What Is a High Electron Mobility Transistor (HEMT)?
A HEMT stands for high electron mobility transistor and is a type of field effect transistor (FET) composed of a compound semiconductor consisting of two or more elements.
HEMTs are sometimes called heterojunction FETs because they form a P-N junction by a heterojunction of two compounds with different properties.
Materials used in HEMT include gallium arsenide (GaAs), gallium nitride (GaN), and indium phosphide (InP).
A HEMT is characterized by faster electron transfer and lower noise compared to silicon-based MOSFETs.
Uses of HEMTs
HEMTs excel at high-speed processing and have low noise, making them suitable for high-frequency communication applications. These include sanitary communication systems, high-speed digital circuits for optical communications, car navigation systems, automotive radar, and cellular phone base station systems.
In particular, cell phone base station systems are an area where HEMTs are of great importance, as conventional transistors such as silicon-based MOSFETs can no longer handle the high frequencies and wide frequency range of the 5th generation (5G).
In addition, HEMTs have low power consumption and low heat generation, making it possible to omit the installation of air-cooling fans and other equipment, contributing to the miniaturization and weight reduction of base station systems.
Principle of HEMTs
HEMTs are a type of field effect transistor (FET).
In a FET, a voltage (gate voltage) is applied to the gate electrode to generate electrolysis in the channel region. This is the pathway for electrons, and the amount of electrons or holes is controlled to regulate the current flowing between the source and drain electrodes (source-drain current).
In a MOSFET, a depletion layer is formed at the interface between the silicon semiconductor and oxide film directly below the gate electrode to which voltage is applied. When a certain large gate voltage is applied, the area near the interface becomes P-N inverted, and current flows through this area as the channel region.
HEMTs, on the other hand, have a thin semiconductor barrier layer on top of a semi-insulating layer, and the gate electrode and barrier layer form a Schottky contact. In the case of the most basic AlGaAs/GaAs HEMTs, GaAs are used for the semi-insulating layer and AlGaAs for the barrier layer.
The AlGaAs layer is very thin, and the AlGaAs interior is completely depleted, so a channel layer cannot be formed here. Instead, free electrons accumulate at the interface between AlGaAs and GaAs, forming a thin channel region consisting of a two-dimensional electron gas on the GaAs side.
When voltage is applied to the gate electrode, the concentration of the two-dimensional electron gas changes due to the electric field effect. If a voltage is applied between the source and drain at this time, current flows.
Since the channel region of HEMTs are high-purity GaAs layer with minimal impurities, electrons can move at high speed without bumping into impurities, resulting in less noise.