True Random Number Generators Using Electrical Noise

True random number generators (TRNGs) are a fundamental resource in information security and can guarantee the absolute security of information in principle. Entropy source is the most critical part of TRNGs, which provides the unpredictability and is the root of security for TRNGs. Electrical noise, which is inevitable and unpredictable in electronic systems, is always used as entropy source for TRNGs. This review discusses the different methods to harvest electrical noise in TRNGs, including the early amplify noise based on amplifier, phase jitter based on oscillator, the effect of electrical noise on the metastable behavior and amplify noise based on chaos circuits. Each method has its own strengths in aspect of speed, cost, complexity and portability. Finally, some post-processing technologies and TRNG evaluation methods are also discussed. With this review, we hope the current spots for TRNGs using electrical noise are summarized and some possible future directions are pointed out.