# On Improving Reliability of SRAM-Based Physically Unclonable Functions
## 2 Background  
### SRAM PUF and Reliability  
SRAM cells that are constituent of embedded memories typically consist of cross-coupled inverters connected by access transistors. Figure 1 shows a typical 6-Transistor SRAM cell. Due to intrinsic process variations, an SRAM cell on start-up would, typically, consistently settle in either _logic-0_ or _logic-1_ values. The settlement state is determined by mismatch in process variations in the cell transistors. Settlement to consistent yet random states allow values from multiple cells to be collected for use as a key or identifier. An SRAM PUF is expected to produce this key each and every time during power-up operation. Unfortunately, noise during start up can impact the settlement state of the PUF resulting in unreliability. Specifically, cells with low mismatch due to process variations are more sensitive to noise than cells with greater mismatch. Cells with greater mismatch produce sufficient differential drive to overcome any impact of noise. Along with various noise sources, variations in ambient conditions and supply voltage and parametric changes due to aging of the transistors also impact reliability. As the properties of a PUF depend on process variations, long-term device degradation due to aging impacts PUF reliability. Reliability of a PUF is also influenced by ambient conditions, supply voltage and various sources of noise. Among the various noise sources, power supply noise, crosstalk, thermal noise, shot noise and random telegraphic noise play a role. In addition to environmental variations, aging of circuits due to negative-bias temperature instability (NBTI), hot-carrier injection (HCI) and time-dependent dielectric breakdown (TDDB) decreases life-time reliability of a PUF. Thus, a PUF is considered unreliable whenever it produces a response that is different from the enrolled ideal response.  
As mentioned earlier, PUF circuits can be broadly classified as Weak and Strong PUFs. Weak PUFs such as SRAM PUFs typically have few CRPs, while Strong PUFs such as Arbiter PUFs have an exponentially large number of CRPs. A Strong PUF circuit may produce errors for certain CRPs.  
Similarly, a Weak PUF may produce an identifier in which only a few bits of the identifier have a high error rate. Due to the difference in CRP count and usage model of Strong and Weak PUFs, the errors have to be handled differently. As Strong PUFs have exponential CRPs and are used for authentication applications, multiple challenges are applied and responses are obtained. Authentication using these error-prone responses can be carried-out successfully by setting a success threshold for responses. For example, a Strong PUF with 2% intrinsic error rate can be successfully authenticated if 97% responses are correct. In contrast, Weak PUF responses are typically used for cryptographic key generation (or identification) where the responses are expected to be extremely reliable. Hence, alternative solutions such as error-correcting codes are more critical for Weak PUFs. As the reliability requirements of Weak PUFs are more demanding, we focus on improving their reliability for the remainder of this paper.