4.5.2 Device Structure and Operations

Figure 4.43 shows the schematic structure of the proposed SOI SA-LIGBT. The

-layer is introduced to the

-anode region to achieve a shorted-anode structure. As can be seen in the figure, the

- and

-areas are separated by the trench oxide.

**Figure 4.43:** Schematics of the proposed SOI SA-LIGBT with a trench oxide at the drainanode.
$\begin{figure}\begin{center} \psfig{file=figures/ligbt/trenchDCONT.eps, width=... ....6,4.0) \end{pspicture}\end{picture} \vspace*{-0.0cm} \end{center}\end{figure}$

The device is designed to achieve a BV of 120V with an SOI thickness $t_\mathrm{soi}$ of 2.0 $\mu$ m and with a buried oxide thickness $t_\mathrm{ox}$ of 1.0 $\mu$ m. The design parameters used for this analysis are listed in Table 4.5.

**Table 4.5:** The technological and geometrical parameters considered for device simulation.
Parameter	Value
-drift doping $N_\mathrm{D}$	1.0 $\times$ $10^{16}$ $\mathrm{cm}^{-3}$
-drift length $L_\mathrm{d}$	8.5 $\mu$ m
SOI thickness $t_\mathrm{soi}$	2.0 $\mu$ m
N-substrate doping	5.0 $\times$ $10^{18}$ $\mathrm{cm}^{-3}$
Buried oxide thickness $t_\mathrm{ox}$	1.0 $\mu$ m
drain length $\mu$ m	2.0 $\mu$ m
anode length $\mu$ m	6.0 $\mu$ m
Trench oxide depth $\mu$ m	0.5 - 1.5 $\mu$ m

The maximum BV of the SOI SA-LIGBT is limited by the thickness of the buried oxide. An optimal drift length and doping must be ensured to get the best trade-off between the on-resistance and the BV. As shown in the table the

-drift length is 8.5 $\mu$ m, the doping amounts to 1.0 $\times$ $10^{16}$ $\mathrm{cm}^{-3}$ , and the trench oxide depth is 1.0 $\mu$ m. A highly doped

-buffer is added at the drain/anode region which helps to prevent punch through at this region.

**Figure 4.44:** Current flow arrows of the proposed SA-LIGBT at $V_\textrm {G}$ 12 V and $V_\textrm {A}$ 10 V.
$\begin{figure}\begin{center} \psfig{file=figures/ligbt/curbjt1.eps, width=0.7\... ...makebox(0,0){Substrate}} \end{picture} \vspace*{0.0cm} \end{center}\end{figure}$

To suppress the NDR the length of the

-anode of the conventional SA-LIGBT must be increased. To overcome this drawback we introduce a trench oxide at the drain/anode region. The

-drain length of 2.0 $\mu$ m and the

-anode length of 6.0 $\mu$ m are used through out all the simulations. With the structure proposed it is possible to suppress the NDR without increasing the

-anode length.

As shown in Figure 4.43, the device has a hybrid LDMOSFET-LIGBT structure with a common drift region. The

-anode provides conductivity modulation of the

-drift region. The

-drain defines a lateral DMOS structure and an electron extraction path during turn-off of the device. As a result two different modes of on-state operation can be seen, which depend on the bias conditions.

At low anode voltages the device exhibits MOSFET operation. Only the

-region at the drain/anode contributes to the current conduction in the on-state and significant conductivity modulation of the

-drift region cannot be seen. As the anode voltage increases, the potential underneath the

-anode starts to fall and makes the

-anode and

-drift junction forward biased. Considerable injection of holes from the

-anode to the

-drift region takes place, resulting in a lower forward voltage drop than that of the SOI-LDMOSFET. Figure 4.44 shows the current flow of the proposed SA-LIGBT at $V_\mathrm{G}$

12V and $V_\mathrm{A}$

10V. The electron current at the

drain region, and the hole current at the

-anode and under the cathode (

-body region) can be seen simultaneously.