Consider a silicon DMOS power MOSFET shown in Figure 15.20 ....

Consider a silicon DMOS power MOSFET shown in Figure $15.20 .$ The source doping concentration is $5 \times 10^{17} \mathrm{~cm}^{-3}$ and the base doping concentration is $10^{15} \mathrm{~cm}^{-3}$. (a) Design the drain doping concentration, channel length, and drain drift region width to support a blocking voltage of $200 \mathrm{~V} .(b)$ Repeat part $(a)$ such that the blocking voltage is $80 \mathrm{~V}$.

Consider a silicon DMOS power MOSFET shown in Figure $15.20 .$ The source doping concentration is $5 \times 10^{17} \mathrm{~cm}^{-3}$ and the base doping concentration is $10^{15} \mathrm{~cm}^{-3}$.
(a) Design the drain doping concentration, channel length, and drain drift region width to support a blocking voltage of $200 \mathrm{~V} .(b)$ Repeat part $(a)$ such that the blocking voltage is $80 \mathrm{~V}$.

Key Concepts

Design Trade-Offs in Power MOSFETs

Designing power MOSFETs involves inherent trade-offs, particularly between optimizing on-resistance and ensuring high breakdown voltage. For instance, increasing the drift region width and reducing its doping concentration helps achieve higher blocking voltages but may lead to increased on-resistance. Conversely, for lower voltage devices, a shorter drift region with a higher doping level can minimize conduction resistance. Balancing these parameters is crucial for tailoring device performance to specific voltage and conduction requirements.

Channel Length Design

The channel length in a MOSFET influences both the conduction properties and the electric field management within the device. In power MOSFETs like the DMOS, the channel length is carefully designed to balance current conduction and voltage blocking ability. A suitably chosen channel length ensures that sufficient channel conduction is maintained while also supporting the high-voltage requirements imposed by the drift region and overall device geometry.

Doping Concentration

Doping concentration in semiconductor devices is a key parameter that controls the electrical properties of different regions. In the context of power MOSFET design, the doping level in the drift region is particularly critical because it influences the breakdown voltage and on-resistance. Lower doping levels in the drift region allow for higher depletion widths and thus enable the device to support higher blocking voltages, while higher doping can reduce the device resistance during conduction.

DMOS Structure

The DMOS (Double-diffused Metal-Oxide-Semiconductor) structure is a semiconductor device architecture used primarily in power electronics. It features a specially designed drift region that allows for the management of high voltages while maintaining low on-resistance in the conduction channel. This structure is optimized by diffusing dopants into the semiconductor substrate in two steps, which creates a gradual profile that aids in sustaining high electric fields and achieving a high blocking voltage without compromising conduction performance.

Drift Region Design and Breakdown Voltage

The design of the drift region is central to achieving the desired breakdown voltage in power MOSFETs. This region must be tailored in terms of both doping concentration and physical dimensions, such as width, to extend the depletion region under reverse bias. A carefully optimized drift region prevents premature avalanche breakdown by evenly distributing the electric field, allowing the device to block high voltages. The relationship between the drift region width, its doping level, and the breakdown voltage is a fundamental aspect in power device design.

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