EC-6.3 The P-N Junction

Work through these

• What happens in the first instant two doped regions meet

  Electrons diffuse from the n side and holes from the p side, and each leaves behind a fixed ionised impurity it can no longer neutralise. Those fixed charges build a field that opposes further diffusion until the two effects balance.

  NPTEL: Introduction to Semiconductor Devices · Course

• The depletion region and the built-in potential

  The region either side of the junction is left with almost no mobile carriers, and the fixed charge in it supports a voltage across the junction even with nothing connected. That voltage is set by the doping and by the material.

• Forward bias: lowering the barrier

  Applying a voltage that opposes the built-in field narrows the depletion region and lets diffusion resume, so current rises steeply once the barrier is nearly cancelled. This is why a silicon diode appears to turn on around two thirds of a volt.

  NPTEL: Solid State Devices · Course

• Reverse bias: widening it, and the small current that remains

  The applied voltage adds to the built-in field, widening the depletion region and stopping diffusion almost completely. A tiny current still flows, carried by minority carriers generated inside the region.

• The diode equation, and what its exponential really says

  Current depends exponentially on applied voltage, which is why a diode has no single resistance and why a small voltage change makes a large current change. The thermal voltage in the exponent is where temperature dependence enters.

  MIT OpenCourseWare 6.012: Microelectronic Devices and Circuits · Course

• Junction capacitance, and why a diode is also a capacitor

  The depletion region separates charge across an insulating gap, so it stores charge and its width changes with voltage. That gives a voltage-dependent capacitance, deliberately exploited in tuning circuits and a nuisance everywhere else.