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High-Electron-Mobility Transistors
Published in Vinod Kumar Khanna, Introductory Nanoelectronics, 2020
In a HEMT made from materials such as GaAs/AlGaAs that do not carry a net charge induced by interfacial polarization, application of a positive voltage may be necessary for attracting electrons to the channel The thickness of the depletion region formed in the barrier layer under the gate electrode determines whether the device will operate in the normally off or normally on mode. If the thickness of the barrier layer (AlGaAs) in the HEMT structure is taken to be very small, not only the barrier layer is fully depleted, the depletion region extends further beyond and the adjoining channel in the well layer is also depleted. Operation of this HEMT reminds us of the normally off or enhancement-mode MOSFET. So, this HEMT is called an enhancement HEMT or eHEMT. But if the barrier layer (AlGaAs) of the HEMT is sufficiently thick, only part of this layer is depleted and the channel is not affected. So, a channel exists under normal conditions and a negative bias is applied to vary the channel thickness providing a depletion mode operation. Such a HEMT is known as dHEMT.
Introductory Concepts
Published in Dragica Vasileska, Stephen M. Goodnick, Gerhard Klimeck, Computational Electronics, 2017
Dragica Vasileska, Stephen M. Goodnick, Gerhard Klimeck
The epitaxial structure of a basic HEMT is illustrated in Figure 2.56. Similar to the MESFET, the HEMT structure is grown on a semi-insulating GaAs substrate using molecular beam epitaxy (MBE), or less commonly, metal–organic chemical vapor deposition (MOCVD). Table 2.5 contains the common MESFET, HEMT, and pHEMT epitaxial structures. The buffer layer, also typically GaAs, is epitaxially grown on the substrate in order to isolate defects from the substrate and to create a smooth surface upon which to grow the active layers of the transistor. Many pHEMT structures contain a superlattice structure to further inhibit substrate conduction. A superlattice structure is a periodic arrangement of undoped epitaxial layers used to realize a thicker epitaxial layer of a given property. For example, alternating layers of AlxGa1−xAs and GaAs form a typical pHEMT superlattice. The AlxGa1−xAs has a larger band gap than GaAs, making it superior to GaAs as a buffer. However, due to strain problems, the AlxGa1−xAs layer thickness is limited. To resolve this problem, the AlxGa1−xAs is grown to just below its critical thickness limit and a thin layer of GaAs is grown on top. The GaAs relieves the strain and allows another layer of AlxGa1−xAs to be grown. This process is typically repeated 10 to 15 times, creating a layer that is “essentially” a thick buffer of AlxGa1−xAs.
Advanced Devices
Published in Chinmay K. Maiti, Introducing Technology Computer-Aided Design (TCAD), 2017
An HEMT is a FET that operates very similar to a metal–semiconductor field-effect transistor (MESFET). Electron flow across the carrier channel from source to drain is modulated by changing gate voltage. The main difference between a MESFET and a HEMT is the device structure. HEMTs use different compounds grown in layers to optimize and extend the performance of the MESFET. The different layers form a heterojunction. Figure 8.16 shows the basic HEMT structure.
2-D optimisation current–voltage characteristics in AlGaN/GaN HEMTs with influence of passivation layer
Published in International Journal of Ambient Energy, 2021
Abdelmalek Douara, Abdelaziz Rabehi, Bouaza Djellouli, Abderrezzaq Ziane, Hamza Abid
The basic concept in a HEMT is the aligning of a wide and narrow bandgap semiconductor adjacent to each other in order to form a hetero-junction (Chan et al. 1990). In a typically doped AlGaN/GaN HEMT structure, the AlGaN donor (carrier supply) layer supplies electrons to the 2DEG (Meng et al. 2012). The 2DEG is formed at the AlGaN/GaN interface even if all the layers are grown without intentional doping. In fact, the contribution of the doping to the 2DEG sheet carries concentration is reported to be less than 30% due to the stronger piezoelectric effect in the material system (Kumar and Bindu 2012).