What is Junction diodes

Chapter 2.7 Junction diodes

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 58CHAPTER 2. SOLID-STATE DEVICE THEORY• A reverse biased PN junction conducts almost no current. The applied reverse bias at-tracts majority carriers away from the junction. This increases the thickness of the non-conducting depletion region.• Reverse biased PN junctions show a temperature dependent reverse leakage current.This is less than a µA in small silicon diodes.2.7Junction diodesThere were some historic crude, but usable semiconductor rectifiers before high purity materi-als were available. Ferdinand Braun invented a lead sulfide, PbS, based point contact rectifierin 1874. Cuprous oxide rectifiers were used as power rectifiers in 1924. The forward voltagedrop is 0.2 V. The linear characteristic curve perhaps is why Cu2O was used as a rectifier forthe AC scale on D’Arsonval based multimeters. This diode is also photosensitive.Selenium oxide rectifiers were used before modern power diode rectifiers became available.These and the Cu2O rectifiers were polycrystalline devices. Photoelectric cells were once madefrom Selenium.Before the modern semiconductor era, an early diode application was as a radio frequencydetector, which recovered audio from a radio signal. The “semiconductor” was a polycrystallinepiece of the mineral galena, lead sulfide, PbS. A pointed metallic wire known as a cat whiskerwas brought in contact with a spot on a crystal within the polycrystalline mineral. (Figure 68,2.31)The operator labored to find a “sensitive” spot on the galena by moving the cat whisker about.Presumably there were P and N-type spots randomly distributed throughout the crystal dueto the variability of uncontrolled impurities. Less often the mineral iron pyrites, fools gold,was used, as was the mineral carborundum, silicon carbide, SiC, another detector, part of afoxhole radio, consisted of a sharpened pencil lead bound to a bent safety pin, touching a rustyblue-blade disposable razor blade. These all required searching for a sensitive spot, easily lostbecause of vibration.Replacing the mineral with an N-doped semiconductor (Figure 68,2.32(a) ) makes the wholesurface sensitive, so that searching for a sensitive spot was no longer required. This device wasperfected by G.W.Pickard in 1906. The pointed metal contact produced a localized P-type regionwithin the semiconductor. The metal point was fixed in place, and the whole point contact diodeencapsulated in a cylindrical body for mechanical and electrical stability. (Figure 68,2.32(d) ) Notethat the cathode bar on the schematic corresponds to the bar on the physical package.Silicon point contact diodes made an important contribution to radar in World War II, de-tecting giga-hertz radio frequency echo signals in the radar receiver. The concept to be madeclear is that the point contact diode preceded the junction diode and modern semiconductorsby several decades. Even to this day, the point contact diode is a practical means of microwavefrequency detection because of its low capacitance. Germanium point contact diodes were oncemore readily available than they are today, being preferred for the lower 0.2 V forward voltagein some applications like self-powered crystal radios. Point contact diodes, though sensitive toa wide bandwidth, have a low current capability compared with junction diodes.Most diodes today are silicon junction diodes. The cross-section in Figure 68,2.32(b) looks abit more complex than a simple PN junction; though, it is still a PN junction. Starting at thecathode connection, the N+ indicates this region is heavily doped, having nothing to do with