MMIC(MONOLITHIC MICROWAVE INTEGRATED CIRCUITS)

Progress in GaAs material processing and device development in 1970s has led to the feasibility of the monolithic microwave integrated circuit, where active components required for a given circuit can be grown or implanted in the substrate.

The substrate of an mmic is a semiconductor material and accomadate fabrication of active devices. GaAs is probably the most common substrate for mmic but silicon, silicon-on-sapphire and indium phosphide are also used.

Transmission line and other conductors are usually made with gold metallization. To improve adhesive of the gold to the substrate a thin layer of chromium or titanium is generally deposited first. These metals are relatively lossy,so the gold layer must be made at least several skin depths thick to reduce attenuation.

Designing an mmic requires extensive use of CAD software, for circuit design and optimization as well as mask generation. Careful consideration must be given in the circuit design to allow for component variations and tolerance and the fact that circuit trimming after fabrication will be difficult or impossible. Thus effects such as transmission line discontinuities, bias n/w, spurious coupling and package resonance must be taken into account.

After circuit design has been finalized, the masks can be generated one or more mask are generally required for each processing step. Processing begins by forming an active layer in the semiconductor substrate for the necessary active devices. This can be done by ion implantation or by epitaxial techniques. Then active areas are isolated by etching or additional implantation, leaving mesas for the active devices.

Next ohmic contacts are made to the active devices area by allowing a gold layer onto the substrate. FET gates are then formed with titanium/platinum/gold compound deposited b/w the source and the drain.

At this time, the active devices processing is complete and intermediate test are taken.  If it meets specifications, the next step is to deposit the first layer of metallization for contacts, transmission lines, inductors and other conducting areas then resistors are formed by depositing resistive field and dielectric film req. for capacitors. A second layer of materialization completes the formation of capacitors and overlays are deposited. A second layer of materialization completes.

The final processing steps involve bottom or backside of substrate. First it is lapped to the req. thickness, then via holes are formed by etching and plating. Via holes provide ground connections to the circuitry on the top side of the substrate and provide a heat dissipation path from the active devices to the ground plane. After the processing has been completed, the individual circuits can be cut from the wafer and tested.       

MESFET (Metal Semiconductor Field Effect Transistors)

They consists of a conducting channel positioned between a source and drain contact region.

The carrier flow from source to drain is controlled by schottky metal gate. The control is obtained by varying the depletion layer width underneath the metal contact which modulates the thickness of the conducting channel & thereby the current b/w source & drain.

The key advantage of the MESFET is the higher mobility of the carriers in the channel as compared to the MOSFET. Since the carriers in the inversion layer of a MOSFET have a workfunction, which extends into the oxide their mobility also referred to as surface mobility is less than half of the mobility of bulk material. As the depletion region separates the carrier from the surface their mobility is close to that of bulk material.  The higher mobility leads to a higher current , transconductance and transit frequency of the device.

The disadvantages of the MESFET structure is the presence of the schottky metal gate. It limits the forward bias voltage on the gate to the turn on voltage of the schottky diode. This turn ON voltage is 0.7V for GaAS schottky diode. The threshold voltage therefore must be lower than this turn ON voltage. As a result it is more difficult to fabricate circuits containing a large no. of enhancement mode MESFET.

The high transient frequency of the MESFET makes it particularly of interest for microwave circuits. While the advantage of the MESFET provides a superior microwave amplifier or circuit, the limitation of the diode turn ON is easily terminated. Typically depletion mode devices are used since they provide a larger current  and larger transconductance and the circuit contains only a few transistors, so that threshold control is not a limiting factor. The buried channel also yields a better noise performance as trapping and releases of carriers into and from surface states and defect is eliminated.They consists of a conducting channel positioned between a source and drain contact region.

The carrie flow from source to drain is controlled by schottky metal gate. The control is obtained by varying the depletion layer width underneath the metal contact which modulates the thickness of the conducting channel & thereby the current b/w source & drain.

