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Gallium arsenide semiconductors

Updated April 30, 2010

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New for August 2009! We've split our page on microwave semiconductor technologies page into multiple pages.

Gallium arsenide is the orginal microwave semiconductor that helped spawn the revolution in personal communications that we all take for granted. But now GaAs is an older gentleman, soon to be content spending days on the park bench of microwave power amplifiers, watching gallium nitride take over this end of the business...

GaAs MESFET

GaAs PHEMT

GaAs MHEMT

GaAs HBT

GaAs VPIN diode

GaAs MESFET

Gallium arsenide MESFET was the original answer to "how can we make amplifiers at microwave frequencies?" The first GaAs MMICs demonstrated in the 1970s. Including HEMT and HBT technologies, literally billions of dollars have been spent extending fmax of GaAs products up into 100s of GHz.

The semi-insulating properties of GaAs substrates and the 12.9 dielectric constant make it an EXCELLENT media for microstrip or CPW design. GaAs devices operate reliably up to 150C channel temperature, although some suppliers will tell you it is good for 175C or even higher. It is "radiation hard" for space applications. GaAs substrates are available up to six inches (150 mm) in diameter, which has been a long development since the first 2-inch wafers were available in the late 1970s. Sadly, GaAs MESFET MMICs will NEVER be cheaper than silicon, due to the starting material cost ($100s of dollars). GaAs parts are more fragile than silicon, and the thermal dissipation factor is not that good. GaAs MESFETs may be extinct in five years, because it doesn't cost much more to fabricate PHEMT or MHEMT on GaAs, and these technologies offer higher performance.

Advantages: Disadvantages
  • Mature technology
  • Optical gates (usually) means low cost
  • Great microwave substrate (12.9 Er, low loss tangent, high bulk resistivity)
  • Six inch wafers available
  • Photonic properties
  • 16-20 volt breakdown possible
  • Relatively cheap to produce (but always more than silicon)
  • Channel temperatures up to 150C possible
  • Limited to Ku-band or lower
  • Noise figure and power performance not as good as GaAs PHEMT
  • Positive and negative voltage typically needed (VGS and VDS).

Examples

M/A-COM Roanoke foundry (to be moved in 2010), TriQuint Oregon

GaAs PHEMT

GaAs PHEMT was the second MMIC technology to be perfected, in the 1990s. Breakdown voltages of PHEMT up to 16 volts make high-power/high efficiency amps possible, and noise figure of tenths of a dB at X-band means great LNAs, and made the DISH network possible, you lucky dogs!

PHEMT stands for pseudomorphic high electron mobility transistor. "Pseudomorphic" implies that the semiconductor is not just GaAs, perhaps AlGaAs/InGaAs/GaAs or some other secret recipe of 11 herbs and spices. Here's some further info on the the use of pseudomorphic in this context (sent in by some M101 fans!)

Actually, "pseudomorphic" means that the hetero layers are thin enough not to keep their own crystal lattice structure, but assume the structure (lattice constants especially) of surrounding material (lots of stress is involved),

If you look at a two dimensional cross section of the layer, you'll see that while it assumes the lattice constant of the bulk structure in the X direction, it tries to keep its original lattice constant in the vertical direction. This layer is indeed strained. For a GaAs pHEMT, indium is added to improve mobility and form a quantum well. Indium wants to growth the lattice and the typical range for useful thicknesses would be 10-25% on GaAs. You can also do strain compensation with the Schottky or cap layer.

The purist nerds of semiconductors often capitalize "PHEMT" as pHEMT. To them we offer this advice: get over it, or we will beat you up like we used to do on the playground, remember?

Advantages: Disadvantages
  • Useful through Q-band, especially if thinned to 2 mils and individual source vias are used
  • Excellent power and efficiency (greater than 60% PAE)
  • Breakdown 12 volts at best, typical operate at 5-6 volts
  • Channel temperatures up to 150C possible.
  • E-beam gates (increases cost)
  • Positive and negative voltage typically needed (VGS and VDS)

Examples:

TriQuint Texas, Northrop, Raytheon (in general all foundries catering to military products)

GaAs MHEMT

Recent work on metamorphic MHEMT has made premium InP HEMT performance possible (amps up at 100 GHz) at the same price as "regular" GaAs PHEMT. You can get noise figure and fmax equal to indium phosphide by using MHEMT, if you use a reputable foundry and indium content is high. You can actually exceed InP RF performance with indium content greater than 55%! The down side to all that indium is reduced operating voltage.

MHEMT stands for metamorphic high-electron mobility transistor. The channel material is InGaAs. "Metamorphic" implies that the lattice structure of GaAs is buffered using epitaxial layers to gradually transform the lattice constant so it lines up with InGaAs. InGaAs is normally grown on InP, which is expensive and fragile compared to GaAs. "Metamorphic" is changing the lattice constant by bond breaking as opposed to "pseudomorphic" which means just straining the heck out of it!

Advantages: Disadvantages
  • Extremely low noise figure
  • Incredibly high fmax (more than 100 GHz)
  • Extremely low on-resistance, makes great switches, but not as good as PIN diodes.
  • Channel temperatures up to 150C possible.
  • Breakdown voltage much lower than PHEMT
  • Low operating voltage (1 to 2 volts)
  • Positive and negative voltage typically needed (VGS and VDS)

Examples:

BAE, Win Semi

GaAs HBT

The heterojunction bipolar transistor (HBT) is a new development, and can decrease the cost of GaAs amplifier products because the emitters are formed optically. GaAs HBT devices operate vertically, compared to the horizontal operation of FETs. However, for very high frequency, the emitter size must be made quite small, and the InGaAs layer is thick and is a thermal insulator, so these devices tend to run HOT. Typical HBT amps are "gain blocks", used in the UHF to C-band frequency ranges.

Typical supporters of HBTs will tell you that wafer yield up to 99% is possible.

Advantages: Disadvantages
  • Single power supply polarity
  • All-optical process
  • Heat dissipation can be problem at small emitter size
  • Typically, reverse isolation is not as high as with PHEMT amplifiers, leading to poor amplifier directivity.
  • Collector resistors are required to stabilize amplifiers. These cut into your power efficiency.

Examples

RFMD

GaAs VPIN diode

PIN diodes make great switching elements. Vertical PINs (VPINs) are offered on some MMICs, but this is truly a niche market. As far as we know, nobody offers VPIN diodes and amplifier devices such as FETs on the same wafer.

Advantages: Disadvantages
  • The lowest on-resistance for the least amount of off-capacitance.
  • Huge power handling.
  • Two terminal device means you must create bias tees to bring in DC control signals.
  • Expect DC current up to 20 mA to create a good RF short circuit.

Examples:

M/A-COM (Cobham), TriQuint Texas

 


 
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