characterization, modeling, and design of esd protection circuits

136 Chapter 4. Simulation: Calibration and Results
will be approximately the same regardless of the number of fingers chosen, so we will
choose to build the device with five parallel poly fingers, each 50µm long. Since the total
width from the drain contacts to the source contacts of a finger is approximately two times
CGS, the total area will be about 50µm X 50µm. With five poly fingers, there will be three
fingers coming off of the input pad into the protection device (refer to Fig. 2.19).
Since the measured and simulated grounded-gate trigger voltage of the protection
structures is very close to 12V, a gate-bounce resistor should be employed to provide a
margin of safety against dielectric failure of the input gates. The simulated results of Vt1 vs. gate resistance in Fig. 4.48 show that a lumped gate resistance of 50kΩ between the
gate electrode and the grounded source will reduce the trigger voltage by 1.2V for a
50µm-wide device subjected to a pulse rise time of 16V/ns. Since the device being
designed has five fingers which are each 50µm wide and the drain-gate overlap
capacitances add in parallel, a proportionately smaller gate resistance, i.e., 10kΩ, can be
used to achieve the same amount of gate bounce. This resistance can most easily be
created by placing a well resistor or tie-off transistor with a resistance of 10kΩ between
the common gate and the source or substrate pad. The gate bounce should not be made too
great because if the gate potential remains significantly high after a finger snaps back, the
high current in the finger will be concentrated at the surface and cause severe heating at a
much lower current level than if the current is distributed evenly along the vertical
junction profile. The reduction in Vt1 of 1.2V created by the 10kΩ resistor, which makes
the value of Vt1 10.6V, is probably a reasonable value.
Assuming the fingers turn on one at a time, which is the worst-case scenario but is also the
most probable scenario considering the random finger-to-finger variations in layout and
the very brief (~1ns) turn-on time, after the first finger turns on the input (drain) device
voltage, Vdev , will rise with device current, Idev , as (refer to Fig. 4.41)
Vdev = Vsb + Rsb ⋅ Idev, (4.40)
where Rsb is the snapback resistance of one finger. For the device to work properly, a
second finger must turn on (snap back) before Idev reaches the failure level for one finger,
641mA. In terms of the device parameters,
Idev =
( Vt1 – Vsb) ⁄ Rsb < 641mA . (4.41)