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FD-SOI, The Disruptive Innovation Samsung Foundry is Leading to Overcome the Limits

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In this article, we will learn more about two kinds of SOI technology: PD-SOI and FD-SOI. What are PD-SOI and FD-SOI? Silicon on Insular (SOI) technology is categorized into two types, Partially Depleted SOI (PD-SOI) and Fully Depleted SOI (FD-SOI), based on the thickness of the Buried Oxide Layer (BOX) and the thickness of the monocrystalline silicon for the channels.

What are the advantages and disadvantages of PD-SOI? PD-SOI is applied to analog products such as power devices, with the thickness of the monocrystalline silicon for the channel ranging from 50nm-100nm, and the thickness of the BOX ranging from 100-200nm. As mentioned in the first article of this series, it has the advantage of blocking leakage current through the junction when compared to bulk transistors, and it also reduces the capacitance generated between the source, drain and body. However, no voltage is applied to the substrate, causing a floating body effect. Electrons move toward the drain, but the holes generated by the hot carrier effect have nowhere to go and are accumulated in the channel silicon. This causes a positive voltage to be applied to the channel silicon, resulting in hysteresis — lagging of the ferromagnetic material’s magnetization behind variations of the magnetic field — which changes the characteristics of the device after operation, compared to its state before initial operation. Additionally, when the number of accumulated holes reaches a certain level, the positive voltage lowers the threshold voltage (Vth) of the device, resulting in an increased off current when the device is turned off, with a kink effect occurring when the drain current suddenly increases from the point where the Vth is lowered.

What are the advantages of FD-SOI? FD-SOI’s structure features exceptionally thin channel silicon, known as Ultra-Thin Body (UTB) SOI, with a body thickness of approximately 10nm and BOX thickness ranging from 20-25nm. The thinness of the channel silicon makes it possible to be a fully depleted device (FDD), resulting in an entirely depleted charge layer. In an FDD, the gate has a greater ability to control the channel, which can reduce the short-channel effect, a phenomenon in which the leakage current in the channel increases as the distance between the source and drain decreases. Generally, FDD’s key methodology is reduction of the thickness of the channel silicon. There are similar technologies such as Fin Field Effect Transistor (FinFET) or Gate-All-Around (GAA), but they have a higher complexity due to their 3D structures, while FD-SOI has a 2D planar structure that allowed it to be introduced earlier. Vth is the voltage across the gate sized to turn the transistor on, so a high Vth requires a larger gate voltage to keep the device on, and a low Vth can cause channel leakage current to flow in an incomplete off-state. This is why it is important to keep Vth at a reasonable level. As previously mentioned, since FD-SOI does better at allowing the gate to control the channel, short-channel effects can be reduced, so doping to make up the occurring Vth drop is not necessary, or at least the concentration of doping can be lowered to a significant level. This can improve the carrier’s mobility because scattering (due to doping) between carriers and impurities is prevented. It can also reduce Vth fluctuations caused by random dopant fluctuations that occur as the number of impurities increases. FD-SOI does currently have the limitation of the self-heating effect. Since the BOX used for insulation is an excellent insulator (SiO2), dissipating heat generated during operation is difficult. Consequently, the body temperature rises and the mobility of the device decreases, resulting in decreased channel current. In this second part of our series on SOI, we have learned about FD-SOI, one of the key technologies of Samsung Foundry. We have been leading the semiconductor industry by constantly overcoming technological limitations, and we look forward to the bright future of Samsung Electronics, which will continue to discover and develop future technologies while implementing ultra-fine semiconductor processes. Comparison of FD-SOI and PD-SOI
Item Advantages Disadvantages
PD-SOI • Easy wafer production
• Low production cost
• Floating-body effect
• Hysteresis
• Off current
• Kink effect
FD-SOI • Prevention of short-channel effect
• Rapid operation speed
• Utilization of body bias effect
• Self-heating effect
• High wafer cost