Cutoff Region
The cutoff region is the operating condition in which a transistor is turned off: for a bipolar junction transistor both the base-emitter and base-collector junctions are reverse biased (or at least the base-emitter junction is not forward biased enough to conduct), so the collector current falls to a tiny leakage value and the device behaves like an open switch between collector and emitter. It is the "off" half of every switching circuit, the opposite end of the operating range from the saturation region, with the amplifying active region lying between the two.
In a bipolar transistor, cutoff means that carrier injection from the emitter has effectively stopped. With the base-emitter voltage well below the roughly 0.6 to 0.7 volt needed to conduct appreciably in silicon, the emitter injects almost no carriers into the base, so there is almost nothing for the collector to collect. Because the collector current depends exponentially on the base-emitter voltage, the fall is steep: at room temperature each 60 millivolts or so below the conduction level reduces the current roughly tenfold, and a base held at 0 volt or a few tenths of a volt below the emitter makes the transistor practically indistinguishable from an open circuit for most purposes. A positive base voltage of 0.4 volt in a silicon device is thus usually described as cutoff, even though it is not strictly a reverse bias.
The residual current that does flow in cutoff is leakage, specified on datasheets as ICBO and ICEO. ICBO is the collector-base leakage current with the emitter open, essentially the reverse current of the collector junction. ICEO is the collector-emitter leakage with the base open, and it is larger, because the leakage entering the base is amplified like any base current: ICEO is approximately (β + 1) × ICBO. Silicon transistors at room temperature have leakage in the range of nanoamperes or less, while the germanium devices of the 1950s leaked microamperes, which was a real design problem. Leakage roughly doubles for every 8 to 10 degrees Celsius of temperature rise, so circuits that were comfortably off at room temperature could drift toward conduction in a hot enclosure, one of the mechanisms behind thermal runaway in early germanium amplifiers.
Cutoff is what makes an ideal switch ideal at its off end, and designing for it means managing leakage and noise at the base. A switching transistor is usually driven off by taking the base to the emitter voltage, often through a resistor that also gives stored charge and leakage a path to drain, because a floating base lets stray pickup and ICBO bias the device partly on. For a relay driver, the off state should leave the coil with essentially no current, and the transistor must withstand the supply plus inductive kickback while off, which is why the collector-emitter breakdown rating matters just as much in cutoff as saturation voltage does in the on state. Pushing the base too far negative risks exceeding the small reverse base-emitter breakdown rating, typically only a few volts to about 7 volts for a silicon bipolar device, so a deliberately negative off-drive must be limited.
In a MOSFET, the corresponding off state is not a sharp cutoff but subthreshold conduction. When the gate-source voltage is below the threshold voltage, the channel is not strongly inverted, but a small drain current still flows, rising exponentially with gate voltage; ideally the swing is about 60 millivolts of gate voltage per tenfold change in current at room temperature, and real devices are somewhat worse. Added to junction and gate leakage, this subthreshold current is why modern processors, with billions of transistors that are nominally off, dissipate static power even when idle, and why techniques such as higher threshold voltages, power gating and new gate-stack and fin-shaped channel structures were developed. A common misconception is that "off" means zero current, whereas in every real device cutoff is a very small, temperature-dependent leakage whose total can become large when multiplied across a chip.
Read cutoff region as the transistor's open-switch state, in which injection has stopped and only leakage remains, a current negligible in one device but, when billions of devices and high temperatures conspire, one of the central power problems of modern electronics.