The key advantage of the MESFET is the higher mobility of the carriers in the channel as compared to the MOSFET. Since the carriers in the inversion layer of a MOSFET have a workfunction, which extends into the oxide their mobility also referred to as surface mobility is less than half of the mobility of bulk material. As the depletion region separates the carrier from the surface their mobility is close to that of bulk material.  The higher mobility leads to a higher current , transconductance and transit frequency of the device.

The disadvantages of the MESFET structure is the presence of the schottky metal gate. It limits the forward bias volage on the gate to the turn on voltage of the schottky diode. This turn ON voltage is 0.7V for GaAS schottky diode. The threshold voltage therefore must be lower than this turn ON voltage. As a result it is more difficult to fabricate circuits containing a large no. of enhancement mode MESFET.

The high transient frequency of the MESFET makes it particularly of interest for microwave circuits. While the advantage of the MESFET provides a superior microwave amplifier or circuit, the limitation of the diode turn ON is easily terminated. Typically depletion mode devices are used since they provide a larger current  and larger transconductance and the circuit contains only a few transistors, so that threshold control is not a limiting factor. The buried channel also yields a better noise performance as trapping and releases of carriers into and from surface states and defect is eliminated. 

HIGH FREQUENCY ISSUES

There are a number of physical effects that are negligible at lower frequencies becomes increasingly important at higher frequencies. Two of these effects are the skin effect and radiation effect.

SKIN EFFECT

The skin effect is caused by the finite penetration depth of an electromagnetic field into conducting material . this effect is a function of frequency and is given by

Therefore  decreases with the increase in frequency and so the electromagnetic fields are confined to regions increasingly near the surface as the frequency increases.  This results in themicrowave currents flowing exclusively along the surface of the conductor, significantly increasing the effective resistance of metallic interconnects.

RADIATION LOSS

For conductors and other components of comparable size to the signal wavelengths ,standing waves caused by reflection of the electromagnetic waves from the boundry of the component can greatly enhance the radiation of electromagnetic energy. These standing waves can be easily established either intentionally or unintentionally  

LTE TECHNOLOGY

LTE ( Long Term Evolution) or 4G LTE, is a standard for wireless communication of high-speed data for mobile phones and data terminals. It is based on the GSM/EDGE and UMTS/HSPA network technologies,
increasing the capacity and speed using new modulation techniques.The world's first publicly available LTE
service was launched by TeliaSonera in Oslo and Stockholm on 14 December 2009. LTE is the natural upgrade path for carriers with GSM/UMTS networks, but even CDMA holdouts such as Verizon Wireless, who launched the first large-scale LTE network in North America in 2010.


 LTE is anticipated to become the first truly global mobile phone standard, although the use of different frequency bands in different countries will mean that only multi-band phones will be able to utilize LTE in all countries where it is supported.

LTE is a standard for wireless data communications technology and an evolution of the GSM/UMTS standards. The goal of LTE was to increase the capacity and speed of wireless data networks using new
DSP (digital signal processing) techniques and modulations that were developed around the turn of the millennium. A further goal was the redesign and simplification of the network architecture to an IP-based system with significantly reduced transfer latency compared to the 3G architecture. The LTE wireless interface is incompatible with 2G and3G networks, so that it must be operated on a separate wireless
spectrum.

LTE was first proposed by NTT DoCoMo of Japan in 2004, and studies on the new standard officially commenced in 2005. In May 2007, the LTE/SAE Trial Initiative (LSTI) alliance was founded as a global collaboration between vendors and operators with the goal of verifying and promoting the new standard in order to ensure the global introduction of the technology as quickly as possible. The LTE standard was finalized in December 2008, and the first publicly available LTE service was launched by TeliaSonera in Oslo and Stockholm on December 14, 2009 as a data connection with a USB modem

The LTE specification provides downlink peak rates of 300 Mbit/s, uplink peak rates of 75 Mbit/s and QoS provisions permitting a transfer latency of less than 5 ms in the radio access network. LTE has the ability to manage fast-moving mobiles and supports multi-cast and broadcast streams. LTE supports scalable carrier bandwidths, from 1.4 MHz to 20 MHz and supports both frequency division duplexing (FDD) and time-division duplexing (TDD). The IP-based network architecture, called the Evolved Packet Core (EPC) and designed to replace the GPRS Core Network, supports seamless handovers for both voice and data to cell towers with older network technology such as GSM, UMTS and CDMA2000.[16] The simpler architecture results in lower operating costs.

