Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Junction Properties
Rectifiers
The most obvious property of a p-n junction is its unilateral nature; that is, to a good approximation it conducts current in only one direction. We can think of an ideal diode as a short circuit when forward biased and as an open circuit when reverse biased.
Rectifiers:
- The p-n junction diode does not quite fit this description, but the I- V characteristics of many junctions can be approximated by the ideal diode in series with other circuit elements to form an equivalent circuit.
- For example, most forward-biased diodes exhibit an offset voltage E0 (see Fig. given below), which can be approximated in a circuit model by a battery in series with the ideal diode (Fig. b).
- The series battery in the model keeps the ideal diode turned off for applied voltages less than E.
- We expect E0 to be approximately the contact potential of the junction.
- In some cases the approximation to the actual diode characteristic is improved by adding a series resistor R to the circuit equivalent (Fig. c).
- The equivalent circuit approximations illustrated in Fig. are called piecewise-linear equivalents, since the approximate characteristics are linear over specific ranges of voltage and current.
- An ideal diode can be placed in series with an a-c voltage source to provide rectification of the signal.
- Since current can flow only in the forward direction through the diode, only the positive half-cycles of the input sine wave are passed.
- The output voltage is a half-rectified sine wave. Whereas the input sinusoid has zero average value, the rectified signal has a positive average value and therefore contains a d-c component.
- By appropriate filtering, this d-c level can be extracted from the rectified signal.
- The unilateral nature of diodes is useful for many other circuit applications that require waveshaping.
- This involves alteration of a-c signals by passing only certain portions of the signal while blocking other portions.
- Junction diodes designed for use as rectifiers should have I- V characteristics as close as possible to that of the ideal diode.
- The reverse current should be negligible, and the forward current should exhibit little voltage dependence (negligible forward resistance R).
- The reverse breakdown voltage should be large, and the offset voltage E0 in the forward direction should be small.
- Unfortunately, not all of these requirements can be met by a single device; compromises must be made in the design of the junction to provide the best diode for the intended application.
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The doping concentration on each side of the junction influences the avalanche breakdown voltage, the contact potential, and the series resistance
of the diode. - If the junction has one highly doped side and one lightly doped side (such as a p -n junction), the lightly doped region determines many of the properties of the junction.
- We see that a high-resistivity region should be used for at least one side of the junction to increase the breakdown voltage Vbr.
- However, this approach tends to increase the forward resistance R of Fig.c, and therefore contributes to the problems of thermal effects due to I2R heating.
- To reduce the resistance of the lightly doped region, it is necessary to make its area large and reduce its length.