**3. Working principle of HEMTs**

[13]. Hence, InP‐based HEMTs show high electron mobility, high electron saturation velocity, and high electron concentration. The device usually consists of an InGaAs/InAlAs composite cap layer for enhanced ohmic contact, an undoped InAlAs as Schottky barrier and an InGaAs/ InAs composite channel for superior electron transport properties as depicted in **Figure 3**.

**Figure 3.** Structure of InP‐based HEMTs.

**Figure 2.** Structure of GaN‐based HEMTs.

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

HEMTs are essentially heterojunctions formed by semiconductors having dissimilar band‐ gaps. When a heterojunction is formed, the conduction band and valence band throughout the material must bend to form a continuous level. The wide band element has excess elec‐ trons in the conduction band as it is doped with donor atoms (or due to polarization charge in GaN‐based HEMTs). The narrow band material has conduction band states with lower energy. Therefore, electrons will diffuse from wide bandgap material to the adjacent lower bandgap material as it has states with lower energy. Thus, a change in potential will occur due to movement of electrons and an electric field will be induced between the materials. The induced electric field will drift electrons back to the conduction band of the wide bandgap element. The drift and diffusion processes continue until they balance each other, creating a junction at equilibrium like a p‐n junction. Note that the undoped narrow bandgap material now has excess majority charge carriers, which yield high switching speed. An interesting fact is that the low bandgap undoped semiconductor has no donor atoms to cause scattering and thus ensures high mobility.

Another interesting aspect of HEMTs is that the band discontinuities across the conduction and valence bands can be engineered to control the type of carriers in and out of the device. This diffusion of carriers leads to the accumulation of electrons along the boundary of the two regions inside the narrow bandgap material. The accumulation of electrons can lead to a very high current in these devices. The accumulated electrons are also known as 2DEG. **Figure 4** shows the generalized band diagram formed at the heterojunction for typical HEMTs. Both

**Figure 4.** Generalized energy band diagram of HEMTs.

the conduction band (Ec ) and valence band (Ev ) bend with respect to the Fermi level (EF ) resulting in a quantum well filled with 2DEG and eventually, a conducting channel is formed.
