This study presents a true random number generator (TRNG) harvesting random bits from delay variations of ambipolarity-based ring oscillator, designed using 20â€.nm InAs Tunnel FET (TFET). Exploiting the TFET transmission gate (TG) functional failure, TFET ambipolarity-based ring oscillator design has been proposed. Random variations are observed in the oscillating frequency of proposed ring oscillator by changing the TFET device ambipolarity. Exploring the same, a TFET ambipolarity-based TRNG circuit has been demonstrated. XOR gate-based post-processing unit is designed to further enhance the unpredictability and randomness of the output bits. The proposed TRNG has passed various NIST tests performed at a supply voltage of 0.5â€.V. In 20â€.nm, the proposed TFET TRNG has an area as low as 90â€.pm2 and consumes 5.4â€.pJ/bit at 0.5â€.V supply voltage. Ambipolarity-based circuit design makes the proposed TRNG robust against reverse engineering attacks.