DIODE

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Diodes

 Most diodes are made from a host crystal of silicon (Si) with appropriate impurity elements introduced to modify, in a controlled manner, the electrical characteristics of the device. These diodes are the typical PN-junction (or bipolar) devices used in electronic circuits. Another type is the Schottky diode (unipolar), produced by placing a metal layer directly onto the semiconductor. The metal-semiconductor interface serves the same function as the PN-junction in the common diode structure. Other semiconductor materials such as gallium-arsenide (GaAs) and silicon-carbide (SiC) are also in use for new and specialized applications of diodes. The electrical circuit symbol for a bipolar diode. The polarities associated with the forward voltage drop for forwarding current flow are also included. Current or voltage opposite to the polarities indicated are considered to be negative values with respect to the diode conventions.

The characteristic curve is representative of the current-voltage dependencies of typical diodes. The diode conducts forward current with a small forward voltage drop across the device, simulating a closed switch. The relationship between the forward current and forward voltage is approximately given by the Shockley diode equation.

where Is is the leakage current through the diode, q is the electronic charge, n is a correction factor, k is Boltzmann’s constant, and T is the temperature of the semiconductor. Around the knee of the curve is a positive voltage that is termed the turn-on or sometimes the threshold voltage for the diode. This value is an approximate voltage above which the diode is considered turned “on” and can be modeled to a first degree as a closed switch with constant forward drop. Below the threshold voltage value, the diode is considered weakly conducting and approximated as an open switch. The exponential relationship means that the diode forward current can change by orders of magnitude before there is a large change in diode voltage, thus providing the simple circuit model during conduction. The nonlinear relationship also provides a means of frequency mixing for applications in modulation circuits.

 

The reverse voltage applied to the diode causes a small leakage current (negative according to the sign convention) to flow that is typically orders of magnitude lower than the current in the forward direction. The diode can withstand reverse voltages up to a limit determined by its physical construction and the semiconductor material used. Beyond this value, the reverse voltage imparts enough energy to the charge carriers to cause large increases in current. The mechanisms by which this current increase occurs are impact ionization and a tunneling phenomenon (Zener breakdown). Avalanche breakdown results in large power dissipation in the diode, is generally destructive and should be avoided at all times. Both breakdown regions are superimposed for comparison of their effects on the shape of the diode characteristic curve. Avalanche breakdown occurs for reverse applied voltages in the range of volts to kilovolts depending on the exact design of the diode. Zener breakdown occurs at much lower voltages than the avalanche mechanism. Diodes specifically designed to operate in the Zener breakdown mode are used extensively as voltage regulators in regulator integrated circuits and as discrete components in large regulated power supplies. During forwarding conduction, the power loss in the diode can become excessive for large current flow. Schottky diodes have an inherently lower turn-on voltage than pn-junction diodes and are therefore more desirable in applications where the energy losses in the diodes are significant (such as output rectifiers in switching power supplies). Other considerations such as recovery characteristics from forwarding conduction to reverse blocking may also make one diode type more desirable than another. Schottky diodes conduct current with one type of charge carrier and are therefore inherently faster to turn off than bipolar diodes. However, one of the limitations of Schottky diodes is their excessive forward voltage drop when designed to support reverse biases above about 200 V. Therefore, high-voltage diodes are the pn-junction type.