13 September 2012
My interests include high-voltage low-power consumption devices and their models. Recently my studies have focused on improving the breakdown voltage and reducing the specific on-resistance of power lateral double-diffused metal-oxide-semiconductor field-effect-transistors (LDMOSFETs), especially for silicon-on-insulator (SOI) power LDMOS. My main interests are to explore the device operating mechanism, propose new structures and simplify the process. Modelling helps deepen our understanding, and simulation can be used to verify the validity of the model. Improving the device performance is the ultimate goal, which includes not only improving the performance of a single device, but also considering device integration matters to improve the whole performance of circuits and systems.
As is well-known, incorporating an oxide trench into the drift region is a good way to reduce the cell pitch and the specific on-resistance while maintaining the same high breakdown voltage for LDMOSFETs. But it could be very hard to manufacture if the trench is deep and narrow when a high breakdown voltage is required. I’ve been considering whether the trench filling dielectric has influence on the device characteristics and finally found the trench could be shallower and wider, and therefore easier to manufacture, if filled with a high dielectric coefficient material.
There are two main conclusions I have drawn in this Letter: 1) high-k dielectric is suitable to fill a shallow and wide trench while low-k dielectric is suitable for a deep and narrow trench; and 2) SOI LDMOS with variable-k dielectric trench has a lowered specific on-resistance than that of conventional SOI LDMOS. The optimal area of the trench permittivity is also given in this Letter.
They will provide convenience for device engineers as they can be more flexible in choosing the trench materials when designing these devices; more kinds of materials (mainly materials which have a relative dielectric coefficient below 10) can be selected to fill the trench. To continue this work, first the characteristics of the interface layer between silicon/high-k or low-k materials still need to be researched. Secondly, the modelling of SOI TLDMOS or VKTLDMOS is still meaningful; quantitative analysis gives us a deeper understanding of how the dielectric trench affects the device operation, and may provide us with an effective optimisation design method. Finally, we’ll choose a material other than oxide to do the experiment, to see whether or not these devices have application prospects.
I have interests in other kinds of power devices, such as the power diode, power bipolar transistor, vertical double-diffusion metal-oxide-semiconductor (VDMOS), insulated gate bipolar translator (IGBT), MOS control thyristor (MCT) etc. Improving the breakdown voltage and reducing the conduction loss is their common demand. Low turn-off loss must be kept for all the devices mentioned above other than VDMOS. In addition, the hot carrier effect is also one of my important research areas because there always exists a high electric field which inevitably generates hot carriers in high voltage applications. I also did some research into RF LDMOS. The study focused on reducing gate charge, on-resistance and decreasing the parasitic capacitance (mainly for the output capacitance).
I think high temperature, high speed, high power capacity and low power dissipation is the development tendency for power semiconductor technology. More intelligence and systematisation is the demand of our customers in the future. As manufacturing technology has entered into the deep sub-micron era, the silicon power device with new structures and new manufacturing processes is very close to the silicon material theory limit. Wide bandgap semiconductors are undoubtedly the power device’s next development direction. The most typical two representatives are SiC and GaN. They have a high critical electric field, high thermal conductivity, high electron saturation velocity, low leakage current, etc., which are exactly the characteristics we need in the future.
The Letter presenting the results on which this interview is based can be found on the IET Digital Library.
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