1. Depletion Region ECE 2204

2. Depletion Region As electrons diffuse from the n region into the p region and holes diffuse from the p region into the n region, the ionized donors and acceptors that created the electrons and holes are left behind as the donors and acceptors are bound in place by their bonds with the Si atoms in the crystal. ▫These fixed charges induce an electric field in the semiconductor, which produces a drift currents that counters the diffusion currents. ▫The depletion region is the distance over which the electric field is induced.

3. Depletion Region The number of ionized acceptors in the depletion region is equal to the number of ionized donors. ▫In the diagram below, the n-type region is more lightly doped (has fewer donors per cubic centimeter) and the p-type region. ▫Therefore, the depletion width extends deeper into the n-region and the p region.  x = 0 is the metallurgical interface between the p-type material and the n-type material. -x p and x n are the edges of the depletion region in the p-type and n-type material, respectively.

4. Junction Capacitance Any time a depletion region exists, a capacitor is formed. ▫In the case of the p-n junction  The p region on side of the depletion region and the n side on the other form the parallel plates of the capacitor  The width of the depletion region is the thickness of the insulator between parallel plates.

5. Depletion Region (Va ≠ 0V) When a voltage is applied across the p-n junction, the width of the depletion region W changes. It increases when the diode is reverse biased, V a < 0 V. W ceases to exist when V a = V bi. ▫The junction capacitance changes with applied voltage. There is another capacitance that is present in the diode when the diode is forward biased. Thus, the diode capacitance is not infinite when V a = V bi.

6. Depletion Region (Va = 0V) ▫The depletion region is inversely proportional to the doping concentration of the more lightly doped side of a p-n junction.

7. Junction Capacitance With no applied voltage ▫where  r is the relative dielectric constant of the semiconductor,  o is the vacuum permittivity, W is the depletion width, and A is the cross-sectional area of the contact between the p and n regions.  The o in the subscript for the junction capacitance indicates that there is no applied voltage.