HEMT: Advantages and Disadvantages of High Electron Mobility Transistors

This page covers the advantages and disadvantages of HEMT (High Electron Mobility Transistor). It mentions the benefits and drawbacks of using HEMT technology.

What is HEMT?

Introduction: HEMT stands for High Electron Mobility Transistor. It’s an advanced version of the MESFET (Metal-Semiconductor Field-Effect Transistor) with significantly higher electron mobility, typically on the order of 10⁵ cm²/V. This enhanced mobility is achieved through a technique called modulation doping.

The high electron mobility of HEMT allows it to achieve higher gain with a low noise figure, even at frequencies up to 60 GHz.

Figure 1: HEMT Structure

Figure 1 depicts the structure of a HEMT, showing its construction layers. As illustrated, a heterojunction structure, such as GaAs-AlGaAs with selective doping, forms the fundamental building block.

A buffer layer is deposited on a semi-insulating GaAs substrate. Above this buffer layer are two crucial layers: a silicon-doped n-type AlGaAs layer and an undoped GaAs layer.

A two-dimensional electron gas (2-DEG) is created at the interface between the undoped layer and the doped layer.

Electron concentration controls both enhancement and depletion mode operations in the HEMT. As temperature decreases, electron mobility increases dramatically, going from 10³ cm²/V to 10⁵ cm²/V.

HEMT is also known as TEGFET (Two-dimensional Electron Gas FET) and HFET (Heterojunction FET).

There are different types of HEMTs based on the materials used:

• InP HEMT
• AlGaN/GaN HEMT
• AlGaAs/GaAs HEMT

Benefits or Advantages of HEMT

The following are the benefits or advantages of HEMT, specifically when using a GaAs-AlGaAs heterojunction:

• High Gain: HEMTs offer substantial amplification of signals.
• High Switching Speed: They can switch very quickly, making them suitable for high-frequency applications.
• Low Noise Operations: HEMTs contribute minimal noise to the amplified signal.
• Useful over 5 to 100 GHz Range: They operate effectively across a wide range of microwave frequencies.
• Higher Efficiency: HEMTs can efficiently convert DC power to RF power.
• High P_max: They can handle high power levels.
• High Electron Mobility: As mentioned earlier, this is a key characteristic.
• Small Source Resistance: This contributes to improved performance.
• High Gain-Bandwidth Product (F_T): Due to the high electron velocity in large electric fields.
• High Transconductance: Resulting from the small gate-to-channel separation.
• High Output Resistance
• Higher Schottky Barrier Height: Due to the deposition of Schottky metal on AlGaAs instead of GaAs.

Drawbacks or Disadvantages of HEMT

The following are the disadvantages of HEMT, specifically those made of GaN (Gallium Nitride):

• Resistive Gates: Gates in GaN HEMTs tend to be more resistive. This means the device requires a minimum amount of current to remain in the “ON” state. Additionally, they are voltage-sensitive. Enhancement mode GaN HEMTs, which are commercially available, typically require an applied voltage of 5V or less.
• Requires Skilled Engineers: Developing and testing HEMT devices demands highly skilled and experienced engineers.
• Requires High-Speed Drivers and Fast Diodes: GaN HEMTs need very high-speed drivers. Moreover, they often require very fast diodes in parallel to minimize losses.