Diode in forward bias
pp 30-36
In this chapter we discuss the structure and function of transistors. To the end
chapter, the student must:
• Learn and understand the mechanism of diffusion of holes and electrons in a contact PN.
• Designs the region Strippers
• Connect the diode to correct and reverse polarity and
designs the characteristic curve,
• Knows the function and connectivity of Varicap diode and diode Zener,
• Uses Zener diode circuits, Voltage,
• Applies the diode PN production imianorthomenis and rectified voltage, and a clamp circuit, voltage multiplication etc.
3.1 Contact / PN diode 3.1.1 Physical Function
As mentioned in the previous chapter, semiconductors with impurities are of two types. The N-type semiconductors have more negative entities, ie they have an excess of electrons and are therefore called Type N (negative). Instead, the P-type semiconductor (positive) have an excess of positive carriers or holes. The holes are lack of electrons. Several times in the literature doped semiconductors are referred to as type and p-type n.
When small piece N type semiconductor make contact with P-type semiconductor piece, then creates a compound PN or PN contact which is a very useful electronic component called PN diode. These pieces of semiconductor may not be different, but parts of the same piece of silicon crystal where one side is created with appropriate P type, the other type P.
The contact PN is shown in Figure 3.1.1. The point of the compound represented by a vertical dashed line. The department consists of N-type positive ions pentavalent element and free electrons. There is also a small number of holes. In section P-type negative ions are trivalent element, several holes and a small number of electrons.
C = holes = electrons
= Positive ions (-) = negative ions
Figure 3.1.1. Contact PN
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At the time of its creation PN contact, electrons from the N-type semiconductor lying near the compound will move towards the p-type semiconductor in order to reconnect with the holes there. This creates reconnection holes and electrons in two parts, the right and left of the contact point and the N-type semiconductor while creating a section with only positive ions without electrons in the p-type semiconductor it creates a part with negative ions only, without holes. These two sections are "stripped" from their bodies and together constitute the stripping area as shown in Figure 3.1.2 (a).
Outside the region of stripping the semiconductor structure is not changed and consists of ions and carriers. This is because to be able to reconnect an electron with a hole or vice versa, must be overcome by stripping the region but with the concentration of ions therein, constitutes a barrier and creates a barrier potential. The potential barrier Vo represented in Figure 3.1.2 (b) and a potential difference is the polarity opposes the diffusion operators.
Contact PN created the above way is called PN diode because lets electric current passes from it only in one direction as we shall see below.
(A)
(B)
Figure 3.1.2
(A) Area Stripper (b) Potential barrier
GENERAL ELECTRONICS
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3.1.2 Notation and capacitance diode barrier.
In order to create a diode PN connected the ends of two semiconductor portions P and N type with metal contacts, thus creating the anode of the diode-side semiconductor P and the cathode of diode side P. The diode PN denoted by an arrow in P side and a line side N for reasons to be explained below.
As mentioned above, in stripping are negative and positive charges, consisting of ions, the two segments. These loads combined with the potential barrier also exists at the ends of this region creates a capacitor whose capacitance is given by (3.1.1)
(A)
Metal contact
(B)
Figure 3.1.4.
(A) PN diode (b) symbol.
3.1.1
The potential difference at the ends of the region depends on the stripping external voltage V that may be applied across the diode and the potential barrier Vo according to the relation (3.1.2)
3.1.2
This capacity called capacitance contact or static capacity barrier and depends, as each capacitor by the geometry of the region that the stripping length L, the surface S and the dielectric constant of the semiconductor, e, according to the relation (3.1.3):
3.1.3
The value of the contact capacity CT ranges from 1-100 pF.
3.2 PN diode and accurate reverse polarity
In the previous section the contact PN was open, that there was no external voltage at its ends. When applied external voltage across a diode, there are two ways to connect the source: in the correct and reverse time.
3.2.1 Polarization at the proper time.
A diode is biased in the forward direction if the external source is connected in the circuit to the positive pole of the part to be the passage P and the negative pole of the section S of the diode, as shown in Figure 3.2.1 (a). Known as a power source a large number of electrons on the negative pole and a large number of positive charges to the positive pole. With the conductive connection of the source with the passage at the proper time, the positive charges of the positive pole of the source go to portion P of the passage and the electrons go to the N respectively.
Figure 3.2.1
Forward bias diode PN
Therefore the positive charges will move from the positive terminal of the source to the P part of the passage. H movement that gives the current Ip of the holes within the section P. H movement of electrons from the negative pole of the source to the portion N of the passage will give the electron current In in the portion N, which has the same direction as the current ratio of the holes of negative charge electrons.
In this way the concentration of holes in the portion P grows, the stripping area becomes narrower and some holes with high kinetic energy manage to overcome the barrier potential and to enter the portion N of the passage. The same procedure applies to the electrons in the region N entering in Section F.
(A).
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While the external voltage increases as the region becomes smaller stripping until reset and have current flowing in the circuit, called direct current or forward current or current diffusion lF (forward current) and has been established to have a direction opposite that of the electrons (Fig. 3.2 .1. (b).
The value of the external voltage to be applied in the passage to pass current in the circuit must be larger than the potential barrier which is a few tenths of Volt (V> V0 = 0,1 to 0,5 V and germanium for silicon). The current has a small value to a voltage called threshold voltage <VL knee, after which increases exponentially. The trend knee while germanium is 0,3 V for silicon is not 0,7 V. As for the current of the external circuit is:
where Ic is said reverse current saturation and the current derived from the thermal excitation of the semiconductor and the value of the order of microamperes (uA).
The electro-equivalent Ku-VR is shown in Figure 3.2.2:
The following table shows the trends barrier threshold Figure 3.2.2
and resistance in the diode forward bias when the diode is ideal when trade:
Figure 3.2.2
Equivalent circuit diode p N
Vg = voltage knee
Rf = good wear resistance.
circuit of the diode in forward bias.
Table 3.2.1. Characteristic values Diode
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Example 3.2.1
An ideal silicon diodes connected in forward bias voltage source with Vi = 12 V, and load resistance RL = 1 KO.
a) To design the circuit.
b) Calculate the current flowing through the circuit.
c) Calculate the current when the diode is not ideal and has resistance RF = 200 O
Solution
Figure 3.2.3
b) Because the diode is ideal we have RF = 0, V0 = 0 so we have the diode voltage drop and applying Ohm's law for the resistance load, we have:
c) For non-ideal diode Vg = 0.7 V and Rf = 200 Oh so have the
following equivalent circuit:
Figure 3.2.4
