  Magnetic tunnel junctions (MTJs), which are the fundamental building blocks
of spintronic devices, have been used to build true random number generators
(TRNGs) with different trade-offs between throughput, power, and area
requirements. MTJs with high-barrier magnets (HBMs) have been used to generate
random bitstreams with $\lesssim$ 200~Mb/s throughput and pJ/bit energy
consumption. A high temperature sensitivity, however, adversely affects their
performance as a TRNG. Superparamagnetic MTJs employing low-barrier magnets
(LBMs) have also been used for TRNG operation. Although LBM-based MTJs can
operate at low energy, they suffer from slow dynamics, sensitivity to process
variations, and low fabrication yield. In this paper, we model a TRNG based on
medium-barrier magnets (MBMs) with perpendicular magnetic anisotropy. The
proposed MBM-based TRNG is driven with short voltage pulses to induce
ballistic, yet stochastic, magnetization switching. We show that the proposed
TRNG can operate at frequencies of about 500~MHz while consuming less than
100~fJ/bit of energy. In the short-pulse ballistic limit, the switching
probability of our device shows robustness to variations in temperature and
material parameters relative to LBMs and HBMs. Our results suggest that
MBM-based MTJs are suitable candidates for building fast and energy-efficient
TRNG hardware units for probabilistic computing.
