Trench Split Gate MOSFET’s Inductive Switching

A trench split gate metal-oxide-semiconductor field-effect transistor (MOSFET) inductive switching is analyzed by adopting six-terminal method. Owing to the buried source terminal in the trench oxide, conventional three-terminal (gate, source, and drain) analysis has a limitation for investigating the detailed time-dependent current flow in the drift region, channel, as well as each terminal. However, a mixed-mode simulation tools enable us to look into the complicated current flow mechanisms in the device by dividing the gate terminal into the gate-to-source and the gate-to-drain terminals and the source terminal into the <inline-formula> <tex-math notation="LaTeX">${n} +$ </tex-math></inline-formula>, the <inline-formula> <tex-math notation="LaTeX">${p} +$ </tex-math></inline-formula>, and the shielded source terminals. The six-terminal method enables us to understand the fundamental turn-on and turn-off switching mechanisms that we have not found out so far from the measurement.

targeted at dc-dc power supplies, and ac-dc converters [1]- [3]. 23 To reduce the specific resistance (R sp ) and the breakdown volt- the gate-to-drain capacitance (C GD.Sh ) will not be ignorable 32 owing to the large shielded area [1]. For   instead of the gate as shown in Fig. 1(a). Therefore, a split-36 gate configuration, which is now very popular in industries, 37 finally lowers both the R sp and the C GD.Sh .

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It is believed that the inductive switching behaviors of split-39 gate MOSFETs are well known because the switching charac-40 teristics are very similar to the conventional planar gate power 41 MOSFETs. However, in reality, the knowledge is very limited 42 to the parasitic capacitance models which have been widely 43 used in power electronics [4]. More specifically, power MOS-44 FETs in power electronics features only three representative 45 capacitance: gate-to-source (C GS ), gate-to-drain (C GD.Sh ), and 46 drain-to-source (C DS ). During the turn-off inductive switching, 47 as the drain voltage increases, some portion of the drain current 48 will flow from the drift region to the shielded source region 49 across the thick oxide [dotted arrow, C DS displacement current 50 in Fig. 1(a)]. At the same time, a part of the C DS displacement 51 current will be transformed into a source-to-gate displacement 52 current because the gate and the shielded source region are 53 separated by an oxide. If the displacement current path (dotted 54 arrow) shown in Fig. 1(a) is redrawn to the conventional power 55 electronics model, the current path will be like Fig. 1(b), 56 i.e., with a conventional three-terminal measurement, those 57 complicated current paths are nearly impossible to be detected. 58 In 2014, Roig et al. [5] schematically modeled the shielded 59 gate with a parasitic C GD.Sh . However, the detailed current flow 60 mechanism across the shielded gate was not explained. Other 61 studies have been focusing on mostly lowering the static and 62 dynamic figures of merits (FOMs) or improving the dynamic 63 ruggedness [6]- [8].

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To secure a thorough understanding and deeper insight into 65 the split-gate's switching behavior, the contact of the device 66 needs to be specified as six terminals as shown in Fig. 1(a). 67 The gate current (I G ) is consisting of the gate-to-source (I GS ) 68 and gate-to-drain (I GD ) current, respectively [9], [10], The operating temperature is 300 K.

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The physical models in the simulation include Shockley-      current level is higher than the source current level (I S ). Fig. 8 154 shows the current flow in the trench MOSFET during t 2 -t 3 .

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After t 3 : Once the drain voltage reaches its turn-on voltage 156 level, the recession of the depletion in the drift region stops, 157 and the gate voltage increases until it becomes the driving 158 voltage level (10 V). A part of the gate charging current 159 (displacement current) flows across the channel, and the other 160 part flows through the shielded source terminal as shown in 161 Fig. 9. i.e., according to Kirchhoff's rules, the sum of the input current 168 (I D ) is the same as the same of the output current (I S + I G ). 169 Fig. 11 shows the voltage and the current waveforms during 170 the turn-off inductive switching.    Schematic turn-off current flows in the trench MOSFET during t 0 -t 1 . Fig. 13.
Schematic turn-off current flows in the trench MOSFET during t 1 -t 2 . t 1 -t 2 : Once the gate voltage reaches its Miller potential, the 179 drain-to-source voltage (V DS ) starts increasing and the deple-180 tion in the drift region continuously expands. The expansion 181 of the depletion in the drift region requires the displacement 182 current to charge the two parasitic capacitances: 1) the capaci-183 tance between the drift region and the shielded source terminal 184 and 2) the capacitance between the p-body ( p+ terminal on 185 the source) and the drift region. Therefore, some part of the 186 drain current flows through the S.S and the p+ terminal. The 187 detailed current flow profiles are shown in Fig. 13. Schematic turn-off current flows in the trench MOSFET during t 2 -t 3 .  The final current flow process is in Fig. 15. Six-terminal analysis for the inductive switching of a trench 212 split gate MOSFET was carried out by adopting a mixed-mode 213 simulation. The six-terminal approach gave us insight into 214 the detailed current flow across the shielded source terminal. 215 More specifically, the displacement current flow from the 216 drain and gate terminals to the shielded source terminal is 217 nearly impossible to be depicted in the conventional power 218 electronics model. From the thorough investigation of the 219 detailed current flows, the understanding of the inductive 220 switching mechanism in a trench split gate MOSFET was 221 improved.