**8. Applications**

Explosion of the internet multimedia communications has speedily spread over the world, which urgently demands the proliferation of transmission network capacity. HEMTs‐ based devices are the most attractive choices for breaking through the speed limit and high gain and noise free mechanism. Different companies worldwide develop and manufac‐ ture HEMT‐based devices, and many possible applications have been suggested for these devices. Without considering all of those possibilities, some key applications are summa‐ rized in this section.

#### **8.1. Broadband communication**

Cellular communication has got the most important nonmilitary applications of HEMT devices by replacing Si transistors. For such broadband/multiband communication applications, we get a lot of advantages. The increase in relative bandwidth for a given power level is one of those. Some new circuit and system concepts provide bandwidth with increased efficiency. Linearity has been improved for the same output power. Reduction of memory effects is also found by using GaN HEMT devices [36].

#### **8.2. Radar components and space applications**

From the past, it can be anticipated that, researching on new device models and structures of HEMTs will definitely result in new insights into the often bizarre physics of quantized electrons. ZnO, SiGe, and GaN have shown fractional quantum Hall effect (FQHE), the greatest exponent for impeccable purity and atomic order, which ensure the bright future of HEMT devices [29]. The concept of different kinds of physical and biosensors are still very new to these kind of devices. The ultra‐high mobility that is possible in InAlSb/InAsSb‐based system enables high‐ sensitivity micro‐Hall sensors for many applications including scanning Hall probe microscopy and biorecognition [30]. Three‐axis Hall magnetic sensors have been reported in micromachined AlGaAs/GaAs‐based HEMTs [31]. These devices may be used in future electronic compasses and navigation. THz detection, mixing and frequency multiplication can also be used by 2DEG‐based devices [32]. GaN and related materials have strong piezoelectric polarization, and they are also chemically stable semiconductors. Combining functionalized GaN‐based 2DEG structures with free‐standing resonators, there is a possibility of designing sophisticated sensors [33]. These can offer methods of measurements of several properties such as viscosity, pH, and temperature.

58 Different Types of Field-Effect Transistors - Theory and Applications

Without references, expansion of this technology in the machine to machine (M2M) field is expected to be used in cloud networking‐based various sensing functions. Diverse applica‐ tions such as environmental research, biotechnology, and structural analysis can be greatly benefited with the help of newly emerged sensing technology which has high speed, high mobility, and high sensitivity characteristics. HEMT technology is expected to make a great change in the intelligent social infrastructure from the device level. A smart city system, transport system, food industry, logistics, agriculture, health welfare, environmental sci‐ ence, and education systems are examples where this technology can make exceptions [34]. The rise of III‐N‐based solid‐state lighting will lead to a continuous development of materials, substrates, and technologies pushed by a strong consumer market. In an analogy, III‐N optoelec‐ tronics will challenge the light bulbs, while III‐N electronics will challenge the electronic equiva‐

Explosion of the internet multimedia communications has speedily spread over the world, which urgently demands the proliferation of transmission network capacity. HEMTs‐ based devices are the most attractive choices for breaking through the speed limit and high gain and noise free mechanism. Different companies worldwide develop and manufac‐ ture HEMT‐based devices, and many possible applications have been suggested for these devices. Without considering all of those possibilities, some key applications are summa‐

Cellular communication has got the most important nonmilitary applications of HEMT devices by replacing Si transistors. For such broadband/multiband communication applications, we get a lot of advantages. The increase in relative bandwidth for a given power level is one of

lent, the tubes [35].

**8. Applications**

rized in this section.

**8.1. Broadband communication**

High gain and low noise amplifiers are the main characteristics for making radar compo‐ nents. GaN HEMTs are one of the first choices for such components. Active electronic sensor arrays are built from GaN‐based HEMTs, which are used for airborne radars, ground‐based air defense radars, and naval radars [37]. Ka‐band missile applications at 35 GHz are also being discussed in literature [38]. Discrete HEMTs are almost always used as the preampli‐ fier in a typical DBS receiver, followed by one or more GaAs MESFET monolithic micro‐ wave integrated circuits (MMICs) due to their excellent low‐noise characteristics. The use of the low‐noise HEMT preamplifier has resulted in substantial improvements in system performance at little additional cost. A low‐noise down‐converter consisting of a 0.25 pm HEMT and three GaAs MMIC chips has shown a system noise figure less than 1.3 dB with a gain of about 62 dB from 11.7 GHz to 12.2 GHz, which is phenomenal for a commercial, system [39]. Microwave equipment used for space applications are very expensive as they need extra protection from harsh environment in space to survive. Moreover, spacecraft shall be launched, and this implies that the equipment should also sustain without damage at high levels of vibrations and shocks. HEMTs can be fabricated to survive these conditions and have been extensively used in various fields. Generally, a microwave component for space applications is ten to hundred times more expensive than for commercial applica‐ tions. Workers at the National Radio Astronomy Observatory (NRAO) have used the excel‐ lent cryogenic performance of HEMTs to receive signals during the Neptune flyby of the voyager spacecraft.

#### **8.3. Sensor applications**

In the recent decade, chemical sensors have gained importance for applications that include homeland security, medical and environmental monitoring, and food safety. The desirable goal is the ability to simultaneously analyze a wide variety of environmental and biological gases and liquids in the field and be able to selectively detect a target analyte with high speci‐ ficity and sensitivity. The conducting 2DEG channel of HEMTs is very close to the surface and very sensitive to adsorption of analytes. Hence, HEMT sensors can be a good alternative for detecting gases, ions, and chemicals [40].

#### **8.4. DNA detection**

Au‐gated AlGaN/GaN HEMTs functionalized in the gate region with label free 3'‐thiol modi‐ fied oligonucleotides serves as a binding layer to the AlGaN surface, which can detect the hybridization of matched target DNAs. XPS shows immobilization of thiol modified DNA covalently bonded with gold on the gated region. Drain‐source current shows a clear decrease of 115 µA as this matched target DNA is introduced to the probe DNA on the surface, show‐ ing the promise of the DNA sequence detection for biological sensing [41].

#### **8.5. Protein detection**

Using amino‐propyl silane in the gate region, ungated AlGaN/GaN HEMT structures can be activated, which can serve as a binding layer to the AlGaN surface for attachment of biotin. Biotin has a very high affinity to streptavidin proteins. When the chemicals are attached to AlGaN/GaN HEMTs, the charges on the attached chemicals affect the current of the device. The device shows a clear decrease of 4 µA as soon as this protein is collected at the surface, showing indication of protein sensing [41].

#### **8.6. pH detection**

The use of Sc<sup>2</sup> O3 gate dielectric produces superior results to either native oxide or UV ozone‐ induced oxide in the gate region. The ungated HEMTs with Sc<sup>2</sup> O3 in the gate region exhibit a linear change in current between pH 3–10 of 37µA/pH. The HEMT pH sensors show stable operation with a resolution of <0.1 pH over the entire pH range. The results indicate that HEMTs may have application in monitoring pH solution changes between 7 and 8, the range of interest for testing human blood [40].
