Junction capacitance

Chapter Junction capacitance

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • are pulled towards the positive charge, away from the junction. In the P-type material,holes are pulled toward the negative charge, also away from the junction. The electrons(in the N-type material) and holes (in the P type) are the majority charge carriers. Theybecome depleted in the vicinity of the P-N junction (Fig. 19-5B). A shortage of majoritycarriers means that the semiconductor material cannot conduct well. Thus, the deple-tion region acts like an insulator.Junction capacitanceSome P-N junctions can alternate between conduction (in forward bias) andnonconduction (in reverse bias) millions or billions of times per second. Other junctionsare slower. The main limiting factor is the capacitance at the P-N junction during con-ditions of reverse bias. The amount of capacitance depends on several factors, includ-ing the operating voltage, the type of semiconductor material, and the cross-sectionalarea of the P-N junction.By examining Fig. 19-5B, you should notice that the depletion region, sandwichedbetween two semiconducting sections, resembles the dielectric of a capacitor. In fact,the similarity is such that a reverse-biased P-N junction really is a capacitor. Some semi-conductor components are made with this property specifically in mind.The junction capacitance can be varied by changing the reverse-bias voltage, be-cause this voltage affects the width of the depletion region. The greater the reversevoltage, the wider the depletion region gets, and the smaller the capacitance becomes.In the next chapter, you’ll learn how engineers take advantage of this effect.Avalanche effectThe greater the reverse bias voltage, the “more determined an insulator” a P-N junctiongets—to a point. If the reverse bias goes past this critical value, the voltage overcomesthe ability of the junction to prevent the flow of current, and the junction conducts as ifit were forward biased. This avalanche effect does not ruin the junction (unless thevoltage is extreme); it’s a temporary thing. When the voltage drops back below the crit-ical value, the junction behaves normally again.Some components are designed to take advantage of the avalanche effect. In othercases, avalanche effect limits the performance of a circuit.In a device designed for voltage regulation, called a Zener diode, you’ll hear aboutthe avalanche voltage or Zener voltage specification. This might range from a coupleof volts to well over 100 V. It’s important in the design of voltage-regulating circuits insolid-state power supplies; this is discussed in the next chapter.For rectifier diodes in power supplies, you’ll hear about the peak inverse voltage(PIV) or peak reverse voltage (PRV) specification. It’s important that rectifier diodeshave PIV great enough so that avalanche effect will not occur (or even come close tohappening) during any part of the ac cycle. Otherwise, the circuit efficiency will becompromised.366 Introduction to semiconductors