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Transistor Fault model is a 
In the example picture, a faulty PMOS transistor in a CMOS NAND Gate is shown (M3-highlighted transistor). If M3 is stuck-open, then in case we apply A=1 and B=0 then the output of the circuit will become Z. And if M3 is stuck-short, then the output will always be connected to 1, and it also may short VCC to GND in case we apply A=B=1.
Digital electronics
Electronic design
Electronic circuit verification
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Fault model
A fault model is an engineering model of something that could go wrong in the construction or operation of a piece of equipment. From the model, the designer or user can then predict the consequences of this particular fault. Fault models can be ...
used to describe faults for CMOS
Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
logic gates. 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). Stuck-short will usually produce a short between VDD and VSS.
In the example picture, a faulty PMOS transistor in a CMOS NAND Gate is shown (M3-highlighted transistor). If M3 is stuck-open, then in case we apply A=1 and B=0 then the output of the circuit will become Z. And if M3 is stuck-short, then the output will always be connected to 1, and it also may short VCC to GND in case we apply A=B=1.
Digital electronics
Electronic design
Electronic circuit verification
{{Electronics-stub