How do you design CMOS circuits to minimize hot-carrier injection effects?

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Hot-carrier injection (HCI) is a phenomenon that causes degradation and failure of CMOS devices over time. It occurs when high-energy electrons or holes are injected into the gate oxide or the substrate, creating interface traps, oxide charges, and parasitic currents. HCI can reduce the performance, reliability, and lifetime of CMOS circuits, especially in high-speed, low-voltage, and scaled-down applications. How can you design CMOS circuits to minimize HCI effects? Here are some tips and techniques to consider.

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1 Use lower supply voltage

One of the main factors that determine the energy and velocity of carriers in CMOS devices is the supply voltage. A higher supply voltage increases the electric field across the channel and the drain, accelerating the carriers and making them more likely to overcome the potential barrier and enter the oxide or the substrate. By using a lower supply voltage, you can reduce the HCI stress and extend the device lifetime. However, lowering the supply voltage also reduces the speed and the noise margin of the circuit, so you need to find a trade-off between performance and reliability.

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Vaishnav Kumar

Senior, Applications Engineer at Synopsys
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HCI was predominant reliability issue in the beginning as the supply voltages were high. But since now the supply voltages are low it’s expected to almost disappear right, since the supply voltages are ultra low?? But no! these issues are projected to get even worse in future generations of devices. Cause of ultra-thin architecture of MOSs.

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Dr. Kumar K - MD (AM), MPC, PGPC

As a Holistic Mental Health & Wellness Coach, I empower individuals and businesses to navigate challenges, optimize performance, achieve goals through personalized guidance, mentorship, therapy, and strategic consulting.
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Hot-carrier injection is a silent killer of CMOS device reliability. This phenomenon, caused by high-energy carriers damaging the gate oxide, leads to performance degradation and premature circuit failure. To combat this, focus on design strategies that reduce electric field strength in the channel. Employing larger channel lengths, lower supply voltages, and optimized transistor sizing can significantly mitigate hot-carrier effects. Additionally, advanced process technologies like gate oxide engineering and strain engineering offer promising avenues to enhance device robustness against hot-carrier stress.

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2 Use shorter channel length

Another factor that affects the HCI stress is the channel length of the MOSFET. A shorter channel length reduces the distance between the source and the drain, lowering the resistance and increasing the speed of the device. However, it also increases the electric field and the drain-to-source voltage, enhancing the HCI effect. By using a shorter channel length, you can improve the performance and the scaling of the circuit, but you need to compensate for the HCI degradation by using other techniques, such as channel engineering or feedback control.

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Vikas Balikai

Semiconductor Professional
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Particles propelled by a strong electric field to a very high kinetic energy are known as "hot carriers." These energetic carriers can be injected into normally forbidden regions of the device, as the gate dielectric, where they can get trapped or cause interface states to be generated. Subsequently, MOS device transconductance degradation and threshold voltage shifts are caused by these flaws. There are various device design modifications that can be made to prevent, or at least mitigate, hot carrier deterioration. A few examples of these include graded drain junctions, double diffusion of the source and drain, and longer channels.

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3 Use channel engineering

Channel engineering is a technique that modifies the doping profile and the geometry of the channel to optimize the device characteristics and reduce the HCI stress. For example, you can use lightly doped drain (LDD) structures, which insert a lightly doped region between the channel and the drain, reducing the electric field and the carrier injection at the drain end. You can also use halo implants, which create a high-doped region around the channel, increasing the threshold voltage and the subthreshold slope, and suppressing the short-channel effects. Channel engineering can improve the performance and the reliability of CMOS devices, but it also introduces additional process complexity and variability.

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4 Use feedback control

Feedback control is a technique that monitors the device parameters and adjusts the bias conditions to compensate for the HCI degradation. For example, you can use a sense amplifier or a comparator to detect the threshold voltage or the current level of the device, and use a feedback loop to regulate the gate voltage or the supply voltage accordingly. You can also use a calibration circuit or a reference device to compare the device performance with a predefined value or a standard device, and use a feedback loop to correct the deviation. Feedback control can maintain the functionality and the accuracy of CMOS circuits, but it also consumes more power and area.

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5 Use circuit redundancy

Circuit redundancy is a technique that adds extra devices or components to the circuit to provide backup or alternative paths for the signal or the current. For example, you can use parallel or series transistors, which share the load and reduce the stress on each device. You can also use spare or switchable devices, which can replace or bypass the degraded devices. Circuit redundancy can increase the robustness and the fault tolerance of CMOS circuits, but it also reduces the efficiency and the density of the circuit.

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