A p–n diode is a type of semiconductor diode based upon the p–n junction. The diode conducts current in only one direction, and it is made by joining a p-type semiconducting layer to an n-type semiconducting layer. Semiconductor diodes have multiple uses including rectification of alternating current to direct current, in the detection of radio signals, and emitting and detecting light.
Structure The figure shows two of the many possible structures used for p–n semiconductor diodes, both adapted to increase the voltage the devices can withstand in reverse bias. The top structure uses a mesa to avoid a sharp curvature of the p+-region next to the adjoining n-layer. The bottom structure uses a lightly doped p-guard-ring at the edge of the sharp corner of the p+-layer to spread the voltage out over a larger distance and reduce the electric field. (Superscripts like n+ or n− refer to heavier or lighter impurity doping levels.)
Electrical behavior
The ideal diode has zero resistance for the forward bias polarity, and infinite resistance (conducts zero current) for the reverse voltage polarity; if connected in an alternating current circuit, the semiconductor diode acts as an electrical rectifier. The semiconductor diode is not ideal. As shown in the figure, the diode does not conduct appreciably until a nonzero knee voltage (or turn-on, cut-in, or threshold voltage) is reached, whose value depends on the semiconductor (listed in Diode § Forward threshold voltage for various semiconductors). Above this voltage the slope of the current-voltage curve is not infinite (on-resistance is not zero). In the reverse direction the diode conducts a nonzero leakage current (exaggerated by a smaller scale in the figure) and at a sufficiently large reverse voltage below the breakdown voltage the current increases very rapidly with more negative reverse voltages. As shown in the figure, the on or off resistances are the reciprocal slopes of the current-voltage characteristic at a selected bias point:
r D = Δ v D Δ i D | v D = V bias , {\displaystyle r_{\text{D}}=\left.{\frac {\Delta v_{\text{D}}}{\Delta i_{\text{D}}}}\right|_{v_{\text{D}}=V_{\text{bias}}}\ ,}
where r D {\displaystyle r_{\text{D}}} is the resistance and i D {\displaystyle i_{\text{D}}} is the current change corresponding to the diode voltage change Δ v D {\displaystyle \Delta v_{\text{D}}} at the bias V bias . {\displaystyle V_{\text{bias}}\,.}
Operation
Here, the operation of the abrupt p–n diode is considered. By "abrupt", it is meant that the p- and n-type doping exhibit a step function discontinuity at the plane where they encounter each other. The objective is to explain the various bias regimes in the figure displaying current-voltage characteristics. Operation is described using band-bending diagrams that show how the lowest conduction band energy and the highest valence band energy vary with position inside the diode under various bias conditions. For additional discussion, see the articles Semiconductor and Band diagram.
Zero bias
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