Diode in reverse bias
pp 37-42
A PN junction diode is reverse biased during the time when the positive pole of the external source is connected to the N part of the diode and the negative pole to Section P, as shown in Figure 3.2.5 (a). After conducting connection, the positive charges from the positive pole of the source into the N part as holes and recombine with electrons present in high concentration there. By the same mechanism, electrons from the negative pole of the source entering the part P and recombine with the holes are there in high concentration.
With those reunions stripping increases the area because it creates more "stripped" positive and negative ions (Fig. 3.2.5 (b)).
Some electrons have enough kinetic energy as derived by splitting the bonds between the atoms of the semiconductor, and overcome the new area and thus stripping the circuit current I0 is called reverse saturation current is very small (of the order of MA). The current can be increased by providing external energy such as heat, electricity or light. Therefore:
3.2.2 Polarization in reverse time
Figure 3.2.5 (,)
Reverse biasing PN diode (a) V <V0 (v) V'0> V0.
In the case of reverse bias potential new barrier is:
As a consequence of increasing the potential barrier and increasing the length of the stripping area (L,> L), reduce the capacity barrier CT getting a new value:
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where S = surface area stripper.
The resistance of the diode in reverse bias is the ideal diode Rr = ¥, while trade routes takes very large values (some MO).
The connection of the diode in the circuit is equivalent to an open circuit <Resistance great honor.
Ideal diode L.
Non-ideal diode
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KPYITAMOAIOAOI
3.3 Characteristic curve and load line 3.3.1 PN diode characteristic curve.
As mentioned previously, in the correct polarity of the diode by increasing the voltage will be in principle a small current and voltage after knee BA a big increase in power. In contrast, in reverse time there will be a very small current, almost constant, which will increase the minimum rate until the voltage reaches a certain price, called the breakdown voltage or Zener, so the current increases sharply. So there are three areas in a typical diode IV curve: the area of good polarization, the polarization reversed region and the region division. Figure (3.3.1) seem typical characteristic of silicon and germanium.
Figure 3.3.1.
Characteristic
curves (I - V) Si and Ge.
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In the vertical axis present current ID in mA, while the horizontal trend in VD Volt. At every point A of the characteristic, in the correct polarization, can be defined as dynamic resistance RF, ac as the quotient of a small change in voltage from the point A to the corresponding change of current. These changes are the vertical sides of a right triangle with hypotenuse tangent to the curve at the given point A (Fig. 3.3.2). Also specifies the static resistance RF, dc as the quotient of the voltage at point A on the same stream.
3.3.1
Figure 3.3.2.
Dynamic and static wear resistance Good
Figure 3.3.3.
Resistance reverse direction.
Similarly defined graphics and resistance reverse direction or polarization RR as the quotient of the change in voltage at point B, the reverse polarity to the corresponding change of flow (Figure 3.3.3):
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KRYSTLLLODIODOI
where q is the electron charge of the constant K Boltsman, T the absolute
temperature coefficient idanikotitas n, n = 1 for an ideal diode and
n = 2 to poor diode. For real diodes 1 <n <2.
At ambient temperature Q = 25 ° CH T = 298 ° since then the relation (3.3.3)
simplifies and becomes (for n = 1)
3.3.4
The approximate curve and in the proper polarization is:
3.3.6
that has slope T / Rp while in reverse bias is:
3.3.7
Free reading:
The mathematical relationship of the above characteristic, without splitting the region, given by an exponential function:
3.3.3
where l0 is the reverse saturation current of VD, ID is the voltage across and current of the diode and e is the base of the logarithm neperion.
The correct polarity of the characteristic can be approximated by segment. Several times, mainly used for solving exercises, the approximate characteristic curve that occurs if that part extend them to a turning point until tmisei axis of trends. The intersection gives the knee voltage Vr So the characteristic consists of two segments in the correct polarity as shown in Figure 3.3.4. The area consists of the reverse direction by two segments: Power:
3.3.5
Figure 3.3.4.
Approximate curve diode.
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IA 30mA 3 x 10r2A
Example 3.3.1:
Give the characteristic curve of a diode. To obtain the static resistance at point A and the dynamic resistance at point B.
ln (mA)
Solution
The static resistance at point A is:
Figure 3.3.5.
The diode characteristic curve of the example
The dynamic resistance at point B is:
(60-40) mA 20 mA 2 x 10r2A = 5 Z
3.3.2 Straight burden
In paragraph 3.2.1 stated that to resolve a circuit containing a diode proper polarization, using the equivalent circuit. But there is a second way to solve this circuit, which is not graphic and computing, and is as follows:
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In a circuit of a diode D connected in correct polarity of direct current source to V | and load resistor RL. Emphasis is also the characteristic of the diode (Figure 3.3.6).
(A) (b)
Figure 3.3.6. Graphic resolution. (A) Circuit (b) A Straight burden
If VD is the voltage across the diode current and lD circuit, applying the rule Kirchnoff trends will be:
V; = VD + VL where VL = ID RL
3.3.8
Replacing it will be:
3.3.9
Equation (3.3.8) is a linear equation of first instance and represents the change in voltage across the diode VD as a function of change of current ID. Schematically this equation is represented by a straight line, called the load line (Figure 3.3.6v).
To find the intersection of the straight line with the burden of stress should be put in relation (3.3.9), ID = 0 so:
Vd = V | - 0 RL = V;
3.3.10
That is, the intersection of the line load on the horizontal axis
trends is the point where the voltage is equal to the voltage source.
