Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Field effect transistor
Reexamination Certificate
1993-07-09
2001-04-10
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Field effect transistor
C257S194000
Reexamination Certificate
active
06215136
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to integrated circuits capable of low-noise and high-power microwave operation.
BACKGROUND OF THE INVENTION
Field Effect Transistors (FETs) are well known to be ideal for use in applications requiring amplification or switching at radio or microwave frequencies. FETs fabricated primarily of GaAs are particularly suited for high frequency uses because of the high electron mobility characteristic of this compound semiconductor. In the past, FETs have utilized a Shottky barrier gate structure (hence the common name Metal-semiconductor Field Effect Transistor, or MESFET) and have been fabricated on semi-insulating GaAs substrates with all dopants being ion implanted.
Recently, the performance demands of modern radar and telecommunications equipment have outstripped the capabilities of traditional MESFET technology. Consequently, FETs have evolved into largely epitaxial structures where semiconductor layers are precisely grown and doped in situ in the growth process. This has allowed the use of highly doped, precisely defined, channel regions buried beneath lightly doped buffer layers, which in turn results in “low-high” FETs with a highly linear relationship between transconductance and gate voltage, a characteristic that is important in minimizing distortion in a high-frequency transistor amplifier.
Additionally, FETs have evolved to permit operation with higher breakdown voltages and therefore at higher power than was possible with traditional MESFETs. One method to achieve higher breakdown voltages in the past has been to incorporate an AlGaAs buffer layer atop a GaAs channel layer. The AlGaAs layer is undoped or lightly doped and separates the highly doped GaAs channel from the gate contact placed on top of the AlGaAs layer. This device is known generally as a MISFET (Metal Insulator Field Effect Transistor) because of the “insulating” AlGaAs layer.
Radar and telecommunications systems commonly require the low-noise highly linear performance of “low-high” FETs for receiver amplifiers, while also requiring the high power, robust structure characteristic of a MISFET type transistor amplifier for transmitting applications. This has traditionally required a system designer to have integrated circuits for power amplifiers, integrated circuits for low-noise amplifiers, and even integrated circuits for the switching and phase shifting functions commonly used in these systems.
SUMMARY OF THE INVENTION
In the past, high frequency systems have been of a generally modular nature, having separate integrated circuits for power, low-noise, switching and phase shifting functions. This approach was made necessary largely by the process and epitaxial material structure requirements of the FETs used in the integrated circuits available to the system designer. The necessity to use separate integrated circuits for each system function drives up system cost and adversely impacts the reliability of the system because of the inter-chip connections required. It is these limitations that the present invention is intended to address.
There have been efforts in the past to integrate FETs having different performance advantages, but these circuits required epitaxial regrowth, i.e. a multilayer epitaxial structure was grown for a first type of device but then the epitaxial processing was stopped and other processing done (e.g. epitaxial material was then etched away in areas of the wafer in which a second device type was needed), and then epitaxial processing was resumed with a second epitaxial material structure being grown for the second device type. It is very difficult to obtain a good second epitaxial growth because of contaminants introduced into an epitaxial material growth chamber when it is opened. This process is also time-consuming and requires expert supervision. Consequently, a structure requiring only one epitaxial processing cycle and conventional processing would be desired to overcome these limitations. The present invention is intended to address these limitations.
In one form of the invention, an integrated circuit is disclosed for providing low-noise and high-power microwave operation conprising: an epitaxial material structure comprising a substrate, a low-noise channel layer, a low-noise buffer layer, a power channel layer, and a moderately doped wide bandgap layer, a first active region comprising a first source contact above the wide bandgap layer, a first drain contact above the wide bandgap layer, wherein the first source contact and the first drain contact are alloyed and thereby driven into the material structure to make contact with the low-noise channel layer, and a first gate contact to the low-noise buffer layer, and a second active region comprising a second source contact above the wide bandgap layer, a second drain contact above the wide bandgap layer, wherein the second source contact and the second drain contact are alloyed and thereby driven into the material structure to make contact with the power channel layer, and a second gate contact to the wide band-gap layer; wherein the first active region and the second active region are electrically isolated from one another, and whereby the integrated circuit is formed with all epitaxial layers formed during a single epitaxial growth cycle and is capable of providing low-noise, high-power, and switching operation at microwave frequencies.
In another form of the invention, an integrated circuit is disclosed for providing low-noise and high-power microwave operation comprising: an epitaxial material structure comprising a semi-insulating GaAs substrate, a GaAs buffer layer, a GaAs highly-doped low-noise channel layer, a GaAs lightly-doped low-noise buffer layer, a GaAs highly-doped power channel layer, a moderately doped AlGaAs buffer layer, and a GaAs highly-doped cap layer, a low-noise field effect transistor comprising a first source contact to the cap layer, and a first drain contact to the cap layer, wherein the first source contact and the first drain contact are alloyed and thereby driven into the material structure to make contact with the low-noise channel layer, a first gate contact to the GaAs lightly-doped low-noise buffer layer, the first gate contact being formed with a double recess etch; a power field effect transistor comprising a second source contact to the cap layer, and a second drain contact to the cap layer, wherein the second source contact and the second drain contact are alloyed and thereby driven into the material structure to make contact with the power channel layer, and a second gate contact to the AlGaAs buffer layer, the second gate contact being formed with a double recess etch; and wherein the first active region and the second active region are separated by a region of the material structure rendered semi-insulating by ion implantation.
In yet another form of the invention, a method is disclosed for fabricating an integrated circuit for providing low-noise and high-power microwave operation comprising: depositing a buffer on a substrate; depositing a low-noise channel layer above the buffer, depositing a low-noise buffer layer above the low-noise channel layer, depositing a power channel layer above the low-noise buffer layer, depositing a wide bandgap layer above the power channel layer, depositing a cap layer above the wide bandgap layer, forming a first transistor configuration by etching the cap layer, the wide bandgap layer, and the power channel layer in a first pattern to form a first recess exposing the low-noise buffer layer, forming a second transistor configuration by etching the cap layer in a second pattern to form a second recess exposing the wide bandgap layer, etching a third recess inside of the first recess that extends slightly into the low-noise buffer layer, etching a fourth recess inside of the second recess that extends slightly into the wide bandgap layer, depositing a first gate metallization in the third recess; depositing a second gate metallization in the fourth recess; depositing a first drain contact on t
Saunier Paul
Tserng Hua Quen
Brady III W. James
Crane Sara
Skrehot Michael K.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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