characterization, modeling, and design of esd protection circuits

2.2. Transmission Line Pulsing 27
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Fig. 2.8 A screen capture of a Tektronix TDS 684A digitizing oscilloscope
displays the device voltage (Ch1) and current (Ch2) response of a
50 ⁄ 0.6µm device to a 4.6V, 150ns input pulse. After some initial
ringing, the device voltage settles to a value approximately equal to the
input voltage and the device current is very small. The current probe
registers 5mV per 1mA of current.
flowing. If the input voltage is stepped carefully enough, the voltage drop due to second
breakdown can also be captured (Fig. 2.10).
It is important to note that beyond snapback, the curve resulting from plotting the current
points vs. the voltage points in Fig. 2.9 is different from the overall TLP curve of Fig.
2.6a. Notice that while snapping back the voltage does not drop all the way to Vsb and
then rise back up to its final level, but rather just drops to the final level. Also, for reasons
discussed in Section 2.3, the peak voltage will probably be less than Vt1 because the
voltage rise, as measured in V/ns, is faster for larger pulse heights. In this respect the TLP
curve below the second-breakdown point really is a dc curve which doesn’t account for
device heating. However, it still represents how the device responds to an ESD stress
because it reveals the operating points after the initial turn-on transient. Since Vt1 is
dependent on the voltage ramp rate, it is equal to the maximum input voltage during an
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