Analysis of E/DMOS structure manufactured by double diffusion method

Double diffusion method, referred to as DMOS process. The substrate uses P^-silicon wafer (called χsubstrate), such a low concentration of P^-type substrate, it is easier to make N-channel depletion typeMOS devices. In order to make an enhanced device, boron can be diffused in the source region to turn it into a P region, and then phosphorus is diffused to make the source and drain regions. In this way, the impurity distribution from source to drain is N⁺-PP^--N⁺, the threshold voltage of the enhanced N-channel is determined by The impurity concentration of the diffused P region is determined, and the effective channel length between the source and drain is also adjusted by the impurity diffusion. N-channel MOS transistors without P-type impurity diffusion in the source region are depletion type. As shown in Figure 6-24.

This structure has the following advantages:

①The impurity distribution in the enhancement mode MOS channel region is uneven, and the closer the source region is, the higher the impurity concentration. Therefore, the distribution of movable carriers in the channel is not uniform. When carriers are generated at the source, a large number of carriers have been generated at the drain. Therefore, the on-current of DMOS devices is smaller than that of ordinary MOS devices. Therefore, for a certain on-resistance, a smaller gate area can be used to make the input capacitance smaller.

②The threshold voltage of enhanced MOS devices can be controlled by the P diffusion area, which allows greater flexibility in device parameter selection.

③Since the channel length is determined by diffusion, a channel length of 1μm is easy to obtain.

④Using the χ substrate can also reduce the junction capacitance and increase the surface mobility of carriers. At the same time, it can also reduce the output voltage caused by the Back gate effect.

The disadvantage of the double diffusion process is that the precise channel length of the enhanced device cannot be known, because the size of the lateral diffusion immediately below the surface must be accurately calculated in actual work. More difficult.

In addition to the above three methods of manufacturing E/DMOS circuits, epitaxy and isolation diffusion can also be used to manufacture E/DMOS, but the process is complicated. Among these methods, the use of ion implantation to manufacture E/DMOS is the most convenient and effective.

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