Application
RF-SOI
Lower Ron·Coff, leakage and cleaner harmonics for RF switches — on thinner top silicon, in the same process flow.
MST for RF-SOI
RF-SOI switches and low-noise amplifiers sit in every phone's front end. Thinner top silicon and shorter gates cut off-capacitance, but the switch then loses breakdown voltage as its body pinches off.
A stack of MST layers traps boron right next to each layer, building a super-steep retrograde (SSRW) ground plane next to the buried oxide under a nearly undoped channel. It also holds back the lateral spread of the source/drain dopants. The switch gets thinner, smaller and faster without giving up breakdown.
The switch problem
Four numbers decide an RF switch
An RF switch is a stack of SOI FETs. On, it should look like a small resistor; off, like a small capacitor that holds the full RF swing without leaking or distorting.
Improving one of these numbers usually costs another — which is why Ron·Coff has improved only slowly from one RF-SOI generation to the next.
Switching efficiency
Ron·Coff
Insertion loss and isolation. Every femtosecond lets the designer shrink the switch or improve the RF path.
Off-state robustness
Body current, GIDL, BV
Stacked FETs must hold the RF voltage swing while off. Leakage and breakdown set the stack height and so the area.
Linearity
H2 / H3 harmonics
Antenna and tuning switches are checked for harmonics at high antenna-node stress.
Power handling
Pmax
Any gain in Ron·Coff must not cost power handling.
How MST helps
MST keeps the implanted Boron and Arsenic doping through downstream thermal processing
MST layers capture the silicon interstitials that carry boron and arsenic, so the doping set by implant survives the thermal budget of the whole flow.
In an RF-SOI switch that gives two things at once: a boron ground plane at the BOX that keeps breakdown up in a thinner film, and tighter source/drain junctions that cut the off-state capacitance.
At the BOX
boron ground plane held next to the buried oxide through the full thermal cycle (silicon + MSTcad™)
~25% lower R_DSON
at the same breakdown, measured on RF-SOI switches; or ~1 V more breakdown at the same R_DSON (silicon)
Lower C_OFF
farther-apart source/drain junctions: a better Ron·Coff (TCAD)
Silicon data
MST delivers the gains on Silicon as predicted by MSTcad™ simulations
Lower Ron·Coff
at the same gate length, with much thinner top silicon
Lower body current (GIDL)
at the same switch area
Cleaner harmonics
less off-state second-harmonic distortion
Lower LNA power
the same gain at lower drain current
Roadmap fit
Additive to the rest of the RF-SOI roadmap
MST is an epitaxial film grown on the incoming SOI wafer. It needs no new fab line and no package change, and it adds to the other performance levers rather than replacing them.
3D-stacked RF-SOI
Stacking separates switch and PA/LNA dies to shrink the footprint. MST improves the switch and LNA devices inside the stack and enables the thinner films.
Strained top silicon
Strain raises mobility but makes boron up-diffusion harder to control. MST holds the boron in place and responds to strain like silicon.
LNA
Lower drain current at the same transconductance, with room to improve.
Contact us to learn more about what MST can do for your RF-SOI roadmap.
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Partner with Atomera
Evaluate MST in your own process and devices with Atomera's engineering team.
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