pHEMTs consist of different semiconductor materials and are joined to form heterojunctions.
pHEMT technology-based MMICs are popular for providing wideband performance characteristics.
The pHEMT process is the fabrication process that involves steps required to manufacture a pHEMT device.
MMICs are an essential part of wireless communication systems
There is a proliferation of pseudomorphic high electron mobility transistor (pHEMT) technology in microwave integrated circuits (MMICs). The pHEMT technology is preferred over its predecessor MESFET due to advantages such as high electron mobility, increased device current capacity, and better high-frequency performance. The replacement of MESFETs with pHEMTs resulted in the closure of MESFET foundries. The foundries that survived are the ones that converted the MESFET process to the pHEMT process.
In this article, we will focus our discussion on the pHEMT process.
MMIC and pHEMT Technology
MMICs are an inevitable part of wireless communication systems. MMIC technology is of primary importance due to its high reliability, good repeatability, and small size. The use of MMICs is widely promoted among circuit designers, as they provide high performance with reduced costs. MMIC technology is free from the unpredictable parasitics of passive elements, which limit the high-frequency operation of conventional hybrid ICs. The incorporation of pHEMT technology benefits MMICs by boosting performance to even higher frequency ranges with excellent efficiency.
The pHEMT technology-based MMICs are popular for providing wideband performance characteristics such as low noise generation, high amplifier linearity, and high reliability at high-frequency operation. The pHEMT technology is the most sought-after MMIC technology employed for the manufacturing of microwave integrated circuits, as it showcases outstanding high-frequency operation due to the presence of heterojunction and different band gaps in the internal structure.
pHEMTs consist of different semiconductor materials and are joined to form heterojunctions. The heterojunctions lead to different types of energy bands in pHEMTs. The heterojunction prevents Coulomb scattering, thereby speeding up electron mobility. All these construction or fabrication characteristics lead to the better performance of pHEMTS.
The Basic Structure of pHEMTs
The epitaxial structure of a basic HEMT consists of layers (in the order of bottom to top): semi-insulating substrate layer, buffer layer, channel, spacer, and donor. The different layers form heterojunctions. When the structure is grown with slightly varying lattice constants, it forms a pHEMT structure. The epitaxial layer structure of HEMTs and pHEMTs is different from MESFETs. The HEMT structure optimizes and extends the performance beyond MESFETs.
The pHEMT Fabrication Process
The pHEMT fabrication process includes the development of the epitaxial structure of pHEMT devices. The structure is grown on a semi-insulating substrate. Methods commonly used during the pHEMT process for growing layers include molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). The buffer layer is also grown epitaxially on the substrate to isolate defects from the substrate. As a result, a smooth surface is formed on which the active layers are grown.
In pHEMTs, a supper lattice structure is usually incorporated to inhibit substrate conduction. The superlattice layer is a periodic arrangement of undoped epitaxial layers. The superlattice layer growing process is repeated several times to develop essential layer thickness.
The next step in the pHEMT process is the growth of the channel. The channel layer is a very important part of the pHEMT structure, as 2-dimensional electron gas (2DEG) formation occurs in this layer and is responsible for the high electron mobility in pHEMT devices. The rest of the pHEMT process comprises the growth of the spacer layer, donor layer, Schottky contact layer, and highly doped layer to realize low resistance ohmic contacts.
The Importance of the pHEMT Process
The pHEMT process has gained attention, as most modern MMICs are based on pHEMT technology. In wireless applications, power amplification circuits are dependent on pHEMT devices. For example, InGaP pHEMT technology-based power amplifiers are extensively used in wireless applications.
InGaP pHEMT power amplifiers are employed in circuits due to advantages such as excellent etch selectivity over GaAs, lower surface oxidation rates due to no Al in material, lower surface potential, high valence band discontinuity, higher breakdown voltage, lower leakage currents, etc. There is a huge demand for pHEMT devices and MMICs based on pHEMT technology, another indication of the importance of pHEMT processes in the semiconductor industry.
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