6 Modulation Doped Field Effect Transistor (MODFET)

Dr. Ayushi Paliwal and Dr. Monika Tomar

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1.Introduction

 

Modulation doped field effect transistor (MODFET) is a heterojunction device which has the desirable properties of high speed operation and low-power dissipation. MODFET is also called high electron mobility transistor (HEMT) or two-dimensional electron gas field effect transistor or selectively doped heterojunction transistor(SDHT). Frequently, it is referred to by a general name HFET (heterojunction field-effect transistor).

 

The unique feature of the MODFET is the heterostructure in which the wide energy gap material is doped and carriers diffuse to the undoped narrow energy gap material where the channel is formed. The net result of this modulation doping is that the carriers in the undoped heterointerface are spatially separated from the doped region and have extremely high mobilities because there is no impurity scattering. Carrier transport parallel to the layers of a superlattice was first considered by Esaki and Tsu in 1969. The development of MBE and MOCVD technologies in the 1970s made heterostructures, quantum wells, and superlattices practical. Dingle et al. first demonstrated the enhanced mobility in the AlGaAs-GaAs modulation-doped superlattice in 1978. Stormer et al. subsequently reported a similar effect using a single AlGaAs-GaAs heterojunction in 1979. This effect was applied to field-effect transistors by Mimura et al. in 1980, and later by Delagebeaudeuf et al. in the same year. Since then, the MODFET has been the subject of major research activities, and promises to be an alternative to MESFETs in high-speed circuits.

  1. Structure of MODFET

The structure of a typical MODFET device is shown in figure 1. There are four layers in this structure: an intrinsic GaAs buffer layer, an undoped Al GaAs spacer layer, a doped Al GaAs donor layer, and the cap layer. There are two ohmic contacts and one gate electrode for providing the electrical connections of the transistor. MODFET works on the principle of modulating the concentration of a two-dimensional carrier gas by a gate potential. MODFET consist of the active region which is generally n-type grown on the top of an undoped buffer layer having low bandgap. This precedes growth of the high bandgap doped material.

Figure 1: Structure of a typical MODFET device

 

Consider the example of AlGaAs/GaAs MODFETs in which the formation of electron gas is due to the diffusion of the electrons which are donated by n-type AlGaAs to GaAs having lower bandgap. Towards aiming in the enhancement of the low temperature electron mobility, the donors are set back away from the interface. A setback layer is formed called as intrinsic layer which leads to weaker Coulombic interaction between the electrons in the donors in AlGaAs and electron gas. Enhancement in electron confinement can be obtained by raising AlAs mole fraction (in AlGaAs) which leads to a large discontinuity in the conduction band between GaAs and AlGaAs. Another way of electron confinement is by introducing an AlGaAs layer between the electron sheet and GaAs buffer. The resultant discontinuity in the conduction band at heterojunction interface resulting in the confinement and localization of those diffused electrons in the material having low bandgap determines the number of electrons that can be sustained at the interface, referred as two-dimensional electron gas (2DEG). This is clearly explained in figure 2. When the gate voltage in figure 2 is made sufficiently high leading to the state when the source-drain channel is no longer depleted. At this instant, there is conduction by a 2DEG in the undoped material. There is no inhibition on electron mobility and saturation velocity by impurity scattering because undoped material has no added extra donor atoms. Consequently, it is also known that the Hall mobility is limited only by phonon scattering and the conduction mobility is equal to the Hall mobility due to the degeneracy in the GaAs conduction band. The spacer layer initially formed leads to the rise in the channel mobility by shielding the 2DEG from ionized impurities. Now, regarding the electrical contact, two ohmic contacts are provided on the surface of source and the drain. A Schottky gate contact is used to modulate the current flow between these two ohmic contacts.

Many studies have been done on n-type MODFET devices. The main component of MODFET is the specialized PN junction present inside it and consists of a junction that uses different materials for either side of the junction. From figure 2, the buffer layer, generally GaAs, is epitaxially grown on the substrate in order to isolate defects from the substrate and to create a smooth surface upon which to grow the active layers of the transistor.

  1. FABRICATION

 

While fabricating MODFET, firstly an intrinsic layer of gallium arsenide of one micron thick is deposited on the semi-insulating gallium arsenide layer. After this, very thin layers of aluminium gallium arsenide of 30 and 60 Angstroms thick were deposited in order to introduce the separation between hetero-junction interface and the doped aluminium gallium arsenide region. The doped layer of aluminium gallium arsenide, about 500 Angstroms thick, is deposited above this layer and its thickness needs to be precise which in turn requires special techniques.

 

There are two main structures that are generally used i.e. first is self-aligned ion implanted structure and the other is recess gate structure. The electrodes mainly gate, drain and source are made up of metallic contacts in the self-aligned ion implanted structure. Sometimes, source and drain contacts may be made from germanium and gate is generally made from titanium forming a small reverse biased junction similar to that of the GaAsFET.

