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Electric Field in Capacitors and Diodes

  1. Mar 24, 2015 #1
    When a capacitor is connected to a voltage source, electrons will move from one plate and accumulate in the other in accordance to Q=CV. Due to the imbalance of charges (positive and negative ions) between the plates, an electric field exists that flow in the direction of the more negative plate. If we assume an ideal capacitor (no leakage resistor), the electrons at the negative plate will not flow across the electric field into the positive plate.
    When a p and n semiconductor materials are chemically combined together, initially free electrons in the n material diffuse across the junction into the n material. Without prolonging the story, positive ions in the n material and negative ions in the p material around the junction form an electric field in the direction of the p material. This electric field prevents less energetic electrons in the n material from flowing across junction. For this reason, the negative terminal of a power supply must be connected to the n material to allow current through the diode.
    My question
    Now, lets see the similarities between the capacitor and diode.
    Capacitor -- Diode
    +ve plate -- n material
    -ve plate -- p material
    dielectric -- pn junction
    The electric field points from the n (or positive plate of the capacitor) to the p (or negative plate of the capacitor).
    In the case of the diode, the p material has minority carriers (few electrons) and these can easily flow across the electric field into the n material. This constitutes the so-called leakage current.
    In the case of the capacitor, one should expect something similar, that the electrons on the negative plate should flow across the field because the field is oriented in the same direction as that in the diode. But according to all what I know about capacitors, electrons do not flow across the E field. The question is why not? Why electrons can flow across the E field in a diode (from p to n) and cannot in a capacitor?
  2. jcsd
  3. Mar 24, 2015 #2


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    The PN junction of a diode is a semiconductor. The dielectric of a capacitor is an insulator.
  4. Mar 24, 2015 #3
    But why is the pn junction called the depletion region. Doesn't that mean there are no carriers available which implies that pn junction is behaving like an insulator ? An insulator should be a material with no free carriers.
  5. Mar 25, 2015 #4


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