A new structural concept to suppress parasitic lateral carrier injection in insulated-gate thyristors

Abstract

A new Insulated -Gate Thyristor structure with thc N-base and P base regions of the thyristor coupled by a ri-channel MOSFET is described. This configuration eliminates the parasitic lateral bipolar transistor present in previously reported MOS gated thyristor structures. This configuration leads to low on-state drop close to that of a 1-D thyristor and high controllable current density. In1 roduction Thyristor structures with MOS gate control have been attracting great interest recently as replacement for currentdriven Gate Turn-off Thyristor (GTO) for high voltage. high current power switching applications like HVDC transmission. traction (drives etc. Device structures previously reported in this categor). are the MCT [l]. BRT [2]. and IGTH [3]. The equivalent circuit of these thyristor structures is shown in Fig. 1. These thyristor structures have an inherent parasitic latcxal bipolar transistor [4] which shunts part of the base drive of the upper NI" transistor of the thyristor structure increasing the on-state drop of the thyristor. Also. carriers are injected in the on-state: into the N JFET region of the DMOS in the case of BRT/IGTH and into the N emitter region in the case of MCT. This makes depletion of these carriers and formation of inversion layer to form the turn-off path difficult [5] and results in low controllable current. The parasitic lateral carrier injection may also aggravate the cxrrent sharing between dtfierent cells in a multi-cellular device compounding the current filamentation problem in IMCT/BRT. In the BRT. the effect of the parasitic lateral PNP transistor can be reduced by reducing the turn-on area of the BRT. thereby reducing the electron supply for the base drive of the lateral PNP transistor. This has been experimentally shown [6] to improve the maximum cointrollable current of BRT from 75 Akin2 to 125 Ncm'. In the IGTH. a high controllable current in the range of 400 Ncm' is obtained by using a Pdiffusion under the MOS gate. However, the problem of the lateral PNP in regards to on-state drop is still present resulting in on-state drops of 1.3 -1.5 V at 200 Ncm' whtch is higher compared to 1 -1.1 V for a 1-D thyristor (P-i-N rectifier). MCTs are triple diffused structures making it difficult to fabricate. 600V MCTs commercially available have controllable current density in the range of 100-200 Ncm' and the on-state drop of MCTs is in the range of 1.2 -1.4 V at 200 PJcrn?. To fully utilize the benefits of thyristor current conduction, it is desirable to operate thyristor based devices at current density in the range of 200-300 Ncm' with on-state drops in the range of 1 -1.2 V. For this. the maximum controllable current should be greater than 300Ncm'. This paper reports a new structure called BC-IGTH (Base Coupledi Insulated Gate Thyristor) which eliminates the parasitic lateral bipolar transistor to achieve a low onstate drop together with high controllable current density. CA%ODE