Parasitic Components In Mosfets: Latchup, Bjts, And Other Surprises
Metal-oxide-semiconductor field-effect transistors (MOSFETs) are integral components in modern integrated circuit design. However, alongside their intended operation, MOSFETs also contain unintended parasitic components that can lead to detrimental effects if not properly addressed. This article provides an in-depth examination of three key parasitic components in MOSFETs: latchup, parasitic bipolar junction transistors (BJTs), and other latent surprises. We will uncover the mechanisms through which these parasites emerge, detail their hazardous impacts, and discuss mitigation techniques to restrict their unwanted behaviors.
Latchup: The Hidden Threat
Latchup stems from the parasitic pnpn structure inherently formed during the fabrication of CMOS circuits. This path couples the power supply rails and can be inadvertently triggered into a low-impedance state that draws excessive current. The sustaining of latchup can precipitate catastrophic device failure from localized heat generation.
Origins of the pnpn Path
The lateral pnpn structure manifests from the juxtaposition of n-well and p-substrate regions and adjacent p+ and n+ diffusions. The inherent vertical npn bipolar transistor combines with an effective pnp transistor through the common n-well and p-substrate regions. This configuration imposes regenerative feedback that can trigger the pnpn structure into a thyristor-like latchup state.
Triggering Mechanisms
The inadvertent triggering on the latchup pnpn path occurs through an event that can initiate regenerative feedback. Common mechanisms include voltage transients, exceeding the maximum current limit, violation of layout design rules, device overstress events, ionizing radiation, and thermal runaway once conduction has commenced.
Consequences and Impact
The latchup condition produces a low-impedance connection between the power rails, permitting currents to escalate unchecked. The resulting localized heat buildup can become sufficient to induce melting, metallization failure, or junction destruction. Latchup events can thus catastrophically damage an integrated circuit.
Design Considerations
Careful layout and guard ring implementation minimizes latchup sensitivity. Ensuring adequate distance between diffusions sharing a well region enhances latchup robustness. Appropriate discipline in the number of well crossings also forestalls triggering events. Further latchup protection arises through voiding undersized layout dimensions.
Parasitic BJTs: An Uninvited Guest
Parasitic bipolar transistors stealthily emerge within standard CMOS flow and can stimulate disconcerting repercussions if left unchecked. These BJTs manifest through implementations of common structures such as p+/n-well diodes and n+/p-substrate diodes. We will uncover how parasitic BJTs take shape and detail the adverse effects they unleash.
Origination in Standard Structures
Parasitic BJTs coalesce in normal diode-connected diffusion regions tied to well structures. A p+/n-well diode for an nMOS transistor contains the innate makings of a vertical npn BJT. Similarly, an n+/p-substrate diode for a pMOS transistor forms an effective pnp BJT. Thus CMOS circuits intrinsically harbor parasitic BJTs within typical diode-connected regions.
Deleterious Consequences
These parasitic BJTs introduce several disturbing effects such as leakage, latchup activation, noise coupling, thermal runaway, and loss of gate control in the CMOS transistors they infiltrate. Their tendencies to turn on when unintended can initiate injection of minority carriers that trigger more parasitic interactions. Parasitic BJTs markedly degrade robustness and reliability.
Design Strategies
Implementing lightly-doped drain (LDD) structures next to the heavy source/drain regions hinders parasitic BJT formation. Insertion of additional wells also disrupts their origination. Drain engineering through the optimization of implant depths and spacer widths further impedes the emergence of parasitic BJTs.
Other Surprises
Beyond latchup and parasitic BJTs, MOSFETs host an assortment of other unintended components that extract performance penalties. These peripheral parasites manifest from secondary effects in materials, interfaces, and fabrication-induced defects.
Materials-Driven Parasitics
Gate oxide traps and interface states degrade mobility and threshold characteristics. Hot carrier injection breeds interface states and gate oxide charges that disturb normal transistor behavior. High-k dielectrics also introduce interface states and threshold voltage instability from charge trapping. Materials-driven parasitics thus disrupt idealized operation.
Fabrication Defects
Manufacturing imperfections spawn additional parasites such as gate-edge shorting from underlap or gate stringers. Contact spiking can prompt pipe formation that ties source and drain regions together deleteriously. These processing defects beget further parasitics that erode device metrics and reliability.
Design Mitigation
Enforcing guard bands, implementing redundant vias, using restrictive layout rules, and adding well/substrate contacts aids in avoiding fabrication defect driven parasitics. Materials selection, interface passivation, limiting high-field stress improves resilience against materials-induced parasitics.
Mitigating Latchup
As detailed previously, latchup threatens catastrophic failure making robust prevention mandatory. We now overview methods to mitigate latchup triggering and suppress latchup propagation when incited.
Physical and Electrical Isolation Techniques
Encircling diffusion regions with guard rings diverts latchup triggering current. Implanting well regions with low doping retards parasitic BJT current gain. Separating the power rails with impedance diminishes regenerative feedback during latchup. These approaches all aim to contain latchup activation.
Layout Strategies
Maintaining minimum geometry and spacing checks averts underlap defects that ignite latchup. Restricting well crossover density also circumvents latchup triggers. Employing dummy gate structures hinders latchup by impeding silicide encroachment along parasitic BJT current paths.
Dynamic Management
Coupling external ballast resistors inhibits latchup sustenance once commenced by limiting current flow. Integrating current sensors with control logic also facilitates dynamic latchup suppression through temporary gating of power rails. Such techniques avert catastrophic failure from sustained latchup events.
Controlling Parasitic BJTs
As outlined previously, parasitic BJTs emerge innately within standard CMOS processing. We detail approaches now to deter their formation and neutralize their adverse impacts.
Structural Modifications
Introducing LDD regions near transistor terminals cuts off parasitic BJT current paths. Implanting additional wells likewise stifles parasitic BJT genesis. Deepening isolation trench walls prevents minority carrier injection from activating parasitic BJTs. Such structural changes obstruct origination.
Dynamic Clamping and Feedback
Connecting diode clamps from collector to base regions counteracts parasitic BJT turn-on. Employing charge pump circuits further dynamically restrain parasitic BJT conduction when incited. Negative feedback via gate coupling also mitigates gain that sustains parasitic BJT activation.
Optimization and Screening
Performing sensitivity analysis facilitates optimization of spacer widths and LDD dimensions to maximum parasitic BJT resilience. Stress testing over voltage, temperature, and aging screens design susceptibility to parasitic BJT modes. These best practices harden designs against parasitic BJT disruption.
Design Considerations
Rigorously co-optimizing process technology and circuit layout is imperative to curb parasites in MOSFETs. Let us reflect on key insights from our exploration into latchup, parasitic BJTs, and other surprise effects in order to inform prescriptive design guidance.
Adhere to Layout Rules
Following minimum spacing checks between regions tied to wells averts underlapped diffusion that triggers latchup. Limiting well crossings density also prevents latchup ignition. Both rules optimize against parasitic BJT emergence as well.
Incorporate Guard Structures
Guard rings act to contain latchup sustaining current once initiated. Dummy poly gates obstruct silicide migration that activates latchup paths. Such guard structures also disrupt minority carrier injection that turns on parasitic BJTs.
Perform Sensitivity Analysis
Varying layout spacings, well crossover quantity, LDD widths systematically screens for worst-case parasitic sensitivities. Stress testing over voltage, current, and temperature reveals susceptibility boundaries for proactive avoidance in design centres.
