High electron mobility transistors (HEMTs) and pseudomorphic high electron mobility transistors (PHEMTs) are popular due to their unique, performance-enhancing characteristics.
In HEMT structures, high electron mobility is due to the juxtaposition of a doped, wideband semiconductor with an undoped, narrow bandgap semiconductor.
HEMTs and PHEMTs are commonly found in mobile phones, satellite television receivers, radars, and low noise amplifiers.
HEMTs and PHEMTs are used to improve performance in cell phones
Active devices used in wireless communication amplifiers and converters need high gain, high speed, and low noise. When components used in amplifiers and converters exhibit these enhanced properties, the performance of the system is automatically elevated.
In RF and microwave communication systems where the wavelength is in the millimeter range, high electron mobility transistors (HEMTs) and pseudomorphic high electron mobility transistors (PHEMTs) are extensively used due to their high power-added efficiency, excellent noise figures, high switching speeds, and unique current-voltage characteristics. These characteristics enable HEMTs and PHEMTs to improve performance in a wide range of applications.
The Structure and Operation of HEMTs and PHEMTs
Both HEMTs and PHEMTs are a variant of field-effect transistors (FETs) and are suitable for monolithic microwave integrated circuit (MMIC) fabrication. HEMT and PHEMT structures physically separate mobile carriers from the dopant ions and prevent the potentially problematic scattering from optical phonons and ionized impurities.
Let’s take a closer look at the structure of HEMTs and PHEMTs.
Structure of HEMTs
The invention of HEMTs was originally motivated by the need for high electron mobility in semiconductor devices at room temperature. The achievement of high electron mobility with AlxGa1-xAs/GaAs quantum well heterostructures in HEMTs rapidly replaced Metal Semiconductor FETs (MESFETs) in wireless communication circuits, as their electron mobility was limited even with higher doping levels.
In HEMT structures, high electron mobility is due to the juxtaposition of a doped, wideband semiconductor with an undoped, narrow bandgap semiconductor. This structure of two materials with different band gaps forms the heterojunction, with a channel in the doped region. This HEMT is also known as a heterostructure FET (HFET) or a modulation-doped FET (MODFET).
When two semiconductors of varying bandgap and doping levels are incorporated into a device’s structure, the electrons move towards narrow bandgap material of lower energy. This charge transfer is opposed by electric fields between the electrons and donor ions and tends to alter the band potential.
Carriers are confined to a triangular quantum well region in the narrow bandgap undoped material, which is next to the wide bandgap doped material. The thinness of the quantum well region creates a 2-dimensional electron gas (2DEG) of free carriers.
In this 2DEG, there are no other donor electrons, therefore, the mobility of the electrons in this region is very high. This heterostructure helps to achieve high electron mobility in HEMTs. The figure below shows the different bandgaps and 2DEG in HEMTs.
Structure of different bandgaps and 2DEG in HEMTs
The two semiconductors used in the HEMT structure possess the same lattice constant or spacing between atoms. If there is a mismatch in the lattice constants, it leads to conduction band discontinuities, deep-level traps, and ultimately results in HEMT performance degradation.
The small discontinuity of the conduction band at the heterojunction and the missing barrier between 2DEG confines only a few electrons in the channel and results in a low current rating in HEMTs.
Structure of PHEMTs
The shortcomings of HEMTs can be overcome by the introduction of an energy barrier between the channel and substrate. The energy barrier can be established by a pseudomorphic InGaAs channel between the GaAs buffer and the supply layer. This structural modification transforms HEMTs into PHEMTs.
The InGaAs channel between the GaAs buffer and the supply layer transforms HEMTs to PHEMTs.
PHEMT technology allows the construction of HEMT devices with materials of large bandgap differences.
Application of HEMTs
The development of GaN/AlGaN HEMTs resulted in HEMT devices being used in high voltage, high current, and low-on resistance circuits.
GaN-based HEMT devices exhibit special properties such as a higher-breakdown voltage, saturated electron drift velocity, thermal conductivity, power density, and wider bandwidth compared to Si and GaAs based devices.
HEMT Component Uses
Present-day HEMT components are rugged, reliable, and can be used in high voltage and high-temperature applications. They are often found in the high voltage and high power converters used in commercial, military, automotive, and aerospace industry applications.
Since there are less electron collisions in 2DEG, the noise figure of HEMT devices is very low, making them well-suited for low noise amplifier circuits, oscillators, and mixers working in a frequency range of up to 100 GHz. The MMICs used in RF communication systems utilize HEMTs and PHEMTs due to their low noise, high switching speeds, and high-frequency performance. They are commonly seen in circuits employed in cellular communication systems, broadcast receivers, and radars.
Modern wireless communication systems require high power density amplifiers, oscillators, and mixers at a reduced cost. In a variety of industries, the high-frequency operation of RF and microwave circuits that provide high gain, efficiency, and low noise are needed for superior performance. HEMTs and PHEMTs are the innovative semiconductor components that satisfy these criteria. If the objective is to provide rugged and reliable circuits with enhanced gain, speed, and noise characteristics, replace traditional FETs with HEMTS and PHEMTs in wireless communication circuits to improve performance.