Understanding High-Electron-Mobility Transistors (HEMTs/HEM FETs)

Key Takeaways

• High-electron-mobility transistors (HEMTs/HEM FETs)  are field-effect transistors utilizing a heterojunction of materials with different band gaps. They offer enhanced performance in high-frequency applications.

• HEMTs are produced using advanced epitaxial growth methods, with layer designs optimized to reduce defects, leading to faster, smaller, and more efficient semiconductors.

• HEMTs are used in communications, imaging, and power switching due to their high gain and low noise.

High-electron-mobility transistors (HEMTs/HEM FETs)  are a type of field-effect transistor

A high-electron-mobility transistor (HEMT or HEM FET), also known as a heterostructure FET (HFET) or modulation-doped FET (MODFET), is a type of field-effect transistor (FET), that uses an electric field to control the flow of current in a semiconductor. HEMTs incorporate a heterojunction, a junction between two materials with varying band gaps, in place of the conventional doped region found in MOSFETs. 

GaAs and AlGaAs are commonly used materials in HEMTs, but variations exist depending on the device's application. High-electron-mobility transistors perform well in high-frequency applications such as cell phones and radar equipment.

Different Types of High-Electron-Mobility Transistors (HEMTs/HEM FETs)

High-Electron-Mobility Transistor Operation Principles

High-electron-mobility transistors (HEMTs/HEM FETs) operate based on the formation of a two-dimensional electron gas (2DEG). This 2DEG is a medium for the flow of electric current and forms at the heterojunction - the interface between two semiconductor materials with different band gaps. This setup gives the HEMT advantageous electron transport properties and much higher sheet charge densities for high-frequency operations compared to a conventional FET. Key heterostructures used in HEMTs: AlGaN/GaN, AlGaAs/GaAs, InGaAs/GaAs, and Si/SiGe.

Formation of the 2DEG Channel

When equilibrium is achieved, the undoped narrow bandgap material now has excess majority charge carriers, enhancing the semiconductor's switching speed. This electron transfer process can be controlled by applying a potential 1.2 eV below the conduction band to the gate. Since there are no donor atoms in the undoped layer, electrons in the 2DEG channel do not undergo impurity scattering and exhibit high mobility. 

AlGaAs/GaAs HEMTs 2DEG

For AlGaAs/GaAs HEMTs, the 2DEG layer, formed by transferred electrons from the doped AlGaAs layer to the undoped GaAs layer, is localized to a thin (∼10 nm) electron gas layer on the GaAs side. Electron mobilities of a factor of approximately 50–100% higher at room temperature in bulk GaAs are routinely achieved in HEMT structures. Even higher carrier velocities are achieved in the AlInAs/InGaAs and InP/InGaAs materials systems relative to GaAs/AlGaAs.

The electron transfer process between GaAs/AlGaAs layers depletes the AlGaAs layer, which can be controlled by applying a potential to the metal Schottky barrier gate on the AlGaAs layer. Electrons accumulate at the interface and form a 2DEG layer when a positive voltage greater than the threshold voltage is applied to the gate.

AlGaAs/GaAs HEMTs structure diagram

HEMT Manufacturing

High-electron-mobility transistors (HEMTs/HEM FETs) can be manufactured using epitaxial growth of a strained SiGe layer. In this process, the germanium content in the layer is linearly increased to about 40-50%. This specific concentration facilitates the formation of a quantum well structure, which is characterized by a high conduction band offset and a dense population of highly mobile charge carriers. Alternatives to SiGe, such as InGaAs/AlGaAs and AlGaN/InGaN compounds, are also used. Materials like InP and GaN are increasingly preferred over SiGe as the base material in MODFETs due to their superior noise and power ratios.

Primary Growth Methods and Material Quality

The primary epitaxial growth methods are molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). Factors that influence material quality include:

• Heterointerface roughness

• Presence of impurities

• Defects in the material

• Strain from compositional mismatches

Materials used for a heterojunction would ideally have the same lattice constant (spacing between the atoms). However, lattice constants are usually slightly different in reality (e.g., AlGaAs on GaAs), which results in crystal defects, deep-level traps, and reduced device performance.

Layer Design in HEMTs

The standard layer design of HEMTs/HEM FETs usually comprises an n+ capping layer made of a smaller bandgap material, an n+ wide-bandgap donor layer, an undoped wide-bandgap spacer, and an undoped smaller bandgap buffer. This capping layer is removed under the gate area to improve breakdown voltage and prevent parallel conduction. The capping layer ensures:

• Effective ohmic contact

• Reduced source resistance

• Donor layer protection from oxidation, especially in systems like GaAs/AlGaAs. 

• Enhanced electron velocity, transconductance (gm), and frequency (fT) of the device by decreasing the source-to-drain spacing

HEMT Applications

If an application requires high gain and low noise at high frequencies, HEMTs will work well. Their current gain can reach frequencies greater than 600 GHz and power gain can reach over 1THz.

Application of HEMTs

Elevate HEMT Designs With Allegro X

If you are a designer utilizing high-electron-mobility transistors (HEMT/HEM FET) for high-frequency applications, look to Allegro X Advanced Package Designer for your final packaging needs. Once you’ve completed your design, ensure precision and compatibility with Allegro X. Harness its power to create high-frequency, high-performance electronics with ease and accuracy.

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