What is Latch-up in VLSI
Latch-up is create a short circuit or a low impedance channel generated between the power and ground rails of a MOSFET circuit, due to that high current is getting generated and which leads to IC damage. It is caused by the interaction of parasitic PNP and NPN transistors (BJTs).
Latch-up occurs when the voltage at the input or output pins of a device exceeds a critical threshold, triggering the parasitic thyristor structure. This can happen due to various factors, such as high currents, voltage spikes, or improper design and layout considerations.
Latch-up can have severe consequences for VLSI circuits, including circuit malfunction, degradation of device performance, and even permanent damage. It can disrupt the normal functioning of the IC, causing unintended current paths and rendering the device unreliable.
The parasitic structure is frequently compared to a thyristor (or SCR), which is a PNPN structure that serves as a PNP and an NPN transistor combined. During a latch-up, one of the transistors starts to conduct, which causes the other one to follow suit.
As long as the structure is forward-biased and some current runs through it, which is normal until the power is switched off, they both maintain saturation. The totem-pole PMOS and NMOS transistor pair produce the SCR parasitic structure on the output drivers of the gates.
Prevention Techniques for Latch-Up in VLSI:
STI(Shallow trench isolation): Use the STI process that covers both the NMOS and PMOS transistors. The parasitic silicon-controlled rectifier (SCR) structure between these transistors will break. It will also help to reduce the leakage.
Epitaxial Layer: Devices manufactured from epitaxial layers that are minimally doped and are developed on substrates that are substantially doped are less likely to latch-up. The strongly doped layer serves as a current sink, allowing surplus minority carriers to recombine fast.
SOI(Silicon-on-insulator): These devices are latch-up resistant by design. The latch-up connection, which connects the tub to the power supply rails, has a low resistance.
Guard Ring: Guard Ring acts as an ohmic contact or bulk(tap) connection.