User:John R. Brews/Draft

A semiconductor diode is a two-terminal device that conducts current in only one direction, made by joining a p-type semiconducting layer to an n-type semiconducting layer.

Electrical behavior

(PD) Image: John R. Brews
Nonideal pn-diode current-voltage characteristics.

The ideal diode has zero resistance for the forward bias polarity, and infinite resistance (conducts zero current) for the reverse voltage polarity. The pn-diode is not ideal. As shown in the figure, the diode does not conduct appreciably until a nonzero knee voltage (also called the turn-on voltage) is reached. Above this voltage the slope of the current-voltage curve is not infinite, but exhibits a nonzero forward resistance. 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.

Types

Semiconductor diodes come in a large variety of types:

• pn-diode: The pn junction diode consists of an n-type semiconductor joined to a p-type semiconductor.
• Zener diode: The Zener diode is a special type of pn-diode made to operate in the reverse breakdown region, and used often as a voltage regulator. The breakdown voltage in these didoes is sometimes called the Zener voltage. Depending upon the voltage range designed for, the diode may break down by either Zener breakdown, an electron tunneling behavior, or by avalanche breakdown.
• Schottky diode: The Schottky diode is made using a metal such as aluminum or platinum, on a lightly doped semiconductor substrate.
• Metal-oxide varistor: The varistor is intended to provide a voltage controlled resistance. Its resistance under small voltage variations is set by the choice of a bias voltage.
• Tunnel diode: Like the Zener diode, the tunnel diode (or Esaki diode)is made up of heavily doped n- and p-type layers with a very abrupt transition between the two types. Conduction takes place by electron tunneling.
• Light-emitting diode: The light-emitting diode is designed to convert electrical current into light.
• pin-diode: The pin-diode is made of three layers: an intrinsic (undoped) layer between the p'- and n-type layers. Because of its rapid switching characteristicsit is used in microwave and radio-frequency applications.
• Gunn diode: The Gunn diode is a transferred electron device based upon the Gunn effect in III-V semiconductors, and is used to generate microwave oscillations.

Operation

Here, the operation of the simple pn junction diode is considered. The objective is to explain the various bias regimes in the figure.