FinFET vs MOSFET (Planar) Transistor

MOSFETs present issues like leakage current, heat dissipation problems or short channel effects when chips shrink below the 28nm node.
By adding a vertical fin, the MOSFET is converted into a FinFET. FinFETs have lower leakage, better heat dissipation and good scalability properties.
FinFETs are three-dimensional transistors, whereas MOSFETs are planar transistors.

Before delving into FinFETs vs MOSFETs, we must understand how IC transistors work. Transistors have 1) a source: the starting point for the electrical current; 2) a drain: the “exit” point for the electrical current; and 3) a gate: which opens or closes to let electricity flow through the semiconductor. You can think of the gate as a valve that opens or closes depending on the voltage applied to it, allowing or blocking the current flow. The silicon substrate (in gray) supports the transistor’s components and provides the necessary mechanical structure. The channel (red arrows) is the pathway through which electrical current flows when the gate is activated. When a voltage is applied to the gate it creates an electric field that excites the charge carriers (electrons) in the channel, allowing the flow of electricity.

MOSFET: The Planar Transistor

Planar/MOSFET transistors have been the most mainstream transistor of the semiconductor industry for many decades. As Moore’s Law advanced and transistors scaled down reaching nanoscale size, MOSFETs started to face limitations, especially below the 28nm process node.

Leakage current: below the 28nm node, MOSFET transistors face electron leakage between the source and the drain, even when the gate (remember, the valve) is closed. This leads to higher power consumption and lower energy efficiency of the chip using the MOSFET transistor.
Heat dissipation problems: as chips and transistors become smaller and more densely packed, the heat/cm2 generated increases, which can degrade performance and reliability.
Short channel effect: when the channel length of MOSFET transistors is too short it can cause:
- Variations in the threshold voltage: the voltage required to turn the transistor on. If the threshold voltage declines, the transistor could be in ON state when it should be in OFF state.
- Drain-Induced Barrier Lowering (DIBL): With DIBL the threshold voltage goes down. This means it becomes easier for the transistor gate to open and let electrons flow, even when it’s supposed to be closed. This leads to higher power consumption.

FinFET: The 3D Advancement Over MOSFET

In 1999 TSMC’s engineer Chenming Hu discovered that most of the MOSFET problems mentioned above were resolved if a vertical “fin” was added to the gate, giving birth to the FinFET transistor. The main improvements of FinFET vs planar MOSFET are:

Low leakage current: the fin structure of the FinFET transistor allows for better isolation. This means the gate is totally sealed when it is in “off” position. The fin makes it more difficult for electrons to tunnel through the insulating layer of the semiconductor, improving energy efficiency.
Improved heat dissipation: the increased surface area of the fins makes it easier to dissipate heat, which is crucial for chip performance and reliability. Lower leakage currents also result in less heat generated.
Reduced DIBL: the fin structure creates a more effective electric field, providing better control over the channel. We can say that the fin makes it harder for electrons to move when they should stay put.
Better control of the threshold voltage: threshold voltage becomes more consistent around the transistor, which ensures the proper functioning of the gate.

So overall, FinFETs provide better electrostatic control compared to MOSFETs below the 28nm process node, resulting in faster switching speeds and making them more suitable for high-speed applications in mobile devices or High Performance Computing (HPC). However, MOSFETs are still perfectly fine for more mature applications like automotive or consumer electronics due to their lower manufacturing cost and more than adequate performance for less demanding tasks.

We should choose between FinFETs vs MOSFETs design depending on the needs of our application. FinFETs have demonstrated great scalability properties, meaning they have maintained their performance as manufacturers create smaller chips with smaller transistors. Both TSMC and Samsung manufacture 5nm FinFET transistors, and TSMC expects to begin manufacturing of 3nm chips in 2026, although these will use a newer transistor structure.