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Electric field p n
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When the pn junction is forward-biased, the contact potential across the junction is lowered (note that VB acts in opposition to the contact potential) Now, the diffusion of majority carriers is aided by the external voltage source; in fact, the diffusion current increases as a function of the applied voltage, according to equation 85 Id = I0 eqvD /kT (85)
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+ vD _
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where vD is the voltage across the pn junction, k = 1381 10 23 J/K is Boltzmann s constant, q the charge of one electron, and T the temperature of the material in kelvins (K) The quantity kT /q is constant at a given temperature and is approximately equal to 25 mV at room temperature The net diode current under forward bias is given by equation 86 iD = Id I0 = I0 (eqvD /kT 1)
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which is known as the diode equation Figure 88 depicts the diode i-v characteristic described by the diode equation for a fairly typical silicon diode for positive diode voltages Since the reverse saturation current, I0 , is typically very small (10 9 to 10 15 A), equation 87: iD = I0 eqvD /kT (87)
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is a good approximation if the diode voltage, vD , is greater than a few tenths of a volt The ability of the pn junction to essentially conduct current in only one direction that is, to conduct only when the junction is forward-biased makes it
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Drift current Diffusion current Depletion region
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Figure 86 Drift and diffusion currents in a pn junction
+ vD _
p n iD = _I0 (a) Reverse-biased pn junction
_ VB +
p n iD = Id _I0 Id (b) Forward-biased pn junction
+ VB _
Diode equation
(86)
Figure 87 Forwardand reverse-biased pn junctions
8
Semiconductors and Diodes
50 Diode current, mA 40 30 20 10 0 0 02
Diode i-v curve
04 06 Diode voltage, V
Figure 88 Semiconductor diode i-v characteristic
The arrow in the circuit symbol for the diode indicates the direction of current flow when the diode is forward-biased iD + vD _
Circuit symbol
p Ohmic contacts n
pn junction
Figure 89 Semiconductor diode circuit symbol
valuable in circuit applications A device having a single pn junction and ohmic contacts at its terminals, as described in the preceding paragraphs, is called a semiconductor diode, or simply diode As will be shown later in this chapter, it nds use in many practical circuits The circuit symbol for the diode is shown in Figure 89, along with a sketch of the pn junction Figure 810 summarizes the behavior of the semiconductor diode by means of its i-v characteristic; it will become apparent later that this i-v characteristic plays an important role in constructing circuit models for the diode Note that a third region appears in the diode i-v curve that has not been discussed yet The reversebreakdown region to the far left of the curve represents the behavior of the diode when a suf ciently high reverse bias is applied Under such a large reverse bias (greater in magnitude than the voltage VZ , a quantity that will be explained shortly), the diode conducts current again, this time in the reverse direction To explain the mechanism of reverse conduction, one needs to visualize the phenomenon of avalanche breakdown When a very large negative bias is applied to the pn junction, suf cient energy is imparted to charge carriers that reverse current can ow, well beyond the normal reverse saturation current In addition, because of the large electric eld, electrons are energized to such levels that if they collide with other charge carriers at a lower energy level, some of their energy is transferred to the carriers with lower energy, and these can now contribute to the reverse conduction process, as well This process is called impact ionization Now, these new carriers may also have enough energy to energize other low-energy electrons by impact ionization, so that once a suf ciently high reverse bias is provided, this process of conduction takes place very much like an avalanche: a single electron can ionize several others The phenomenon of Zener breakdown is related to avalanche breakdown It is usually achieved by means of heavily doped regions in the neighborhood of the metal-semiconductor junction (the ohmic contact) The high density of charge carriers provides the means for a substantial reverse breakdown current to be sustained, at a nearly constant reverse bias, the Zener voltage, VZ This phenomenon is very useful in applications where one would like to hold some load voltage constant for example, in voltage regulators, which are discussed in a later section To summarize the behavior of the semiconductor diode, it is useful to refer to the sketch of Figure 810, observing that when the voltage across the diode, vD , is greater than the offset voltage, V , the diode is said to be forward-biased and
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