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مُساهمةموضوع: diode characteristic   الثلاثاء 28 أبريل 2009, 1:00 pm

Electronic Devices and Circuits
Engineering Sciences 154

Diode i-v Characteristics



Some preliminaries


      "The Law of the Junction"

      pn(xn) = pn0 exp(VA/VT) and np(xp) = np0 exp(VA/VT)



Diffusion currents components

Jn = Dn dn/dx and Jp = -Dp dp/dx
Drift currents components

Jn = q n mn E and Jp = q p mp E
Einstein relation
Dn/mn = Dp/mp = VT
"Mass Action Law"

pn0 nn0 = pp0 np0 = pi ni = ni2= B T3 exp(-EG/VT)
Shockley equation for the diode i-v characteristic

I(VA) = A q ni2{Dp/Lp Nd + Dn/Ln Na}[ exp(VA/VT) - 1]

I(VA) = Isat [ exp(VA/VT) - 1]







Working model of real silicon diode










Semilog version of the "Shockley" ("ideal") current-voltage characteristic



See supplimentary discussion of reverse bias diode breakdown






Current-voltage characteristic of a "real" silicon diode.
(a) Generation-recombination region. (b) "Ideal" or Shockley diffusion current region. (c) High-injection current region. (d) Series resistance effect. (f) Reverse leakage current due to generation-recombination and surface effects. (After S. M. Sze, Physics of Semiconductor Devices)
See supplimentary discussion of reverse bias diode breakdown




Diode Characteristic


This forward-bias voltage drop exhibited by the diode is due to the action of the depletion region formed by the P-N junction under the influence of an applied voltage. When there is no voltage applied across a semiconductor diode, a thin depletion region exists around the region of the P-N junction, preventing current through it. The depletion region is for the most part devoid of available charge carriers and so acts as an insulator:

If a reverse-biasing voltage is applied across the P-N junction, this depletion region expands, further resisting any current through it:

Conversely, if a forward-biasing voltage is applied across the P-N junction, the depletion region will collapse and become thinner, so that the diode becomes less resistive to current through it. In order for a sustained current to go through the diode, though, the depletion region must be fully collapsed by the applied voltage. This takes a certain minimum voltage to accomplish, called the forward voltage:

For silicon diodes, the typical forward voltage is 0.7 volts, nominal. For germanium diodes, the forward voltage is only 0.3 volts. The chemical constituency of the P-N junction comprising the diode accounts for its nominal forward voltage figure, which is why silicon and germanium diodes have such different forward voltages. Forward voltage drop remains approximately equal for a wide range of diode currents, meaning that diode voltage drop not like that of a resistor or even a normal (closed) switch. For most purposes of circuit analysis, it may be assumed that the voltage drop across a conducting diode remains constant at the nominal figure and is not related to the amount of current going through it.
In actuality, things are more complex than this. There is an equation describing the exact current through a diode, given the voltage dropped across the junction, the temperature of the junction, and several physical constants. It is commonly known as the diode equation:
where,
ID ... diode current
Is ... saturation current
e .... Euler's constant (2.71828...)
q .... charge of electron (1.610-19 As)
V .... voltage across the diode
N .... "non-ideality" coefficient (typ. between 1 and 2)
k .... Boltzmann's constant (1.3810-23)
T .... junction temperature in Kelvin
The equation kT/q describes the voltage produced within the P-N junction due to the action of temperature, and is called the thermal voltage, or Vt of the junction. At room temperature, this is about 26 millivolts. Knowing this, and assuming a "nonideality" coefficient of 1, we may simplify the diode equation and re-write it as such:

You need not be familiar with the "diode equation" in order to analyze simple diode circuits. Just understand that the voltage dropped across a current-conducting diode does change with the amount of current going through it, but that this change is fairly small over a wide range of currents. This is why many textbooks simply say the voltage drop across a conducting, semiconductor diode remains constant at 0.7 volts for silicon and 0.3 volts for germanium. However, some circuits intentionally make use of the P-N junction's inherent exponential current/voltage relationship and thus can only be understood in the context of this equation. Also, since temperature is a factor in the diode equation, a forward-biased P-N junction may also be used as a temperature-sensing device, and thus can only be understood if one has a conceptual grasp on this mathematical relationship.
A reverse-biased diode prevents current from going through it, due to the expanded depletion region. In actuality, a very small amount of current can and does go through a reverse-biased diode, called the leakage current, but it can be ignored for most purposes. The ability of a diode to withstand reverse-bias voltages is limited, like it is for any insulating substance or device. If the applied reverse-bias voltage becomes too great, the diode will experience a condition known as breakdown, which is usually destructive. A diode's maximum reverse-bias voltage rating is known as the Peak Inverse Voltage, or PIV, and may be obtained from the manufacturer. Like forward voltage, the PIV rating of a diode varies with temperature, except that PIV increases with increased temperature and decreases as the diode becomes cooler -- exactly opposite that of forward voltage.

Typically, the PIV rating of a generic "rectifier" diode is at least 50 volts at room temperature. Diodes with PIV ratings in the many thousands of volts are available for modest prices.

  • REVIEW:
  • A diode is an electrical component acting as a one-way valve for current.
  • When voltage is applied across a diode in such a way that the diode allows current, the diode is said to be forward-biased.
  • When voltage is applied across a diode in such a way that the diode prohibits current, the diode is said to be reverse-biased.
  • The voltage dropped across a conducting, forward-biased diode is called the forward voltage. Forward voltage for a diode varies only slightly for changes in forward current and temperature, and is fixed principally by the chemical composition of the P-N junction.
  • Silicon diodes have a forward voltage of approximately 0.7 volts.
  • Germanium diodes have a forward voltage of approximately 0.3 volts.
  • The maximum reverse-bias voltage that a diode can withstand without "breaking down" is called the Peak Inverse Voltage, or PIV rating.



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مُساهمةموضوع: رد: diode characteristic   الثلاثاء 28 أبريل 2009, 2:08 pm

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مُساهمةموضوع: رد: diode characteristic   الخميس 18 يونيو 2009, 11:55 am

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مُساهمةموضوع: رد: diode characteristic   الخميس 18 يونيو 2009, 12:42 pm

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مُساهمةموضوع: رد: diode characteristic   الخميس 18 يونيو 2009, 1:24 pm

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