Self heating of a MOS device will reduce performance. Drain current decreases and long term reliability can be affected. In SOI devices, sel-f heating is an even greater problem due to the buried oxide. By adding a path from SOI to substrate that has a high thermal conductivity and low electrical conductivity , the negative effects of sel-f heating can be reduced.
References:
[ 1] Garner D M, Chen Y, Sabesan L, et al . A novel flash EEPROM cell based on trench technology for integration within Power integrated circuits[ J] . IEEE Electron Device Letters, 21( 5) : 236~ 238.