Ideal switchZero current when open, no matter what
A real transistorAlways passes a trickle: the off current, Ioff
Why it mattersA phone or server holds billions of transistors that sit idle most of the time
Three leaksOver the barrier, through the oxide, and across a junction
Switching energy is the visible cost of a chip. Leakage is the one that never sleeps.
OVER THE BARRIER
Subthreshold leakage
Ioff ≈ Ith × 10−Vth/S
Carriers in the source follow a Boltzmann distribution, so some always have enough energy to cross a barrier the gate has not fully raised. The current falls by a factor of 10 for every S of gate voltage below threshold, and at room temperature S cannot beat about 60 mV per decade. Lower Vth by 120 mV at S = 60 and the leakage is 100 times larger.
A faster, lower-threshold transistor leaks exponentially more.
THROUGH THE OXIDE
Gate tunneling
When the gate oxide is only a few atoms thick, electrons can tunnel straight through it. The leakage falls exponentially with thickness, and in the early 2000s SiO₂ was thin enough (about 1.2 nm) that the gate leakage of a whole chip became a serious fraction of its power. This is what drove the move to high-k dielectrics (Part 4).
Thin oxides give strong control, and the price is current through the gate.
THE BUDGET
Fast or quiet, pick a mix
Junction leakageA reverse-biased drain-to-body diode, and band-to-band tunneling at high doping, add a smaller trickle