Meaning
A specialized silicon-on-insulator wafer incorporates a high density of deep-level electronic defects beneath the insulating oxide layer to capture free charge carriers. By utilizing a trap-rich substrate, RF device manufacturers suppress the parasitic surface conduction that occurs due to the bias voltages applied to active transistors. This suppression prevents the formation of a conductive layer that would otherwise degrade signal linearity and increase substrate losses.
Carrier Immobilization
Free carriers generated by high-frequency signals are trapped by the crystalline defects in the underlying layer. In a trap-rich substrate, these immobilized charges cannot respond to alternating electromagnetic fields, reducing the dielectric loss of the wafer. This carrier immobilization maintains high isolation between adjacent RF ports.
High-Frequency Performance
Harmonic distortion diminishes when the substrate remains electrically passive under high-power signals. Circuits fabricated on a trap-rich substrate show stable insertion loss at high frequencies.
Wafer Manufacture
Manufacturing of these specialized wafers requires the deposition of a polycrystalline silicon layer prior to the bonding of the active silicon layer. In-line testing validates the trap density of the trap-rich substrate by measuring the effective resistivity under a sweep of DC bias voltages. This quality control step ensures that the substrate maintains its high-frequency characteristics throughout the high-temperature processing steps.
