Bio: Ms. Yongju Zheng is a PhD candidate in the Physics department of Auburn University where she works under the supervision of Dr. Sarit Dhar. Her doctoral research is focused on Silicon Carbide device science and technology, especially on improving the dielectric/SiC interface in 4H-SiC MOSFETs. Her research explores key issues in SiC MOSFETs that result in poor channel mobility and instability.
During her PhD studies, Yongju has completed two summer internships at Texas Instruments in Dallas, TX, in 2015 and 2016. These internships expanded her knowledge and skill sets in the science and business of industrial power electronics devices.
Ms. Zheng holds an MS in Physics from Capital Normal University and a BS in Physics from Bohai University in China.
The project Ms. Zheng is working on is called, The Effect of Boro-silicate glass (BSG) gate dielectric and Antimony Surface Doping on Channel Transport of 4H-SiC MOSFETs.
Our previous work demonstrated a shallow surface counter-doping in the channel region of 4H-SiC MOSFETs by Antimony (Sb) improves sub-threshold slope and channel mobility at low electric field with a peak vale of ~110cm2/V•s for lightly doped p-well with lower threshold voltage VT than NO annealing due to the Sb counter-doping effect. It also has been reported that BSG gated 4H-SiC MOSFETs have a high channel mobility of ~100 cm2/V∙s for a wide range of surface electric fields along with a large VT of ~5V in lightly doped p-epitaxial layers. For vertical power MOSFETs with heavily doped p-wells, it is expected that BSG would cause an even further increase of VT (~10 V), which would be undesirable. Therefore, in this project, we combined the Sb surface doping process (presented at ECSCRM 2014), with BSG gate dielectric with a two-fold goal: 1) Tune VT to adequate value with high sub-threshold slope; and 2) Achieve high low-field channel mobility by Sb counter-doping while retaining the high-field mobility characteristics of BSG.
The results from our experiments indicate that these goals were achieved. The ‘Sb+BSG’ process results in a significant improvement of both low-field channel mobility to ~180cm2/V∙s (due to addition of Sb surface doping) and high-field channel mobility to ~90 cm2/V∙s (due to the BSG gate dielectric) along with a tuned threshold voltage of ~2V and a steeper sub-threshold slope than standard NO annealing. Interface trap characterizations on capacitors by C-V and CCDLS both show lower interface trap density with BSG compared to NO annealing. The mechanism of Boron passivation and stability of BSG gated 4H-SiC MOSFETs will be further studied.