NMOS devices, also known as n-channel metal-oxide-semiconductor devices, are a type of field-effect transistor (FET) that are commonly used in digital integrated circuits. These devices are formed in a p-type substrate of moderate doping level. Let's understand why this is the correct option.

Understanding nMOS Devices:
- nMOS devices are made up of three layers: a p-type substrate, an n-type source/drain region, and a metal gate electrode separated from the substrate by a thin insulating layer.
- The p-type substrate acts as the body or bulk of the transistor, while the n-type source/drain regions are heavily doped to facilitate the flow of charge carriers (electrons).
- The metal gate electrode controls the flow of electrons between the source and drain regions by modulating the conductivity of the channel region under the gate.

Explanation of Option C: p-type substrate of moderate doping level
- The correct option, option C, states that nMOS devices are formed in a p-type substrate of moderate doping level.
- This means that the p-type substrate is intentionally doped with impurities to create a moderate concentration of holes (positive charge carriers).
- The doping level is carefully chosen to ensure optimal device performance, including desirable threshold voltage and low leakage current characteristics.
- The moderate doping level of the p-type substrate allows for better control of the device during operation.

Reasons for Option C Being Correct:
1. Threshold Voltage Control:
- The doping level of the p-type substrate affects the threshold voltage of the nMOS device.
- A moderate doping level helps to achieve the desired threshold voltage, which is crucial for proper device operation.
- The threshold voltage determines the minimum voltage required on the gate electrode to turn on the transistor and allow current flow between the source and drain regions.

2. Leakage Current Reduction:
- A moderate doping level in the p-type substrate helps to minimize leakage currents in the nMOS device.
- Leakage currents can negatively impact the device's performance and power consumption.
- By carefully controlling the doping level, the nMOS device can achieve lower leakage currents, enabling more efficient operation.

Conclusion:
- nMOS devices are formed in a p-type substrate of moderate doping level.
- This choice of substrate doping allows for better control of the device's threshold voltage and reduces leakage currents.
- The moderate doping level is carefully selected to optimize the performance of the nMOS device in digital integrated circuits.