Frequency bands

The LTE standard can be used with many different frequency bands. In North America, 700/ 800 and 1700/
1900 MHz are planned to be used; 800, 1800, 2600 MHz in Europe; 1800 and 2600 MHz in Asia; and 1800 MHz in Australia. As a result, phones from one country may not work in other countries. Users will need a multi-band capable phone for roaming internationally.

HEMT(high electron mobility transistors)

High electron mobility transistor (HEMT), also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a field effect transistor incorporating a junction between two materials with different band gaps (i.e., a heterojunction) as the channel instead of a doped region, as is generally the case for MOSFET. A commonly used material combination is GaAs with AlGaAs, though there is wide variation, dependent on the application of the device. Devices incorporating more indium generally show better high-frequency performance, while in recent years, gallium nitride HEMTs have attracted attention due to their high-power performance.

Invention 

The invention of the HEMT is usually attributed to Takashi Mimura  (Fujitsu, Japan). In America, Ray Dingle and his co-workers in Bell Laboratories also played an important role in the invention of the HEMT. In Europe, Daniel Delagebeaudeuf and Trong Linh Nuyen from Thomson-CSF (France) filed for a patent of this device on the 28th of March 1979.

Explanation

To allow conduction, semiconductors are doped with impurities which donate mobile electrons (or holes). However, these electrons are slowed down through collisions with the impurities (dopants) used to generate them in the first place. HEMTs avoid this through the use of high mobility electrons generated using the heterojunction of a highly-doped wide-bandgap n-type donor-supply layer (AlGaAs in our example) and a non-doped narrow-bandgap channel layer with no dopant impurities (GaAs in this case).

The electrons generated in the thin n-type AlGaAs layer drop completely into the GaAs layer to form a depleted AlGaAs layer, because the heterojunction created by different band-gap materials forms a quantum well (a steep canyon) in the conduction band on the GaAs side where the electrons can move quickly without colliding with any impurities because the GaAs layer is undoped, and from which they cannot escape. The effect of this is to create a very thin layer of highly mobile conducting electrons with very high concentration, giving the channel very low resistivity (or to put it another way, "high electron mobility"). This layer is called a two-dimensional electron gas. As High electron mobility transistor 2

with all the other types of FETs, a voltage applied to the gate alters the conductivity of this layer.

Quantum mechanism

Since GaAs has higher electron affinity, free electrons in the AlGaAs layer are transferred to the undoped GaAs layer where they form a two dimensional high mobility electron gas within 100 ångström of the interface. The n-typ AlGaAs layer of the HEMT is depleted completely through two depletion mechanisms:

• Trapping of free electrons by surface states causes the surface depletion.

• Transfer of electrons into the undoped GaAs layer brings about the interface depletion.

The Fermi energy level of the gate metal is matched to the pinning point, which is 1.2 eV below the conduction band. With the reduced AlGaAs layer thickness, the electrons supplied by donors in the AlGaAs layer are insufficient to pin the layer. As a result, band bending is moving upward and the two-dimensional electrons gas does not appear. When a positive voltage greater than the threshold voltage is applied to the gate, electrons accumulate at the interface and form a two-dimensional electron gas.

Versions of HEMTs

• pHEMT

Ideally, the two different materials used for a heterojunction would have the same lattice constant (spacing between the atoms). In practice, e.g. AlGaAs on GaAs, the lattice constants are typically slightly different, resulting in crystal defects. As an analogy, imagine pushing together two plastic combs with a slightly different spacing. At regular intervals, you'll see two teeth clump together. In semiconductors, these discontinuities form deep-level traps, and greatly reduce device performance. A HEMT where this rule is violated is called a pHEMT or pseudomorphic HEMT. This is achieved by using an extremely thin layer of one of the materials – so thin that the crystal lattice simply stretches to fit the other material. This technique allows the construction of transistors with larger bandgap differences than otherwise possible, giving them better performance.

