One of the major drawbacks of power MOSFETs compared to bipolar devices is their high on-state conduction loss for high-voltage applications. Generally, power MOSFETs have a double-diffused structure with a lightly doped drift region to support the high voltage. Figure 3.12 shows a standard power DMOSFET. The on-resistance of the structure shown in the figure is the total resistance between the source and drain terminal in the on-state. This can be determined by the resistance components of the device. The major resistance components for high-voltage DMOSFETs are the channel ( ), accumulation ( ) and drift layer ( ) resistances, and which is the contribution from the drift region between the -base regions (JFET region). The resistances of the -emitter and substrate regions are generally negligible for high-voltage power MOSFETs. For high-voltage devices the drift region contributes a large resistance. It is important to decrease the drift doping concentration to obtain a high blocking voltage. Increasing the BV can result in a significant undesired increase of .
Consider the ideal case where the of power MOSFETs is determined by the drift region only, and assume that the current flows uniformly through the drift region without current spreading effects. Then the relation between the on-resistance and the BV can be expressed as
for -channel devices and
for -channel devices. The resistances of the accumulation and JFET region are not considered in the equations. The mobility of the drift region also affects the on-resistance of the devices.
In deriving the numerical values in these equations, it is assumed that the mobility is that for a relatively low doping concentration ( ). The ideal of a -channel MOSFET is higher than that for the -channel MOSFET due to the lower mobility of holes than electrons in silicon.
In low-voltage MOSFETs below 100V, the channel resistance and the charge accumulation layer resistance are the dominant factors in the total on-resistance of MOSFETs. These resistances account more than 80% of the total resistance in 20V class devices. On the other hand, in the medium- and high-voltage MOSFETs, the resistance is dominated by doping concentration and thickness of the drift region.
Figure 3.13 shows the state-of-the-art trade-off between and BV of standard power MOSFETs and theoretical silicon limit, respectively . The superior switching performance of the power MOSFET ensure that it has become a key device in the field of SMPS. However, as can be seen in the figure, the on-resistance of power MOSFETs increases sharply with the BV. This has prevented the use of power MOSFETs at high voltages. A high blocking voltage of a standard power MOSFET requires a thick and low doped epitaxial layer (-drift region) which causes an increase in the on-resistance. A variety of MOS structures can be used for power MOSFETs. In the medium- and high-voltage applications, reliability and SOA (safe operating area) of the device are more important than the on-resistance. Therefore, planar structure is frequently used in the high-voltage power MOSFETs, and the trench structure is used in the low-voltage MOSFETs (see Figure 3.13).
Jong-Mun Park 2004-10-28