11 Semiconductor diodes
11.5 Forward and reverse bias
When an external voltage is applied to a p-n junction mak- ing the p-type material positive with respect to the n-type material, as shown in Figure 11.8, the p-n junction is forward biased. The applied voltage opposes the con- tact potential, and, in effect, closes the depletion layer.
Holes and electrons can now cross the junction and a cur- rent flows. An increase in the applied voltage above that required to narrow the depletion layer (about 0.2 V for germanium and 0.6 V for silicon), results in a rapid rise in the current flow.
When an external voltage is applied to a p-n junction making the p-type material negative with respect to the n- type material as in shown in Figure 11.9, the p-n junction is reverse biased. The applied voltage is now in the same
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Figure 11.8
Figure 11.9
sense as the contact potential and opposes the movement of holes and electrons due to opening up the depletion layer. Thus, in theory, no current flows. However, at nor- mal room temperature certain electrons in the covalent bond lattice acquire sufficient energy from the heat avail- able to leave the lattice, generating mobile electrons and holes. This process is called electron-hole generation by thermal excitation.
The electrons in the p-type material and holes in the n-type material caused by thermal excitation, are called minority carriers and these will be attracted by the applied voltage. Thus, in practice, a small current of a few microamperes for germanium and less than one microam- pere for silicon, at normal room temperature, flows under reverse bias conditions.
Graphs depicting the current-voltage relationship for forward and reverse biased p-n junctions, for both germa- nium and silicon, are shown in Figure 11.10.
Problem 3. Sketch the forward and reverse charac- teristics of a silicon p-n junction diode and describe the shapes of the characteristics drawn.
Reverse current (µA) Germanium
Reverse voltage (V)
−100 −75 −50 −25 Silicon
Forward current (mA)
−3
−2
−1 2 4 6 8 10
0.2 0.4 0.6 0.8 Germanium
Forward voltage (V)
Figure 11.10
A typical characteristic for a silicon p-n junction is shown in Figure 11.10. When the positive terminal of the battery is connected to the p-type material and the negative termi- nal to the n-type material, the diode is forward biased. Due to like charges repelling, the holes in the p-type material drift towards the junction. Similarly the electrons in the n- type material are repelled by the negative bias voltage and also drift towards the junction. The width of the depletion layer and size of the contact potential are reduced. For applied voltages from 0 to about 0.6 V, very little current flows. At about 0.6 V, majority carriers begin to cross the junction in large numbers and current starts to flow. As the applied voltage is raised above 0.6 V, the current increases exponentially (see Figure 11.10).
When the negative terminal of the battery is connected to the p-type material and the positive terminal to the n-type material the diode is reverse biased. The holes in the p-type material are attracted towards the negative ter- minal and the electrons in the n-type material are attracted towards the positive terminal (unlike charges attract). This drift increases the magnitude of both the contact potential and the thickness of the depletion layer, so that only very few majority carriers have sufficient energy to surmount the junction.
The thermally excited minority carriers, however, can cross the junction since it is, in effect, forward biased for these carriers. The movement of minority carriers results in a small constant current flowing. As the magnitude of the reverse voltage is increased a point will be reached where a large current suddenly starts to flow. The voltage
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at which this occurs is called the breakdown voltage. This current is due to two effects:
(i) the zener effect, resulting from the applied volt- age being sufficient to break some of the covalent bonds, and
(ii) the avalanche effect, resulting from the charge car- riers moving at sufficient speed to break covalent bonds by collision.
Problem 4. The forward characteristic of a diode is shown in Figure 11.11. Use the characteristic to determine (a) the current flowing in the diode when a forward voltage of 0.4 V is applied, (b) the voltage dropped across the diode when a forward current of 9 mA is flowing in it, (c) the resistance of the diode when the forward voltage is 0.6 V, and (d) whether the diode is a Ge or Si type.
(b)
(c)
(a)
0
0.0 0.2 0.4 0.6 0.8
2 4 6 8 10
Forward voltage (V)
Forward current (mA)
0.4V 0.67V
1.9 mA 6 mA 9 mA
Figure 11.11
(a) From Figure 11.11, when V=0.4 V, current flowing, I=1.9 mA
(b) When I=9 mA, the voltage dropped across the diode, V=0.67 V
(c) From the graph, when V=0.6 V, I=6 mA.
Thus, resistance of the diode, R=V
I = 0.6 6×10−3
=0.1×103=100
(d) The onset of conduction occurs at approximately 0.2 V. This suggests that the diode is a Ge type.
Problem 5. Corresponding readings of current, I, and voltage, V , for a semiconductor device are given in the table:
Vf(V) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
If(mA) 0 0 0 0 0 1 9 24 50
Plot the I/V characteristic for the device and identify the type of device.
The I/V characteristic is shown in Figure 11.12. Since the device begins to conduct when a potential of approxi- mately 0.6 V is applied to it we can infer that the semi- conductor material is silicon rather than germanium.
Forward voltage (V)
Forward current (mA)
40 35 mA
(b)
(a) 30
20 16 mA 10
0
0.0 0.2 0.4 0.6
0.65 V 0.76 V 50
0.8
Figure 11.12
Problem 6. For the characteristic of Figure 11.12, determine for the device (a) the forward current when the forward voltage is 0.65 V, and (b) the forward voltage when the forward current is 35 mA.
(a) From Figure 11.12, when the forward voltage is 0.65 V, the forward current=16 mA
(b) When the forward current is 35 mA, the forward voltage=0.76 V
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Now try the following exercise.
Exercise 36 Further problems on semiconductor