4.2.1 Current in the Diode

Chapter 4.2.1 Current in the Diode

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • CHAPTER 4. DIODE CIRCUITS66Initially both semiconductors are totally neutral.The concentration of positive andnegative carriers are quite different on opposite sides of the junction and the thermal energy-powered diffusion of positive carriers into the N-type material and negative carriers into theP-type material occurs. The N-type material acquires an excess of positive charge nearthe junction and the P-type material acquires an excess of negative charge.Eventuallydiffuse charges build up and an electric field is created which drives the minority chargesand eventually equilibrium is reached. A region develops at the junction called the depletionregion. This region is essentially un-doped or just intrinsic silicon.To complete the diode conductor, leads are placed at the ends of the PN junction.4.2.1Current in the DiodeThe behaviour of a diode depends on its polarity in the circuit (figure 4.2). If the diode isreverse biased (positive potential on N-type material) the depletion region increases. Theonly charge carriers able to support a net current across the PN junction are the minoritycarriers and hence the reverse current is very small. A forward-biased diode (positive poten-tial on P-type material) has a decreased depletion region; the majority carriers can diffuseacross the junction. The voltage may become high enough to eliminate the depletion regionentirely.PNa)+I rPNb)+fIFigure 4.2: Diode circuit connections: a) reversed biased and b) forward biased.An approximation to the current in the PN junction region is given by (shown in fig-ure 4.3a)I = I0(eV/VT− 1),(4.1)where both I0 and VT are temperature dependent. This equation gives a reasonably accurateprediction of the current-voltage relationship of the PN junction itself – especially the tem-perature variation – and can be improved somewhat by choosing I0 and VT empirically to fit