Is Smaller Always Better for Transistor Size?

In the field of technology, many groundbreaking advancements are often associated with “more” and “bigger.” However, in the realm of integrated circuits, our pursuit is focused on “small” – specifically, the size of transistors. If the central processing unit is likened to the engine of a computer, then the chip serves as its power system, directly influencing the overall performance of the computer. The transistor, acting as the microscopic engine of the chip, sees a leap in performance with a reduction in size, allowing for the accommodation of more minuscule components. So smaller is better? This is a thought-provoking question!

Transistors at your fingertips

Evolution of Transistor Size

The developmental journey of transistors can be seen as a cyclical interplay between size, integration, and performance. In essence, as transistor size diminishes, chip integration increases, and device performance correspondingly rises, propelling the next round of further transistor size reduction.

In the early days, scientists used terms like small-scale, medium-scale, large-scale, very-large-scale, etc., to describe the number of transistors integrated on chips. However, as technology progressed, this nomenclature became impractical. Instead, feature transistor sizes such as 28 nm, 20 nm, 14 nm, etc., became the norm. These numbers do not directly represent the transistor’s size but reflect advances in the fabrication process, including critical transistor structures such as gate length, gate width, channel length, etc.

The graph below illustrates the evolution of transistor sizes from 1987 to 2019, showcasing technological innovations and the overtime of shrinking transistor sizes.

transistor size over time

For the layperson, “nanometers” may not be a familiar unit of measurement. To explore this, consider examples: the size of human hair ranges from tens to hundreds of μm; red blood cells measure between 6 to 8 μm; bacteria range from hundreds of μm to nm; and DNA’s size is around 2 nm. Describing transistors as being the size of a virus is not an exaggeration.

Impact of Transistor Size on Performance

Understanding the working principle is a good starting point before discussing this issue. Taking MOSFET transistors as an example, fundamentally, they act as switches, with the voltage applied to the gate determining whether the source and drain are conducting. Applying a positive voltage to the gate of an N-type MOSFET causes electrons to gather in the channel between the source and drain, reducing resistance and allowing conduction. When no voltage is applied to the gate, fewer electrons exist between the source and drain, resulting in increased resistance and non-conduction.

Next, by combining N-type and P-type transistors, a complementary metal-oxide-semiconductor (CMOS) inverter circuit is built, achieving basic “not” logic operations – for example, input 0 yields output 1, and input 1 yields output 0.

N-type and P-type transistor combination

However, for complex calculations like rocket launches, a single inverter combination is insufficient, requiring a large number of transistors to be combined. With the chip’s size remaining constant, smaller transistor sizes mean more transistors and more combinations. Additionally, smaller sizes imply shorter electron movement distances within the chip, leading to shorter signal transmission times and improved processing speed. Consequently, it is generally accepted that smaller transistor sizes translate to better performance.

Limitations of Transistor Size

According to Moore’s Law, the number of transistors that can fit on a chip doubles approximately every 18 to 24 months. This law has held true for several decades; however, as technology continues to advance, some signs suggest that Moore’s Law might be broken at some point in the future.

When transistor sizes shrink to 28 nanometers, the shortened gate length weakens the gate’s control over the channel, leading to issues like leakage currents. To overcome this challenge, FinFET transistors were introduced, addressing the gate control problem with a clever design that extends the channel into a fin-like structure, ensuring effective control of current switches even at smaller sizes.

Comparison of FinFET transistors

Transistor sizes have now reached the 5-nanometer threshold, approaching the physical limits. Theoretically, the smallest transistor size could be 1 nanometer, but as transistor size decreases to the atomic level, quantum effects and heat dissipation become more pronounced, potentially limiting the ability to further reduce transistor sizes.

Furthermore, although smaller transistor sizes allow for more transistors on the chip, this doesn’t always directly translate to higher CPU frequencies. As resistance and inductance increase, the speed boost from size reduction becomes less significant. For this reason, many industry experts believe that even if transistor volume can be further reduced, manufacturers may opt for exploring more advanced materials and new technologies instead.

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