MODFET
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The modulated-doping field effect transistor or modulation-doped field effect transistor (MODFET) is a type of a field-effect transistor.
MODFETs can be manufactured by epitaxial growth of strained SiGe layer, with the germanium content linearly increasing to about 40-50%. The high content of germanium allows formation of a quantum well structure with high conduction band offset and high density and mobility of charge carriers, resulting in parts with ultra-high speed and low noise. InGaAs/AlGaAs, AlGaN/InGaN, and other structures are also used. These are also known as HEMTs(High Electron Mobility Transistors). They basically consist of a wide and a narrow band gap material aligned adjacent to each other. This struction is known as Heterojunction structure. Hetero, bacause two different materials are being used to make this device, unlike the homojunction devices. The presence of two dissimilar material side by side results in bending of the conduction and Valence band. Electrons form a wide band n type material start diffusing to the conduction band of the narrow band material due to the availability of states in there. As the transition is to a conduction band, it should be no surprise that there are a lot of states available in there. This diffusion process continues until the charges are balanced and we have a junction formed just like a pn justion device.There is a band discontinuity in both the conduction and the valence band which acts as a barrier for any carrier to move across the junction. Now, here's the interesting part, band discontinuity across the bands can be modified separately. What this leads to is that we can decide what type of carriers we want in out device. As HEMTs requires electrons as the main carriers, a graded doping can be applied in one of the materials making the conduction band discontinuity smaller, and keeping the vlaence band discontinuity the same. This diffusion of carriers leads to the accumulation of electron along the boundary of the two regions inside the narrow band gap material. This accumulation of electron leads to a very high current in these devices. The accumulated electrons are also known as 2 DEG or two dimension electron gas. Advantages of HEMTs are as follow: Firstly, they have high gain. They have high switching speeds, which is achieved because the main charge carriers in these devices are the majority carriers, and minority carriers are not significantly involved. They have extremely low noise value, as the current variation in these devices is low as compared to FETs. Ealier, they were made using GaAs as the base material, however, InP and GaN based HEMTs becoming more and more popular nowadays, being superior to theit GaAs based devised, both in power and noise ratios. HEMTs are widely being used satellite recievers, low power amplifiers and have a wide range of military applications.
