n. Poly Gate log Ids (A) N TRANSISTOR LOCOS THICK OXIDE 100 UA 10 uA TRESHOLD VOLTAGE drain source LEAKAGE 100 and GATE THIN CURRENT OXIDE CROSS SECTION 10 nA LATERAL MOS TRESHOLD LATERAL VOLTAGE LOCOS 10 pt Regular +5 Vgs (V) Transistor LARGE PARASITIC A Vt # tox 1 to Figure 75. Principle of a log Ids-V characteristic for a transistor affected by a lateral leakage. III-86 9.1.2. EXAMPLE OF CONSEQUENCES OF LEAKAGE FOR CIRCUITS Two main consequences are observed: In complex circuits, due to the large number of transistors, the total supply current rapidly exceed limits, and internal voltage drops occurs due to Ohmic loss in supply lines. Before this extremity, signal can be corrupted and margin exceeded. And even before, integrity of sensitive internal analog function such as amplifiers and comparators can be exceeded. In simple circuits, input leakage current can exceed data recommended by the product sheet, being a cause of part rejection. The two following figures show the behavior of the same type of circuits (DRAM from 4 to 64 megabytes). It is remarkable that the loss of functionality remains in the same range of dose (10 to 30 kilorads), despite a decade of time and three generations have passed between them. This will be discussed at the end of this chapter. Meanwhile, we will go further into modeling SO as to prepare this discussion. Co-60 versus 'CCSB& 'CCOP Co-60 versus Functionality POSS/N 1.5Krod/hr 0481 1.5Krod/hr 0485 Krads(Si) ICCSB ICCSB 1st Error - (Dose rate : 1.5 Krad/hr) ICCOP ICCOP 32nd Error-Functional Failure ESA/ESTEC-OCA July '91 Figure 76. Correlation of failure and leakage current in 4 Mb DRAMs (TMS44100DM) (After L. Adam, R. Harboe-Sorensen et al., ESA group, radiation Summary Sheet 5 and 9, 1991). III-87 Micron 4Mb (5V Standard) Micron 16Mb(5V/3.6V) Micron 64Mb (3.3V) Micron 16Mb (3.3V) Dose, krad(Si) Figure 77. Correlation of failure and leakage current in 64 Mb DRAMs [Shaw-95]. Leakage currents strikingly escape the Moore's law, i.e. they still are susceptible to appear in the range