Forward Bias
Forward bias is the polarity of voltage applied across a pn junction with the positive side connected to the p-type material and the negative side to the n-type material, which lowers the potential barrier at the junction and lets a large current flow. It is the on state of a diode and, in a bipolar transistor, the condition applied to the base-emitter junction to make the device conduct.
Forward bias works by shrinking the barrier that diffusion has to climb, and the effect on current is exponential. In equilibrium the depletion region holds a built-in potential of about 0.6 to 0.7 volts in silicon, which stops all but a trickle of carriers from crossing. A forward voltage opposes that built-in potential, narrowing the depletion region and lowering the barrier by the amount applied, so far more electrons from the n side and holes from the p side have enough energy to cross. The ideal diode equation, worked out by William Shockley in his 1949 Bell System Technical Journal paper on pn junctions, says the current equals a small saturation current times the quantity exp(V divided by the thermal voltage) minus one. The thermal voltage is about 0.026 volts at room temperature, so every extra 60 millivolts or so multiplies the current by ten, and a few hundred millivolts of change takes the current from nanoamperes to milliamperes.
The roughly 0.6 to 0.7 volts quoted for silicon is a practical landmark, not a threshold built into the physics. Because the current rises exponentially, there is no sharp knee: a silicon small-signal diode carries microamperes at 0.4 volts, about a milliampere near 0.6 and tens of milliamperes near 0.7 or 0.8, and the voltage that appears across it at a given current changes slowly, which is why the number is quoted as typical. The voltage also falls by roughly 2 millivolts for each degree Celsius of warming at fixed current, a property exploited in temperature sensors. Other materials shift the figure: germanium diodes turn on near 0.2 to 0.3 volts and Schottky diodes at a similar low level, while light-emitting diodes need voltages close to their photon energy, around 1.8 volts for red and above 3 volts for blue and white. At high current the series resistance of the semiconductor and contacts adds a linear term, so real forward curves bend away from the ideal exponential.
In a bipolar transistor the same physics is the heart of amplification. The base-emitter junction is forward biased, with a base-emitter voltage near 0.7 volts for a small silicon transistor in normal operation, and the collector current is then set exponentially by that voltage, so a small voltage change produces a large current change. The same exponential makes the transistor strongly temperature dependent, and it forms the basis of the band-gap voltage references and proportional-to-temperature sensors found in nearly every analog chip. The injected minority carriers, electrons crossing into the base of an NPN transistor, are what the collector then collects.
A common error is to think forward voltage and current are tied by a fixed resistance, or that a diode simply conducts once the supply exceeds 0.7 volts. A forward-biased junction is a nonlinear element: the voltage across it stays nearly pinned in the 0.6 to 0.8 volt range while the current is set by whatever resistance is in the circuit, which is why an LED or a diode connected directly to a supply is destroyed. Forward bias also stores charge, the injected minority carriers that must be removed when the diode switches off, which limits the speed of power rectifiers and gives rise to reverse-recovery effects. In today's chips forward-biased junctions appear in electrostatic-discharge protection diodes, in the substrate diodes that designers must keep from turning on accidentally, and in the temperature sensors and voltage references embedded in processors and accelerators.
Read forward bias as lowering the junction's barrier so that the current climbs exponentially, with about 0.6 to 0.7 volts being where silicon typically becomes visibly conductive.