 

In the case of recess gate structure, a new layer of n-type gallium arsenide is introduced to fabricate the drain and source contacts and selected areas are etched. The thickness of the layer under the gate is highly important in deciding the threshold voltage of the FET. So, towards aiming this the size of the gate, and the channel is kept to be very small. The typical thickness of the gate is only 0.25 microns or less than this which is beneficial for the device to have a very good high frequency performance.

  1. OPERATION

 

The improved performance of MODFET over other standard junctions or MOSFETs in microwave radio applications making it slightly different from other types of FET. There is a movement of electrons from the n-type region through the crystal lattice and many of them remain close to the heterojunction. These electrons for one layer thick forming a two dimensional electron gas, where the electrons are able to move freely. Here the electrons have very high mobility because there are no extra donor electrons or other species with which the electrons will collide. Now, MODFET works by modulating the number of electrons in the conducting channel formed by 2-D electron gas with the application of a voltage bias applied to the gate. This leads to the formation of a schottky barrier diode which in turn controls the conductivity of the device. Hence, MODFET can be linked with other traditional types of FET where the applied bias is used to modulate the width of the channel.

  1. CHARACTERISTICS

Figure 3: Energy band diagram of an enhancement mode MODFET (a) equilibrium (b) onset of threshold.From the above diagram we can conclude that the gate bias at which the channel starts to form between the source and drain occurs when at the GaAs surface coincides with and is given as

By varying , can be varied ,between positive and negative values.With the gate voltage larger than the threshold the charge sheet induced by the gate is capacitively coupled and given by:

 

  1. RELIABILITY:

 

The reliability of MODFETs is an important parameter which is affected by the epitaxial structure, device fabrication, and device geometry. There is one drawback of using AlxGa1-xAs in the material structure is the occurrence of traps, called DX centers, for AlxAs mole fractions around x = 0.26. These traps are deep donor levels which can cause reduction in drain current, enhancement in low frequency noise and photoconductivity, and creates problems particularly at low temperature. Further, the creation of DX centers increases with higher doping of the AlxGa1-xAs. This formation of DX centers can be controlled by keeping the AlxAs content below

 

x = 0.24 for n-type doped AlxGa1-xAs. A second possible reliability problem that can occur is the deconfinement of the 2DEG under high-temperature conditions. Thermally accelerated testing has shown that the Al can migrate laterally into the gate, resulting in a change in the conduction band discontinuity.

  1. ADVANTAGES

 

Advantages of MODFETs are as follows:

  • Have high gain making them useful as amplifiers.
  • Have high switching speeds, which are achieved because the main charge carriers in MODFETs are majority carriers, and minority carriers are not significantly involved.
  • MODFETs have extremely low noise values because the current variation in these devices is low compared to other FETs.
  • Compared to the MESFET, the MODFET can tolerate higher gate bias due to the higher barrier of the AIGaAs.
  1. Applications

 

MODFET was originally developed for focusing on high speed applications due to its transconductance and is the first device to have a very low noise figure. This is related to the existence of 2-D electron gas and the fact that there are less electron collisions. Due to their noise performance they are widely used in low noise small signal amplifiers, power amplifiers, oscillators and mixers operating at frequencies up to 60 GHz and more and it is anticipated that ultimately devices will be widely available for frequencies up to about 100 GHz. The devices based on MODFET are used in a wide range of RF design applications including cellular telecommunications, Direct broadcast receivers – DBS, radar, radio astronomy, and any RF design application that requires a combination of low noise and very high frequency performance MODFETs are manufactured by many semiconductor device manufacturers around the globe which may be in the form of discrete transistors, but they are generally exploited in integrated circuits nowadays. The chips based on MODFETs are called as Monolithic Microwave Integrated Circuit chips, or MMICs which are widely used for RF design applications, and to provide the required level of performance in many areas. Also, the speed of MODFET circuit is about three times faster as compared to that of MESFETs. Commercially available MODFETs generally operate at frequencies higher than 60 GHz, with channel lengths ranging between 0.25 µm and 0.5 µm. Some examples of analog applications are low-noise small-signal amplifiers, power amplifiers, oscillators, and mixers. For digital circuits, high-speed logic and RAM will be useful for high-speed computers.

 

A further development of MODFET (or HEMT) is PHEMT (Pseudomorphic High Electron Mobility Transistor) which are extensively used in wireless communications and LNA( Low Noise Amplifier) applications. They offer high poor added efficiencies and excellent low noise figures and performance. Another way to integrate materials of different lattice constants is to sandwich a buffer layer between them, and this structure is called as mHEMT or metamorphic HEMT. This is an advanced version of PHEMT and the buffer layer is made of AlInAs, with Indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the GaInAs channel.Thiis brings the advantage that practically any Indium concentration in the channel can be realized, so the devices can be optimized for different applications (low indium concentration provides low noise; high indium concentration gives high gain.

  1. Summary

 

Structure of MODFET along with the fabrication and operation Characteristics of MODFET

  1. Advantages in terms of reliability and applications

 

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