Silicon Substrate Surface Conduction and Dielectric Passivation Fundamentals
Trap-rich polysilicon passivation pins interface charges on high-resistivity silicon, suppressing surface conduction and harmonic spur spikes across RF switches.
RF Switch Linearity
Meaning
The ability of a solid-state switch to handle high-power signals without generating unwanted frequency components determines the clarity of a wireless transmission. Evaluating RF switch linearity ensures that the multi-band transceiver can transmit and receive data simultaneously without self-interference. This characteristic prevents the distortion of complex modulation schemes.
Distortion Mechanism
Nonlinear behaviors in the channel resistance of a field-effect transistor generate harmonics and intermodulation products when high-power signals pass through. These spurious signals can fall into adjacent frequency bands, violating regulatory limits on spectral emissions. A high level of RF switch linearity is essential to maintain signal integrity in modern LTE and 5G networks.
Design Technique
Engineers use stacked-transistor topologies to distribute the high RF voltage swing across multiple gates, reducing the voltage stress on each individual transistor. This stacking technique prevents the transistors from turning on partially during the off-state of the switch. Deep n-well isolation and substrate bias voltages are also applied to stabilize the body effect and further improve performance.
Measurement Protocol
Testing for linearity involves applying two closely spaced tones to the input of the switch and measuring the third-order intermodulation products at the output. This test requires highly sensitive spectrum analyzers and low-noise signal generators to ensure the measurement is not limited by the test equipment itself. Designers verify that the third-order intercept point remains well above the maximum transmit power level across the entire operating temperature range.
This verification ensures that the switch will not degrade the system performance when installed in the final mobile device.