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The Capacitances of a Semiconductor Diode

Electrons in Solids | Intrinsic Conduction | Extrinsic Conduction | Structures of Semiconductor Diodes | The Tunnel and Inversed Diodes | Microwave Semiconductor Diodes | Physical Processes in a Transistor | The Basic Circuit Configurations of Transistors | Models of Transistors | Bias Supply and Temperature Compensation for Transistors |


Читайте также:
  1. Barriers, defects, emission, diodes and noise
  2. Compound Semiconductor Heterostructures
  3. Diffusion Currents in Semiconductors
  4. Early Semiconductor Lasers and Light-Emitting Diodes
  5. Light-Emitting Diodes
  6. Microwave Semiconductor Diodes
  7. Schottky diode

The depletion layer has a high resistance and acts as a dielectric on each side of which there are two unlike space charges, + Q R and — Q R, produced by the ionized atoms of the donor and acceptor impurities. Therefore, a p-n junction has a capacitance similar to that of a two-plate flat capacitor. It is called the junction capacitance. In the case of a. c. voltage, it is given by:

Cj = ∆QR /∆UR = , (3.2)

where Cj0 – the equilibrant value of junction capacitance and 0.3< n ≤0.5.

When the reverse voltage is raised, the depletion layer broadens, and Cj decreases. The manner in which Cj varies as a function of UR is shown by the plot in Fig. 3.2.

Fig. 3.2. Junction capacitance as a function of reverse bias voltage

When forward-biased, a p-n junction has what is known as the diffusion capacitance, Cdif, in addition to the junction capacitance. For each value of for­ward voltage there is a certain value for the two equal but unlike space charges, + Qdif and – Qdif, stored in the n- and p -regions owing to the diffusion of carriers across the junction. In the case of a. c. voltage, Cdif is the ratio of charge to potential difference:

Cdif = ∆Qdif / ∆UF = , (3.3)

 
 

where τ – a transition time for minority carrier from base.

The diffusion capacitance of a p-n junction is appreciably greater than its barrier capacitance, but there is no way of putting it to any use because it is shunted by the low forward resis­tance of the diode itself.


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