Pn Diode

In Chapter 5.4, optical ultraviolet radiation sensors are described, including UV-enhanced silicon-based pn diodes, detectors made from other wide band gap materials in crystalline or polycrystalline form, the latter being a new, less costly alternative. Other domestic applications are personal UV exposure dosimetry, surveillance of sun beds, flame scanning in gas and oil burners, fire alarm monitors and water sterilization equipment surveillance. 

Figure 17 Transient current induced in SOI pn junction diode by 15-MeV C-ion irradiation (solid line). The result obtained for bulk Si pn diode is also shown as a dotted line in the figure for comparison. The reverse bias of 10 V was applied to pn diodes during measurements.

The total current in the emitter IE, not too far from the emitter-to-base junction, consists of two contributions the electron current (drift current) 1 e, which proceeds under forward bias toward the emitter-to-base junction, and the much smaller hole current (a diffusion current), which originates in the base and proceeds in the opposite direction, but decays exponentially, as the distance from the junction increases. The total current in the collector, close to the base-to-collector junction, consists of electrons l c (a large fraction of 1 e) that have somehow evaded capture within the base and proceed against reverse bias in the collector region. The rest of the electron current in the collector is what in pn diodes is called reverse saturation current frs, and here it is called collector current with zero emitter current Iqo = hs- l c is the "useful" electron flow in the transistor. 

Electron-hole recombination and light emission from a pn diode under forward bias. Adapted from Tang and Van Slyke [14]. 

Under biases where Vos < we are in cutoff and no current flows (in a silicon FET, because of the back-to-back pn diodes, in an OFET because of no available mobile carriers). 

Chazalviel JN, Ozanam F (1992) Mechanism of electron injection during the anodie oxidation of silicon. In Fauchet PM, Tsai CC, Canham LT, Shimizu I, Aoyagi Y (eds) Mieroerystalline Semiconductors Materials Science Devices. Mater Res Soc Symp Proc 283 359-364 Chen ZL, Bosman G, Ochoa R (1993) Visible-light emission from heavily doped porous silicon homojunction pn diodes. Appl Phys Lett 62(7) 708-710... 

As mentioned on page 141, a diode is an electronic device having two terminals. There are many types, but the most common one is the PN diode. This can "rectify" ac, by only allowing current to flow in one direction, thus converting the ac into dc. 

If an external voltage, from a battery for example, is put across a PN diode so the P type material is made positive and the N-type negative, then current will flow. 

Fig. 14.1 A PN diode, "forward biased," with current flowing. Black dots are electrons, and circles are holes. (A "bias" is a small voltage applied to a device, tending to make a current flow in a certain direction.)...

Fig. 14.2 A "reverse biased" PN diode, with nearly zero current flowing.

It should be noted that there is an appreciable forward voltage drop, about 0.6 volts when the current is around 1 ma. This is typical for silicon PN diodes, but diodes made from the element germanium can be made with "forward voltages" of about 0.3 volts. Completely different kinds of diode, not involving PN junctions can be made, such as Schottky diodes, and some have even lower forward voltages. (The nonlinearity at very low current is "crossover distortion.")... 

Another type of avalanche diode is the "diac." This also has a characteristic curve that is the same in both directions, because it is a symmetrical design with a PNP structure. It can be thought of as two PN diodes, back to back. Each one is designed to be able to avalanche at about 1 ma of current without being damaged. Breakdown can be as low as about 6 volts, and this goes lower after the avalanche has started, even more so than in the neon bulb. We will make use of this device in Chapter 21. As an optional experiment, it can be inserted into the curve tracer to give a curve like that in Fig. 14.4. 

On pages 110 and 138, capacitors were used as snubbers, to absorb the inductive kick from a quickly tumed-off inductor. Another way to do this is to attach a PN diode, not across the switch as on page 138, but across the coil, as in Fig. 14.10. 

CL, Cathodoluminescence PL, photoluminescence n, injection luminescence of pn diodes PC, CL of pressure-treated crystals (pressure cubic, from Ref. 196) RC (RK), CL of electron (ion)-irradiated crystals (radiation cubic, from Ref. 193) GC, CL of common crystals (genanl cubic, from Ref. 187). 

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