• mHEMT

Another way to use materials of different lattice constants is to place a buffer layer between them. This is done in the mHEMT or metamorphic HEMT, an advancement of the pHEMT. The buffer layer is made of AlInAs, with the indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the GaInAs channel. This 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).

Applications

Applications are similar to those of MESFETs – microwave and millimeter wave communications, imaging, radar, and radio astronomy – any application where high gain and low noise at high frequencies are required. HEMTs have

shown current gain to frequencies greater than 600 GHz and power gain to frequencies greater than 1 THz.

Numerous companies worldwide develop and manufacture HEMT-based devices. These can be discrete transistors but are more usually in the form of a 'monolithic microwave integrated circuit' (MMIC).

S PARAMETERS

• Scattering parameters or S-parameters (the elements of a scattering matrix or S-matrix) describe the electrical behavior of linear electrical networks when undergoing various steady state stimuli by electrical signals.
• The parameters are useful for electrical engineering, electronics engineering, and communication systems design,and especially for microwave engineering.
• The S-parameters are members of a family of similar parameters, other examples being: Y-parameters, Z-parameters, H-parameters, T-parameters or ABCD-parameters.They differ from these, in the sense that S-parameters do not use open or short circuit conditions to characterize a linear electrical network; instead, matched loads are used. These terminations are much easier to use at high signal frequencies than open-circuit and Short-circuit terminations. Moreover, the quantities are measured in terms of power.
• Many electrical properties of networks of components (inductors, capacitors, resistors) may be expressed using S-parameters, such as gain, return loss, voltage standing wave ratio (VSWR), reflection coefficient and amplifier stability. The term 'scattering' is more common to optical engineering than RF engineering, referring to the effect observed when a plane electromagnetic wave is incident on an obstruction or passes across dissimilar dielectric media. In the context of S-parameters, scattering refers to the way in which the traveling currents and voltages in a transmission line are affected when they meet a discontinuity caused by the insertion of a network into the transmission line. This is equivalent to the wave meeting an impedance differing from the line's characteristic impedance. 
• S-parameters change with the measurement frequency, so frequency must be specified for any S-parameter measurements stated, in addition to the characteristic impedance or system impedance.S-parameters are readily represented in matrix form and obey the rules of matrix algebra.

Background

The first published description of S-parameters was in the thesis of Vitold Belevitch in 1945. The name used by Belevitch was repartition matrix and limited consideration to lumped-element networks. The term scattering matrix was used by physicist and engineer Robert Henry Dicke in 1947 who independently developed the idea during wartime work on radar.

Working

In the S-parameter approach, an electrical network is regarded as a 'black box' containing various interconnected basic electrical circuit components or lumped elements such as resistors, capacitors, inductors and transistors, which interacts with other circuits through ports. The network is characterized by a square matrix of complex numbers called its S-parameter matrix, which can be used to calculate its response to signals applied to the ports. For the S-parameter definition, it is understood that a network may contain any components provided that the entire network behaves linearly with incident small signals. 

An electrical network to be described by S-parameters may have any number of ports. Ports are the points at which electrical signals either enter or exit the network. Ports are usually pairs of terminals with the requirement that thecurrent into one terminal is equal to the current leaving the other. S-parameters are used at frequencies where the ports are often coaxial or waveguide connections.

The S-parameter matrix describing an N-port network will be square of dimension 'N' and will therefore contain N-Square elements. At the test frequency each element or S-parameter is represented by a unitless complex number that represents magnitude and angle, i.e. amplitude and phase.The S-parameter magnitude may be expressed in linear form or logarithmic form. When expressed in logarithmic form, magnitude has the "dimensionless unit" of decibels. 

Two-Port S-Parameters

The S-parameter matrix for the 2-port network is probably the most commonly used and serves as the basic building block for generating the higher order matrices for larger networks.In this case the relationship between the reflected, incident power waves and the S-parameter matrix is given by:

Expanding the matrices into equations gives:

b1= (S11)(a1) + (S12)(a2),
b2=(S21)(a1)+(S22)(a2), 

Each equation gives the relationship between the reflected and incident power waves at each of the network ports, 1 and 2, in terms of the network's individual S-parameters S11,S12 ,S21 , andS22 . If one considers an incident power wave at port 1 (a1) there may result from it waves exiting from either port 1 itself (b1 ) or port 2 (b2).

