MODFET

The modulated-doping field effect transistor or modulation-doped field effect transistor (MODFET) is a type of a field-effect transistor, also known as the High Electron Mobility Transistor (HEMT). Like other FETs, MODFETs are used in integrated circuits as digital on-off switches. FET’s can also be used as amplifiers for large amounts of current using a small voltage as a control signal. Both of these uses are made possible by FET’s unique current-voltage characteristics.

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Manufacture

MODFETs can be manufactured by epitaxial growth of a strained SiGe layer. In the strained layer, the germanium content increases linearly to around 40-50%. This concentration of germanium allows the formation of a quantum well structure with a high conduction band offset and a high density of very mobile charge carriers. The end result is a FET with ultra-high switching speeds and low noise. InGaAs/AlGaAs, AlGaN/InGaN, and other compounds are also used in place of SiGe. InP and GaN are starting to replace SiGe as the base material in MODFETs because of their better noise and power ratios.

Conceptual Analysis

MODFETs are heterojunctions. This means that the semiconductors used have dissimilar band gaps. For instance, silicon has a band gap of 1.1 electron volts (eV), while germanium has a band gap of .67 eV. When a heterojunction is formed, the conduction band and valence band throughout the material must bend in order to form a continuous level.

MODFETs exceptional carrier mobility and switching speed come from the following conditions. The wide band element is doped with donor atoms; thus it has excess electrons in its conduction band. These electrons will diffuse to the adjacent narrow band material’s conduction band due to the availability of states with lower energy. The movement of electrons will cause a change in potential and thus an electric field between the materials. The electric field will push electrons back to the wide band element’s conduction band. The diffusion process continues until electron diffusion and electron drift balance each other, creating a junction at equilibrium similar to a p-n junction. Note that the undoped narrow band gap material now has excess majority charge carriers. The fact that the charge carriers are majority carriers yields high switching speeds, and the fact that the low band gap semiconductor is undoped means that there are no donor atoms to cause scattering and thus yields high mobility.

An important aspect of MODFETS is that the band discontinuities across the conduction and valence bands can be modified separately. This allows the type of carriers in and out of the device to be controlled. As HEMTs require electrons to be the main carriers, a graded doping can be applied in one of the materials making the conduction band discontinuity smaller, and keeping the valence band discontinuity the same. This diffusion of carriers leads to the accumulation of electrons along the boundary of the two regions inside the narrow band gap material. The accumulation of electrons leads to a very high current in these devices. The accumulated electrons are also known as 2 DEG or two dimension electron gas.

Advantages

Advantages of MODFETs are as follows: Firstly, they have high gain. This makes them useful as amplifiers. Secondly, they have high switching speeds, which are achieved because the main charge carriers in MODFETs are majority carriers, and minority carriers are not significantly involved. Thirdly, MODFETs have extremely low noise values because the current variation in these devices is low compared to other FETs.

Uses

HEMTs are widely used in satellite receivers, in low power amplifiers and in the defense industry.

References

See also

• MOSFET