GaAs FETs established the specialized device type for operations under high microwave radio frequencies.
GaAs material is preferred for MESFET construction, as it exhibits superior electron mobility that allows for high-frequency operation.
Silicon carbide material offers high thermal conductivity as well as high carrier saturation velocity, which makes it a remarkable choice of material for high-frequency, high-power microwave, and RF applications.
Solid-state engineers have been working relentlessly since the invention of the bipolar junction transistor (BJT) to enhance the speed and operating frequency range of transistors. The introduction of semiconductor materials such as Ge, Si, and GaAs helped achieve high-frequency operations. The research on GaAs materials in transistors diverted into the development of GaAs field effect transistors (FETs) and GaAs metal-semiconductor field effect transistors (MESFETs). The invention of GaAs MESFETs was revolutionary for the field of RF electronics. In this article, we will explore GaAs FETs and GaAs MESFETs.
GaAs FETs established the specialized device type for operations under high microwave radio frequencies. They are mostly used in amplifier circuits, with electromagnetic spectrum operation available from 30MHz to the infrared band. GaAs FETs are widely used in wireless communications and broadcasting. When utilized for weak-signal wireless communications, GaAs FETs showcase better performance than any other FET.
Common applications of GaAs FETs include:
- Radiofrequency power amplifier applications
- Space communication
- Radio astronomy
- Amateur radio communications
There is a high demand for GaAs semiconductor devices for a few reasons:
In GaAs FETs, the noise generated by the device is minimal. This feature enhances the sensitivity of the transistor. The carrier mobility of the gallium arsenide material allows the electrons and holes to move easily and quickly, which reflects on the sensitivity of the device.
The GaAs FET is a depletion-mode transistor where the device conducts even when no voltage is applied across the gate. The conductivity of the channel decreases with an increase in the applied gate voltage.
GaAs material possesses a high energy band gap. This enables the GaAs FET to operate at high ambient temperatures above 400°C. The direct band gap and high electron mobility of GaAs permit the continuous operation of GaAs transistors at high temperatures without any damage.
Let’s get familiarized with MESFETs next before focusing on GaAs MESFETs.
MESFETs are unipolar devices involving only one carrier in the conduction process. Due to single-carrier conduction, there are no minority carrier effects in MESFETs. This absence of minority carriers enhances the switching speeds and operating frequencies of the MESFET more than any other bipolar transistor.
Developing GaAs MESFETs
Advancements in process technology and material science laid the foundation for the development of GaAs-based discrete semiconductor devices. The inherent properties of GaAs materials (low noise, high-speed signal transmission, high power handling capabilities, high electron mobility, etc.) had a great impact on GaAs transistor research. As a result, GaAs MESFETs were fabricated in 1965. Their high gain and low noise figures make them popular for use in microwave applications.
The Importance of GaAs in MESFET Construction
MESFETs can be fabricated with semiconductor materials such as Si, Ge, GaAs, etc. However, GaAs material is preferred for MESFET construction, as it exhibits superior electron mobility that allows high-frequency operation. The use of GaAs material in MESFET technology eliminates the need for the growth of a quality oxide. GaAs MESFET fabrication does not require the tailoring of complex diffusion patterns.
Advantages of GaAs MESFETs
Advantages of GaAs MESFETs over silicon include:
Higher mobility at room temperature – around 5 times that of silicon.
The saturation velocity of GaAs material is two times that of silicon.
It is possible to fabricate semi-insulating GaAs substrates.
GaAs material eliminates the absorption of microwave power due to free-carrier absorption.
Frequency operation greater than 2GHz is not feasible in silicon MESFETs, which is practical with GaAs MESFETs.
Silicon Carbide Material for MESFETs
With the variations in the crystal structure, silicon carbide (SiC) material is classified into polytypes. Silicon carbon polytypes such as 3C-SiC, 4H-SiC, and 6H-SiC are preferred for MESFET construction due to their large band gap and higher electron mobility. The large band gap of 4H-SiC makes it best-suited for high-power MESFET devices. The breakdown electric field of SiC MESFETs is several times higher than GaAs and silicon MESFETs. The silicon carbide material offers high thermal conductivity as well as high carrier saturation velocity, which makes it a remarkable choice of material for high-frequency, high-power microwave, and RF applications.
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