However if, according to the definition of S-parameters, port 2 is terminated in a load identical to the system

impedance (Z0 ) then, by the maximum power transfer theorem,b2 will be totally absorbed making a2 equal to zero.

Scattering parameters 4

zero. Therefore

S11=b1\a1
and

S21=b2\a1.
Similarly, if port 1 is terminated in the system impedance then becomes zero, giving

S12=b1\a2
and

S22=b2\a2
Each 2-port S-parameter has the following generic descriptions:

S11 is the input port voltage reflection coefficient

S12 is the reverse voltage gain

S21 is the forward voltage gain

S22 is the output port voltage reflection coefficient

S-Parameter properties of 2-port networks

An amplifier operating under linear (small signal) conditions is a good example of a non-reciprocal network and a matched attenuator is an example of a reciprocal network. In the following cases we will assume that the input and output connections are to ports 1 and 2 respectively which is the most common convention. The nominal system impedance, frequency and any other factors which may influence the device, such as temperature, must also be

specified.

Complex linear gain:- The complex linear gain G is given by

.G=S21

That is simply the voltage gain as a linear ratio of the output voltage divided by the input voltage, all values

expressed as complex quantities.

Scalar linear gain:-The scalar linear gain (or linear gain magnitude) is given by

|G| =|S21|

That is simply the scalar voltage gain as a linear ratio of the output voltage and the input voltage. As this is a scalar quantity, the phase is not relevant in this case.

Scalar logarithmic gain:- The scalar logarithmic (decibel or dB) expression for gain (g)is  

g=20log|S21|dB.

This is more commonly used than scalar linear gain and a positive quantity is normally understood as simply a'gain'... A negative quantity can be expressed as a 'negative gain' or more usually as a 'loss' equivalent to its magnitude in dB. For example, a 10 m length of cable may have a gain of - 1 dB at 100 MHz or a loss of 1 dB at 100 MHz.

Insertion loss:-In case the two measurement ports use the same reference impedance, the insertion loss ( ) is the dB expression of the transmission coefficient . It is thus given by:

IL=-20log|S21|dB.

It is the extra loss produced by the introduction of the DUT between the 2 reference planes of the measurement.

Notice that the extra loss can be introduced by intrinsic loss in the DUT and/or mismatch. In case of extra loss the insertion loss is defined to be positive.

Input return loss

Input return loss ( ) is a scalar measure of how close the actual input impedance of the network is to the nominal system impedance value and, expressed in logarithmic magnitude, is given by


RLin=|20log|S11|| dB.

By definition, return loss is a positive scalar quantity implying the 2 pairs of magnitude (|) symbols. The linear part,|S11| is equivalent to the reflected voltage magnitude divided by the incident voltage magnitude.

Output return loss

The output return loss (RLout) has a similar definition to the input return loss but applies to the output port (port2) instead of the input port. It is given by

RLout=|20log|S22||dB.

Reverse gain and reverse isolation

The scalar logarithmic (decibel or dB) expression for reverse gain is:

g rev=20log|S12|dB.

Often this will be expressed as reverse isolation in which case it becomes a positive quantity equal to the magnitude of and the expression becomes:

Irev=|g rev|=|20log|S12||dB.

Voltage standing wave ratio

The voltage standing wave ratio (VSWR) at a port, represented by the lower case 's', is a similar measure of port match to return loss but is a scalar linear quantity, the ratio of the standing wave maximum voltage to the standing wave minimum voltage. It therefore relates to the magnitude of the voltage reflection coefficient and hence to the magnitude of either for the input port or for the output port.

At the input port, the VSWR ( ) is given by

Sin=(1+|S11|)\(1-|S11|);

At the output port, the VSWR ( ) is given by

Sout=(1+|S22|)\(1-|S22|);



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