The PN junction

Put p-type and n-type silicon in contact and something useful happens at the boundary on its own, with no voltage applied.

• Free electrons diffuse from the n side into the p side and recombine with holes, leaving uncovered ions behind.
• Those two layers of fixed charge form the depletion region, and the field across it stops further diffusion.
• Crossing that field costs energy, and the cost is the barrier potential: about 0.7 V for silicon and 0.3 V for germanium at 25 °C.
• The p side is the anode, the n side is the cathode, and the symbol's arrow points the way conventional current flows.

Three bias conditions

• No bias: the depletion region sits at its natural width and the net current is zero.
• Forward bias, plus to the p side: the depletion region narrows, and once the applied voltage exceeds the barrier potential the diode conducts heavily.
• Reverse bias, plus to the n side: the depletion region widens and only a tiny saturation current flows, until the reverse voltage reaches breakdown.

This is why the V-I characteristic is a near-flat line one way and a steep knee the other.

Working with diodes in a circuit

You almost never solve the exponential diode equation by hand. You pick a model.

• Ideal model: a perfect switch, 0 V when on. Good for a quick sanity check.
• Practical model: a 0.7 V drop when on. This is the one used throughout this course.
• Complete model: adds the bulk resistance rd. Reserved for precision work.
• Load-line analysis solves the circuit and the device together: draw the resistor's line across the diode's curve and read the Q point, VDQ and IDQ, where they cross.

For series and parallel configurations the method is always the same: decide whether each diode is on or off, replace it with its model, then solve what is left with Ohm and Kirchhoff.