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As we continue to push the boundaries of Moore’s Law, scaling down transistor sizes has become both a necessity and a challenge. While smaller transistors promise faster speeds and greater energy efficiency, they also introduce significant issues that degrade the performance of traditional MOSFET devices. 

One of the key factors driving the shift towards FinFET technology is the increasing difficulty in maintaining control over the MOSFET’s Id-Vg characteristics as gate lengths shrink. But what exactly happens to these devices as they get smaller? Let’s dive into the root causes and challenges that are making the transition to FinFET inevitable.

Short-Channel Effects: The Core of the Problem

As gate lengths (L) in MOSFETs reduce from larger dimensions like 1μm down to nanometers, the short-channel effects (SCEs) become more pronounced. These effects severely impact the MOSFET’s ability to regulate current, causing two major issues:

1. Degradation of Subthreshold Swing (S): Subthreshold swing represents how efficiently the transistor switches off as the gate voltage (Vgs) decreases. In ideal conditions, a low S value (around 60 mV/decade) means the device can easily cut off current as the gate voltage is reduced. However, as gate lengths shrink, the subthreshold swing worsens, requiring more voltage to reduce the current. This makes it harder to fully turn off the transistor, leading to higher leakage currents and wasted power.
2. Reduction in Threshold Voltage (Vt): Another side effect of short-channel effects is the reduction in threshold voltage, the minimum voltage needed to turn the transistor on. As gate lengths shrink, the threshold voltage decreases, making it difficult to fully switch off the device. This further contributes to leakage currents, where even when the device is supposed to be off, a small current still flows, reducing overall efficiency.

The above plots illustrate how the subthreshold swing and threshold voltage are impacted as gate lengths decrease, highlighting the worsening of short-channel effects.

These two factors - higher subthreshold swing and lower threshold voltage - mean that traditional planar MOSFETs can no longer effectively manage current as they get smaller. The device’s performance degrades, and the variability introduced by these short-channel effects becomes increasingly problematic in mass production.

Wait for the next blog in this series, where we will explore the root cause of short-channel effects in 20th-century MOSFETs, and the growing challenges faced by traditional transistors as they shrank in size. Then, in the final blog of the series, we’ll discuss Why the Shift to FinFET? and how FinFET technology addresses the issues we’ve uncovered.

Stay tuned!