kink effect

**Kink Effect** is a **characteristic anomaly in the output I-V curves of PD-SOI MOSFETs** — appearing as a sudden increase ("kink") in drain current at moderate drain voltages, caused by impact ionization charging the floating body and reducing the threshold voltage. **What Causes the Kink?** - **Trigger**: As $V_{DS}$ increases, impact ionization generates electron-hole pairs near the drain. - **Body Charging**: Holes accumulate in the floating body (no path to ground), raising body potential $V_{BS}$. - **$V_t$ Drop**: The raised $V_{BS}$ lowers $V_t$ -> more current flows -> kink in $I_D$ vs. $V_{DS}$ curve. - **Appearance**: A visible knee or bump in the saturation region of the I-V characteristic. **Why It Matters** - **Analog Design**: The kink reduces output resistance ($r_o$) -> lower voltage gain. Devastating for op-amp design. - **Output Impedance**: The I-V curve is no longer flat in saturation, making biasing unpredictable. - **Solutions**: Body contacts, FD-SOI, or circuit topologies that are insensitive to $r_o$ degradation. **Kink Effect** is **the signature artifact of floating body physics** — a visible distortion in the I-V curve that reveals the presence of trapped charge in the body.