Editing Microwave (section)

===Microwave integrated circuits===
[[File:LNB dissassembled.JPG|thumb|upright=0.7|[[ku band|k<sub>u</sub> band]] [[microstrip]] circuit used in [[satellite television]] dish]]
Prior to the 1970s microwave devices and circuits were bulky and expensive, so microwave frequencies were generally limited to the output stage of transmitters and the [[RF front end]] of receivers, and signals were [[heterodyning|heterodyned]] to a lower [[intermediate frequency]] for processing. The period from the 1970s to the present has seen the development of tiny inexpensive active solid-state microwave components which can be mounted on circuit boards, allowing circuits to perform significant [[signal processing]] at microwave frequencies. This has made possible [[satellite television]], [[cable television]], [[GPS]] devices, and modern wireless devices, such as [[smartphone]]s, [[Wi-Fi]], and [[Bluetooth]] which connect to networks using microwaves.

[[Microstrip]], a type of [[transmission line]] usable at microwave frequencies, was invented with [[printed circuit]]s in the 1950s.<ref name="Roer" /> The ability to cheaply fabricate a wide range of shapes on [[printed circuit board]]s allowed microstrip versions of [[capacitor]]s, [[inductor]]s, [[Stub (electronics)|resonant stubs]], [[Power dividers and directional couplers|splitters]], [[directional coupler]]s, [[diplexer]]s, [[electronic filter|filters]] and antennas to be made, thus allowing compact microwave circuits to be constructed.<ref name="Roer" />

[[Transistor]]s that operated at microwave frequencies were developed in the 1970s.  The semiconductor [[gallium arsenide]] (GaAs) has a much higher [[electron mobility]] than silicon,<ref name="Roer" /> so devices fabricated with this material can operate at 4 times the frequency of similar devices of silicon. Beginning in the 1970s GaAs was used to make the first microwave transistors,<ref name="Roer" /> and it has dominated microwave semiconductors ever since. MESFETs ([[metal-semiconductor field-effect transistor]]s), fast GaAs [[field effect transistor]]s using [[Schottky diode|Schottky junctions]] for the gate, were developed starting in 1968 and have reached cutoff frequencies of 100&nbsp;GHz, and are now the most widely used active microwave devices.<ref name="Roer" /> Another family of transistors with a higher frequency limit is the HEMT ([[high electron mobility transistor]]), a [[field effect transistor]] made with two different semiconductors, AlGaAs and GaAs, using [[heterojunction]] technology, and the similar HBT ([[heterojunction bipolar transistor]]).<ref name="Roer" />

GaAs can be made semi-insulating, allowing it to be used as a [[wafer (electronics)|substrate]] on which circuits containing [[passive component]]s, as well as transistors, can be fabricated by lithography.<ref name="Roer" /> By 1976 this led to the first [[integrated circuit]]s (ICs) which functioned at microwave frequencies, called [[monolithic microwave integrated circuit]]s (MMIC).<ref name="Roer" /> The word "monolithic" was added to distinguish these from microstrip PCB circuits, which were called "microwave integrated circuits" (MIC).   Since then, silicon MMICs have also been developed. Today MMICs have become the workhorses of both analog and digital high-frequency electronics, enabling the production of single-chip microwave receivers, broadband [[amplifier]]s, [[modem]]s, and [[microprocessor]]s.