**6. Latest research**

method [16] for the current transport equation; and explicit forward differencing "marching" method for calculating the average energy. This model has an improvement over Widiger's

Ueno et al. presented Monte Carlo simulation of HEMTs to analyze 2DEG electron transport [10]. The analysis is based on electron–phonon interaction model proposed by Price [18]. In this framework, the 2DEG electrons are assumed to be scattered by bulk phonons. Thus, wave func‐ tions calculated by self‐consistent analysis are used to evaluate the scattering rate. The chan‐ nel region is not considered uniform and electrons near drain region are considered as three dimensional and near‐source region are considered as two dimensional. In addition, electrons with high energy beyond the barrier height behave as three‐dimensional electrons and are not confined in the quantum well. In these simulations, the initial condition is first evaluated. Then the sheet electron density at each position between the source and the drain are estimated using the current continuity relation along the channel. Next, Monte Carlo simulation is car‐ ried out by dividing the channel into different meshes and evaluating the scattering rates of the electronic states in each mesh. Then taking the potential distribution of the given device from two‐dimensional Poisson equation, the steps are repeated until a steady state is obtained.

Lee and Webb described a numerical approach to simulate the intrinsic noise sources within HEMTs [11]. A 2‐D numerical device solver is used in this model. Spectral densities for the gate and drain noise current sources and their correlation are evaluated by capacitive cou‐ pling. After solving Poisson's and the continuity equations using 2‐D numerical device solver, Green's functions are obtained. Here, Green's functions are used to determine local fluctua‐ tion (in terms of current or voltage at any point in the channel) at the gate and drain terminals. This approximate impedance field concept [19] helps determining the gate and drain noise sources and their correlation. For numerical simulation, the entire device is divided into some orthogonal areas and it is considered that 2‐D simulation results will be consistent with the 3D simulation result. Spontaneous polarization and strain‐induced piezoelectric polarization are also considered. It is assumed that the microscopic fluctuations in each segment are spatially

uncorrelated which are originated from velocity fluctuation (diffusion) noise only.

Hirose et al. proposed a numerical model for AlGaN/GaN HEMT structures where shear stress due to the inverse piezoelectric effect is used to predict high‐temperature DC stress test results [12]. In this model, lattice plane slip in the crystal is assumed to be the initial stage of crack for‐ mation. Shear stress causes the slip, and slip deforms the crystal when the shear stress exceeds the yield stress. In GaN‐based HEMTs, the basal slip plane is (0001) and the slip direction is <1120>. The AlGaN layer is a wurtzite crystal grown in the <0001> direction [20]. Shear stress is assumed to be a result of the inverse piezoelectric effect. The mechanical stress and electric displacement occur due to the piezoelectric effect. Under the assumption of lattice mismatch in AlGaN layer, shear stress relates to the slip in the <1120> direction. However, to calculate shear stress, electric field is obtained from two‐dimensional device simulation based on Poisson's

energy transport model [17] where conduction is ignored in the AlGaAs layer [9].

**5.3. Monte Carlo simulation**

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

**5.4. Noise current using Green's function formalism**

**5.5. High temperature shear stress analysis**

With the upsurge of popularity, research works on HEMT devices are still going on. In this section, some very recent research works published in renowned scientific literature have been briefly highlighted.

### **6.1. GaN HEMT‐based RF tuned cavity oscillator**

Hörberg et al. presented a GaN‐based oscillator for X band tuned by radio frequency micro‐ electromechanical systems (RF‐MEMS) [22]. The phase noise is reported to be reduced between the range of −140 and −129 dBc/Hz at 100 kHz offset, which is significantly low. This oscillator is suitable for reduced noise‐based high frequency modulators.

### **6.2. A compact GaN HEMT power amplifier MMIC**

A compact GaN HEMT‐based X‐band power amplifier MMIC has been reported with detailed performance analysis recently [23]. A good range of output power (47.5–48.7 dBm) can be obtained from this amplifier. Such amplifier can be used to build electronic systems that require airborne phased radar array or satellite transmitters. Improved out‐ put power of the amplifier also improves the stability, reliability, and performance of these electronic systems. **Figure 9** shows the output power performance in both pulse mode and continuous wave (CW) modes with frequency variation in this power amplifier.

#### **6.3. Q‐spoiling based on depletion mode HEMTs**

Q‐spoiling is a process where MRI coils are detuned for safety and protection. Traditionally, such decoupling or Q‐spoiling is done using PIN diodes, which require high current and power drain. Lu et al. proposed an alternative technique of Q‐spoiling, which replaces PIN diodes with depletion mode GaN HEMTs [24]. It is shown that the proposed technology detunes MRI coils effectively with low current and power drain compared to the traditional Q‐spoiling technologies. It also provides suitable safety measures required for detuning the MRI coils.

#### **6.4. GaN HEMT oscillators with low phase noise**

Excellent figure of merit (FOM) has been achieved for low phase noise in designing GaN HEMT‐based oscillators [25]. The design demonstrated that low phase noise can coincide

**Figure 9.** Output power performance of GaN HEMT power amplifier MMIC with frequency variation in pulse and CW modes.

with low bias power. The result is verified designing Colpitt and negative resistance oscilla‐ tors and both of these present so far the best reported FOMs.

#### **6.5. Kink effect in GaN HEMT technology**

Crupi et al. investigated Kink effect (KE) in advanced GaN HEMT technology [26]. For better understanding, KE is studied comprehensively with change of temperature and bias conditions. It is shown that the dependence of KE on operating conditions is mainly due to device transcon‐ ductance. Characterization of anomalous KE would be a useful tool for microwave engineers who need this knowledge of KE for designing and modeling devices with GaN HEMTs.

#### **6.6. 600 V GaN HEMT switches for power converters**

A total of 600 V GaN HEMT switches have been demonstrated experimentally to show per‐ formance comparison with silicon‐based transistor switches such as IGBTs and MOSFETs [27]. HEMT switches, despite being beginners, show excellent performance compared to the matured counterparts, Si‐based MOSFETs. It is shown that GaN switches offer higher boost converter efficiency than the MOSFET switches. Next, GaN switches are compared experimentally with IGBTs. Both Si body and SiC body‐based IGBTs have been considered. It is found that at higher switching frequency, IGBT switches loss efficiency very rapidly, while HEMT switches loss effi‐ ciency monotonically as shown in **Figure 10**. Therefore, HEMTs offer superior performance to Si‐based MOSFETs and IGBTs for high frequency power converter switching applications.

High Electron Mobility Transistors: Performance Analysis, Research Trend and Applications http://dx.doi.org/10.5772/67796 57

**Figure 10.** Comparison of efficiency for GaN HEMT switches with Si body IGBT and SiC body IGBT switches.
