on SiC indicate that 'GaN-on-SiC' technology is very robust, with predicted lifetimes at application conditions in excess of 1 million hours.
Cost reduction is also an important factor for GaN HEMT commercialization. GaN-on-Si technology has been developed that allows cost reduction by adopting a large diameter Si substrate.
Operational Advantages of GaN HEMT technology
- Operation at a relatively high voltage and temperature:
Owing to wide gape(~3.4eV direct band gap), these devices can operate at significantly higher voltages which is further translated into high power operations. Silicon and other common materials have a bandgap on the order of 1 to 1.5eV, which implies that such semiconductor devices can be controlled by relatively low voltages. However, it also implies that they are more readily activated by thermal energy which interferes with their operation. It limits silicon-based devices to operational temperatures below 100°C, beyond which the uncontrolled thermal activation of the devices makes it difficult for them to operate correctly. Wide-bandgap materials typically have band gaps on the order of 2 to 4 eV allowing them to operate at much higher temperatures on the order of 300°C. It makes wide bandgap semiconductors highly attractive for military applications.
In Wide Band Gap Semiconductor (WBGS-RF) trials it was found that GaN HEMT based Solid State Power Amplifiers(SSPA) were capable of operating at 150˚C continuously for almost 100000 hours. Higher temperature tolerance of GaN HEMT leads to lesser cooling requirements which have its own advantages for space and airborne applications because cooling system adds extra weight and power consumption. Owing to the superior thermal conductivity of SiC substrate, removal of heat from the junction is easier and GaN HEMT based SSPA can be operated with a less complex cooling mechanism. Existing Gallium Arsenide (GaAs) HEMT based MMICs require complex liquid coolant circulation.
- High-Efficiency Operation:
Compared to Silicon transistors widely used in amplifiers, GaN HEMT shows low power loss during on state with low Ron . Power amplifiers for transmitters of wireless communication systems and radar systems need high power output and high-efficiency operation. For instance, if a transmitter needs 100w of output, to achieve the power of 100w order, a parallel combination of multiple devices is required if conventional GaAs MESFET or Si LD-MOSFET are used. On the other hand, single GaN HEMT can achieve 100w or higher. In case of RF power amplifiers, efficiency is characterized by drain efficiency that is the ratio of output RF power and DC power supply. Power added efficiency (PAE) is more inclusive and widely used term to describe the efficiency of RF SSPA., it is the ratio of the difference between output-input RF power to DC supply.
 |
| GaN HEMT based Solid State Power Amplifier compared to the power amplifier based on other technologies. (Image: GaN HEMT: Dominant Force in High-Frequency Solid-State Power Amplifiers by James J. Komiak) |
 |
GaN HEMT based SSPA power output-frequency and PAE-frequency curve( Image: GaN HEMT: Dominant Force in High-Frequency Solid-State Power Amplifiers J.J Komiak)
|
- High reliability and ruggedness:
Reliability is characterized by Mean Time To Failure (MTTF), ruggedness is also important to for SSPA to remain operational under heavy RF stress. In WBGS-RF program, RF step stress and VSWR ruggedness tests were conducted which confirmed no significant degradation under a wide range of RF stress. MTTF was estimated to be 1.07 × 106 hours (approximately122 years) at 200°C.
 |
| MTTF vs Channel Temperature curve for GaN HEMT (Image: Development of High-reliability GaN HEMT) |
Application:
GaN HEMT is building the block of transmitter-receiver modules (TRM) used in radar and communication systems. GaN HEMTs are used to implement receive and transmit stage power amplifiers. As GaN HEMT based SSPA has higher power output than GaAs or Si-based PA, required power levels can be achieved by single GaN SSPA instead of using multiple GaAs/Si-based SSPAs with power combining circuits, which makes GaN SSPAs more efficient and obvious candidate for high-speed wireless communication systems (4G/5G), satellite transceivers and active phased array radar systems.
- Wireless Cellular Communication:
Existing mobile base station used for cellular communication employs GaAs based SSPA. L-band GaN HEMT based high power amplifiers which enable high-efficiency operation, can be used to make existing cellular communication system more efficient and energy saving. A higher PAE(Power Added Efficiency) is also important for the efficient performance of a remote radio head(RRH). Among the high PAE HPA design, Class E operation is the most popular approach for L-or S-band which consists of an output matching circuit to achieve high efficiency. Since parasitic capacitance of a GaN HEMT is 23.6 times smaller than that of an LDMOS, it allows GaN HEMT to operate at higher power and frequency.
 |
performance comparison of GaN-based HEMT with transistors based on GaAs and Si.
|
GaN HEMT technology can increase PAE while reducing amplifier size and power consumption which is strongly required for wireless cellular communication and next-generation technologies such as 4G/5G.
Adaptive Active phased array antenna with advanced beamforming capability employing a large number of GaN HEMT based SSPA will be the backbone of 5G mobile communication systems- boosting power and minimizing interference between users (beamforming capability results in highly directional antenna).
 |
Active antenna systems and massive MIMO (Image: EDN)
|
- Space Application:
In space application such as satellite communication, the RF power amplifier is one of the key components and High PAE is essential to reduce the launch cost of the satellite( Higher PAE translates into reduced weight). GaAs based amplifiers cannot typically offer acceptable PAE for many satellite application. GaN HEMT SSPA with its high PAE is a likely candidate to replace TWTA(Traveling Wave Tube Amplifier).
- Radar Application:
Typical Modern Active Phased Array radars use multiple(100-1000s in single antenna) TRMs(Transmitter-Receivers Modules). The first generation of AESA radar employed GaAs based TRM. While it provided radars with very high scanning and frequency hopping rates, it lacked in terms of brute power compared to TWTA based passive phased array radars. However, the second generation of AESA radars features GaN-based TRM which not only improves scanning performance but also outperforms TWTA based phased array radars in terms of range and peak output power. Thanks to higher PAE new phased array radars are lighter and energy efficient.
 |
| GaN-based TRM developed by Fujitsu |
 |
| GaN/AlGaN-on-SiC wafer- DRDO |
Comments
Post a Comment