High Electron Mobility Transistor

The High Electron Mobility Transistor (HEMT), also known as a modulation-doped field-effect transistor (MODFET), is a type of field-effect transistor (FET) that utilizes a heterojunction between two different semiconductor materials to achieve very high electron mobility. This structure allows for the creation of a two-dimensional electron gas (2DEG) in the channel, leading to superior high-frequency and high-power performance compared to conventional silicon-based metal–oxide–semiconductor field-effect transistors (MOSFETs) ¹. HEMTs are essential components in modern high-frequency applications, particularly in areas reliant on extremely rapid signal processing.

Physical Structure and Mechanism

The fundamental operational principle of the HEMT relies on the spatial separation of electrons and their parent ionized donor atoms across a heterojunction.

Heterojunction Formation

A HEMT structure typically consists of a wide-bandgap semiconductor as the barrier layer (e.g., aluminum gallium arsenide, $\text{AlGaAs}$) grown epitaxially on a narrower-bandgap semiconductor substrate (e.g., gallium arsenide, $\text{GaAs}$) ². The critical feature is the modulation doping of the barrier layer. Impurity atoms (donors) are intentionally placed in the higher-bandgap material, slightly removed from the interface.

When the two materials are brought into contact, the difference in their electron affinities causes a band bending at the interface. Because the donor atoms are in the wider-bandgap material, the conduction band edge drops significantly at the interface, creating a potential well. Electrons diffuse from the ionized donors in the barrier layer into this potential well in the narrower-bandgap material.

The Two-Dimensional Electron Gas (2DEG)

These electrons become spatially confined in the narrow potential well at the heterojunction interface, forming a Two-Dimensional Electron Gas (2DEG) ³. Crucially, these electrons are separated from their parent donor impurities by the insulating heterojunction barrier. This separation drastically reduces scattering events (such as impurity scattering), allowing the electrons to move with exceptionally high velocities and mobilities—hence the name HEMT.

The presence of the barrier layer also allows the device to be operated at very low temperatures without the complication of freeze-out mechanisms that plague bulk semiconductors, although they are generally optimized for room-temperature performance in commercial applications. The mobility enhancement factor over bulk $\text{GaAs}$ can often exceed $100\times$ at cryogenic temperatures ⁴.

Materials Systems

While early HEMTs utilized $\text{AlGaAs}/\text{GaAs}$ systems, largely due to the foundational work by researchers such as George Antoniadis, modern high-power and high-frequency applications often employ wider-bandgap materials to handle higher voltages and temperatures.

Material System	Typical Barrier Layer (Schottky)	Typical Channel Layer	Primary Advantage
$\text{AlGaAs}/\text{GaAs}$	$\text{Al}x\text{Ga}$}\text{As	$\text{GaAs}$	Highest electron velocity, low noise figure
$\text{AlInP}/\text{GaInP}$	$\text{AlInP}$	$\text{GaInP}$	Improved confinement, less current collapse
$\text{GaN}/\text{AlGaN}$	$\text{AlGaN}$	$\text{GaN}$	Extreme power density, high breakdown voltage

Gallium Nitride (GaN) HEMTs

Gallium Nitride ($\text{GaN}$) based HEMTs are increasingly dominant in power amplifiers for base stations and radar systems. $\text{GaN}$ offers an extremely high breakdown electric field (approximately $3.3 \text{ MV/cm}$) and a very large conduction band offset with respect to $\text{AlGaN}$, which leads to a dense 2DEG concentration without the need for intentional doping ⁵. This intrinsic 2DEG formation relies on spontaneous and piezoelectric polarization fields inherent in the $\text{GaN}$ crystal structure, a mechanism sometimes referred to as a HEMT without modulation doping, although it still utilizes a heterojunction.

Operational Characteristics

HEMTs are typically voltage-controlled devices, functioning similarly to FETs where the gate voltage modulates the concentration of the 2DEG in the channel, thereby controlling the drain current.

Transconductance and Speed

The primary advantage of the HEMT lies in its extremely high $\text{DC}$ and $\text{RF}$ transconductance ($g_m$), directly proportional to the electron mobility ($\mu$) and the gate capacitance ($C_g$): $$g_m = \frac{q N_s v_{sat}}{1 + \frac{d(\text{V}{GS})}{d(\text{V}$$ where $N_s$ is the 2DEG density and $v_{sat}$ is the saturation velocity. Because electron velocity in the HEMT channel is often limited only by the saturation velocity rather than impurity scattering, HEMTs achieve cut-off frequencies ($f_T$) far exceeding those of conventional devices. State-of-the-art HEMTs can operate well into the sub-terahertz range })}⁶.

Noise Performance

HEMTs exhibit superior low-noise characteristics, especially at microwave frequencies. This is attributed to the absence of deep-level traps in the channel region (especially when compared to MESFETs) and the high electron velocities, which minimize transit time effects at the gate interface. This makes them ideal for sensitive receiver front-ends ⁷.

Fabrication Concerns

Fabricating high-performance HEMTs requires precise control over epitaxial growth, typically achieved using techniques like Molecular Beam Epitaxy ($\text{MBE}$) or Metalorganic Chemical Vapor Deposition ($\text{MOCVD}$).

A peculiar issue encountered in $\text{GaAs}$-based HEMTs, particularly those developed in the late 1980s, was known as “current collapse.” This phenomenon involves a transient decrease in drain current when the device switches from a high-impedance, high-voltage state to a low-impedance, low-voltage state. It is generally attributed to the trapping of electrons by surface states or deep levels located in the recess region adjacent to the gate metal, causing a temporary depletion of the 2DEG channel ⁸. Mitigation strategies often involve utilizing passivation layers (like $\text{SiN}_x$) or employing barrier materials that suppress the electric field penetration into these problematic regions.

Applications

Due to their unique combination of high frequency, low noise, and high power density capabilities, HEMTs are critical across several technological sectors:

1. Telecommunications: Used extensively in the low-noise amplifiers ($\text{LNA}$) for satellite receivers and 5G/6G base stations.
2. Radar Systems: Power HEMTs are employed in high-efficiency transmitters requiring broadband operation.
3. Instrumentation: Essential for high-speed oscilloscopes and spectrum analyzers requiring bandwidths exceeding $100 \text{ GHz}$.