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