4.2.2 The PN Diode as a Circuit Element

Chapter 4.2.2 The PN Diode as a Circuit Element

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

  • CHAPTER 4. DIODE CIRCUITS67a particular diode. However, for a real diode, other factors are also important: in particular,edge effects around the border of the junction cause the actual reverse current to increaseslightly with reverse voltage, and the finite conductivity of the doped semi-conductor ulti-mately restricts the forward current to a linear increase with increasing applied voltage. Abetter current-voltage curve for the real diode is shown in the figure 4.3b.a)IVI0(eV/VT−1)b)IVslope = 1/Rrslope = 1/RfVPNFigure 4.3: Current versus voltage a) in the PN junction region and b) for an actual PNdiode.Various regions of the curve can be identified: the linear region of forward-biasing, anon-linear transition region, a turn-on voltage (VPN ) and a reverse-biased region. We canassign a dynamic resistance to the diode in each of the linear regions: Rf in the forward-biased region and Rr in the reverse-biased region. These resistances are defined as the inverseslope of the curve: 1/R =∆I/∆V . The voltage VPN , represents the effective voltage dropacross a forward-biased PN junction (the turn-on voltage). For a germanium diode, VPN isapproximately 0.3 V, while for a silicon diode it is close to 0.6 V.4.2.2The PN Diode as a Circuit ElementDiodes are referred to as non-linear circuit elements because of the above characteristiccurve. For most applications the non-linear region can be avoided and the device can bemodeled by piece-wise linear circuit elements. Qualitatively we can just think of an idealdiode has having two regions: a conduction region of zero resistance and an infinite resistancenon-conduction region. For many circuit applications, this ideal diode model is an adequaterepresentation of an actual diode and simply requires that the circuit analysis be separatedinto two parts: forward current and reverse current. Figure 4.4 shows a schematic symbolfor a diode and the current-voltage curve for an ideal diode.A diode can more accurately be described using the equivalent circuit model shown infigure 4.5. If a diode is forward biased with a high voltage it acts like a resistor (Rf ) in serieswith a voltage source (VPN ). For reverse biasing it acts simply as a resistor (Rr). These