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Unformatted text preview: Nanyang Technological University School of Physical and Mathematical Sciences Division of Physics and Applied Physics PAP 119 Physics Lab Ib Experiment 3: Semiconductor Diode Background A diode is the simplest type of semiconductor device. Of the many di ff erent types of diodes, the most commonly used diode consists of a PN junction, where an n-type semiconductor and a p-type semiconductor are in electrical contact. It is a two-terminal device (see Figure 1), with an anode ( + ) and a cathode ( − ), and is usually identified as 1NXXXX. Figure 1: Photograph (left) and symbol (right) for a diode. A diode allows current to ﬂow in only one direction, from anode to cathode. When a forward voltage V > V d is established across a diode as shown in Figure 2(a) the junction presents a very low resistance path and a large current ﬂows across the junction. The diode is said to be forward-biased. On the other hand, if the applied voltage across the junction is reversed, as shown in Figure 2(b), no current ﬂows through the diode. The diode is said to be reverse-biased. (a) (b) (c) Figure 2: The diode in (a) forward bias and (b) reverse bias. Also shown in (c) is the I- V characteristic of an ideal diode. Figure 2(c) shows the I- V characteristic of an ideal diode. For an ideal diode, an infinite current will ﬂow as soon as it is forward-biased. The diode is said to be turned on. In practice, when the 1 forward voltage is non-zero, a small diode current starts to ﬂow. The current rises exponentially when the forward-biased voltage exceeds a certain threshold voltage V d . This threshold is about....
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