What Are Transistor Faults?
At transistor level, a transistor may be stuck-short or stuck-open. In stuck-short, a transistor behaves as it is always conducts (or stuck-on), and stuck-open is when a transistor never conducts current (or stuck-off). Transistor Fault model is a fault model used to describe faults for CMOS logic gates.
For stuck-open faulty circuit, AB = 00 produces Z = 1 & AB = 01 will retain Z = 1 without a falling transition. Hence, it requires a sequence of two test vectors rather than a single test vector like stuck-at faults. Stuck-short fault makes a conducting path between Vdd & Vss. For example, if transistor N2 is stuck-short, there will be a conducting path between Vdd & Vss for the test vector 00. This creates a voltage divider at the output node Z where the logic level voltage will be a function of the resistances of the conducting transistors. However, stuck-short transistor faults may be detected by monitoring the power supply current during steady state, referred to as IDDQ.
The circuit in the figure has 2 X 4 = 8 possible single
transistor faults. There are equivalent faults at the transistor level, as
stuck-open faults in a group of series transistor (such as P1 and P2) are
indistinguishable. The same holds true for stuck-short faults in a group of
parallel transistors (such as N1 and N2).
Number of collapsed faults = 2 X Transistors – TS (Series
Transistors) + GS (Group of Series Transistors) - TP (Parallel Transistors) + GP (Group of Parallel transistors)
= 2 X 4 – 2 + 1 – 2 + 1 = 6
See also: Faults in a VLSI design, Stuck at faults, Fault models
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