Planar FOX Edge FOX Edge FOX inter-transistors transistor/wel inter-transistors transistor/well P-substrate N-well P-well N-substrate Trapped charge Trapped charge Parasitic leakage channel Parasitic leakage channel Figure 72. Decomposition of parasitics into edge and planar topology of oxides. Two examples of situations in N-well and P-well technologies, among many other possible. EDGE FIELD OXIDES (E-FOX) MODELING The section is devoted to total-dose modeling with special attention paid to the subtle effects encountered in the actual devices, i.e. the geometry and internal field developed by the trapped charges. Classical models will be reviewed first, and adapted to actual 1D, 2D or even 3D situations, such as encountered in parasitic transistors responsible for leakage. Field oxides, edge parasitics and SOI parasitics will be used as examples. Fig. 73 describes the basic characteristics of leagage current, with build-up with dose and, very commonly, post-irradiation slow partial recovery. Transistor Transistor Before Irradiation After 10krad(SiO ) After 30krad(SiO) After 265H Storage Gate Voltage (V) Figure 73. Typical features of parasitic leakage in a 0.35 um technology irradiated under 3.3 Volts [Chab- 97a]. 9.1.1. LATERAL THICK OXIDES: LOCOS TYPE The "LOCOS bird's beak transistor" gives an example of lateral structure. It can be depicted as a parasitic thick oxide transistor in parallel with the principal transistor with a sharing gate. In the classic LOCOS, the "beak" is the fine part, while the "head" of the bird is the thicker section of the LOCOS [Kooi-91]. Parasitic MOS transistors Leakage path Source Drain Leakage path Field oxide Source Drain Bird's beak Positive trapped charge P-type Substrate Figure 74. Illustration of parasitic transistors of lateral and field oxide, showing leakage paths [Flam-98]. An "knee" in the characteristic log Ids-Vgs reveals this effect. The substhreshold value current can be also raised up to a tenth of the primary current of the transistor after the onset of conductio