1. 11/22/2023 Latch-up and Prevention 1
Latch-up and Its Prevention
Usha Mehta
usha.mehta@nirmauni.ac.in
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What is Latch-up?
• Latchup is the creation of a low impedance path
between the power supply rails (which should neve
exist in CMOS).
• Because of PARACITIC Bipolar transistors (BJTs) in
CMOS ICs
• Triggered when applying voltage or current input,
output, I/O pin, over voltage on Vdd pin.
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• Temporary Latch-Up
• A temporary or transient latchup occurs only while the
pulse stimulus is connected to the integrated circuit and
returns to normal levels once the stimulus is removed.
• Permanent/True Latch-Up
• A true latchup remains after the stimulus has been
removed and requires a power supply shut down to
remove the low impedance path between the power supply
rails
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How does Latch-up occur?
• Latchup is the regenerative process that can occur in a pnpn
structure (SCR-silicon controlled rectifier) formed by a parasitic
npn and a parasitic pnp transistor.
• Important concepts:
• To avoid latchup, vPNPN ≥ VS
• Once the pnpn structure has latched up, the large current
required by the above i-v characteristics must be provided
externally to sustain latchup
• To remove latchup, the current must be reduced below the
holding current
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Latch-Up and its Prevention
• Latch is the generation of a low-
impedance path in CMOS chips
between the power supply and the
ground rails due to interaction of
parasitic pnp and npn bipolar
transistors. These BJTs for a
silicon-controlled rectifier with
positive feedback and virtually
short circuit the power and the
ground rail.
• This causes excessive current
flows and potential permanent
damage to the devices.
• Analysis of the a CMOS Inverter
CMOS depicting the parasitics.
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Latch-Up Continued
• The equivalent circuit shown has
Q1 being a vertical double
emmitter pnp transistor whose
base is formed by the n-well with a
high base to collector current gain
(b1).
• Q2 is a lateral double emitter npn
transistor whose base is formed by
the p-type substrate.
• Rwell represents the parasitic
resistance in the n-well structure
whose value ranges from 1KW to
20kW.
• The substrate resistance Rsub
depends on the substrate
structure.
• Assume the Rwell and Rsub are
significantly large so that they
cause open circuit connections,
this results in low current gains
and the currents would be
reverse leakage currents for both
the npn and pnp transistors.
• If some external disturbance
occurs, causing the collector
current of one of the parasitic
transistors to increase, the
resulting feedback loop causes
the current perturbation to be
multiplied by b1.b2
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Latch-up Continued
• This event triggers the silicon-
controlled rectifier and each
transistor drives the other with
positive feedback eventually
creating and sustaining a low
impedance path between power
and the ground rails resulting in
latch-up.
• For this condition if b1 *b1 is
greater than or equal to 1 both
transistors will continue to
conduct saturation currents even
after the triggering perturbation is
no longer available.
• Some causes for latch-up are:
• Slewing of VDD during start-up
causing enough displacement
currents due to well junction
capacitance in the substrate and
well.
• Large currents in the parasitic
silicon-controlled rectifier in CMOS
chips can occur when the input or
output signal swings either far
beyond the VDD level or far below
VSS level, injecting a triggering
current. Impedance mismatches in
transmission lines can cause such
disturbances in high speed circuits.
• Electrostatic Discharge stress can
cause latch-up by injecting minority
carriers from the clamping device in
the protection circuit into either the
substrate or the well.
• Sudden transient in power or
ground buses may cause latch-up.
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Guidelines For Avoiding Latch-Up
• Reduce the BJT gains by lowering the
minority carrier lifetime through Gold
doping of the substrate (solution might
cause excessive leakage currents).
• Use p+ guardband rings connected to
ground around nMOS transistors and
n+ guard rings connected to VDD
around pMOS transistors to reduce Rw
and Rsub and to capture injected
minority carriers before they reach the
base of the parasitic BJT.
• Place substrate and well contacts as
close as possible to the source
connections of the MOS transistors to
reduce the values of Rw and Rsub.
(solution to be used in your designs)
• Place source diffusion regions for
the pMOS transistors so that
they lie along equipotentials lines
when currents flow between VDD
and p-wells.
• Avoid forward biasing of the
source/drain junctions so as not
to inject high currents , this
solution calls for the use of
slightly doped epitaxial layer on
top of the heanily doped
substrate and has the effect of
shunting the lateral currents
from the vertical transistor
through the low resistance
substrate.
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How does Latch-up sustains?
• The loop gain of the relevant BJT configuration must
exceed unity. βn βp ≥ 1
• A bias condition must exist such that both bipolars are
turned on long enough for current through the “SCR”
to exceed its switching current.
• The bias supply and associated circuits must be
capable of supplying the current at least equal to the
switching current and at least equal to the holding
current to maintain the latched state.
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10. 11/22/2023 Latch-up and Prevention 10
Latch-up in CMOS IC
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Lateral BJTs LT1 and LT2
Vertical BJTs VT1 and VT2
