What is P-N junction – Definition, Explanation, Types – eSaral

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P-N junctions are formed by diffusing trivalent impurity to one-half side and pentavalent impurity to another side. If you want to know what is P-N junction then keep reading this article.

P-N Junction

  • By merely Joining the two pieces a P-N Junction cannot be formed.
  • P-N junctions are formed by diffusing trivalent impurity to one-half side and pentavalent impurity to another side.
  • The plane dividing the two zones is known as a junction.
  • P-N junction is unohmic
  • As P-type semiconductor has a high concentration of holes and N-type semiconductor has a high concentration of free electrons. there is a tendency of holes to diffuse over to the N side and electron to the P side.
  • When the hole diffuses from the P to N side then this will neutralize with free-electron similarly when electron diffuse from the N to P side it will neutralize with the hole. So, a depletion layer is formed near the Jn.What is P-N junction

Depletion layer:

There is a barrier near a junction that opposes the flow of charge carrier is known as depletion layer width of the depletion layer is in micrometer order.

Potential Barrier:

Potential developed in depletion layer is called P.B.

P-side is at lower potential and N-side is at higher potential.

P.B. for Ge  0.3 volt

P.B. for Si  0.7 volt

Electric field:

Electric field due to P.B.


For Ge


$=3 \times 10^{5} \mathrm{~V} / \mathrm{m}$


$\mathrm{E} \approx 10^{5} \mathrm{~V} / \mathrm{m}$

the direction of E due to P.B. N to P-side

P-N Junction with forward bias :

When the Positive terminal of a battery is connected to the P side and the negative terminal to the N side. Then PN Junction is in forwarding Bias

Forward bias reduces the potential barrier. More charge carriers diffuse across the junction.
What is P-N junction
Special Point:
  • Potential barrier reduces
  • Width of the depletion layer decreases
  • P-N junction offers low resistance in forwarding bias.
  • Forward current flow in a circuit
  • The forward characteristic curve is shown in the figure. Forward Bias
  • Forward dynamic resistance $r_{f}=\frac{\Delta V_{f}}{\Delta I_{f}} \cong 100 \Omega$
  • Knee or cut in voltageGe  0.3 VSi  0.7 V
  • Dependence of forward current on bias voltage $\mathrm{I}=\mathrm{I}_{0}\left[\mathrm{e}^{\frac{\mathrm{qV}}{\mathrm{kT}}}-1\right]$$\mathrm{e}^{\frac{\mathrm{qV}}{\mathrm{kT}}}>>1$

    $I \approx I_{0} e^{\frac{+q V}{k T}}$ (Approximate exponential growth)

    I = Forward current

    $\mathrm{I}_{0}$ = reverse saturation current

    k = Boltzman constant

    q = charge of electron

    V = forward voltage

    T = temperature

P-N junction with reverse bias:

When the positive terminal of a battery is connected to the N-side and the negative terminal is connected to the P-side. Holes in the P-region are attracted towards the negative terminal and the electrons in the N-region are attracted towards the positive terminal.
P-N junction with reverse bias
Special Point:
  • The depletion layer increases for reverse biased.
  • Potential barrier increases
  • The reverse characteristic curve is shown in figure reverse bias
  • Very little current called reverse saturation current flows due to minority carrier flow.For Silicon = $10^{-9}$ AFor Germanium = $10^{-6}$ A
  • In reverse biased condition, junction behaves as a capacitor of few picofarads.
  • In reverse biased condition, junction behaves like high resistive material between two regions.
  • In reverse biased P-N diode behaves like an insulator.
  • Reverse resistance $\mathrm{R}_{\mathrm{B}}=\frac{\Delta \mathrm{V}_{\mathrm{B}}}{\Delta \mathrm{I}_{\mathrm{B}}} \cong 10^{6} \Omega$$\frac{R_{B}}{R_{f}}=10^{3}$ : 1 for Geand

    $\frac{R_{B}}{R_{f}}=10^{4}$ : 1 for Si
  • Dependence of reverse current on bias volt. $\mathrm{I}_{\mathrm{r}}=\mathrm{I}_{0}\left[\mathrm{e}^{-\frac{\mathrm{q} \mathrm{V}}{\mathrm{kT}}}-1\right]$$\mathrm{e}^{-\frac{\mathrm{qV}}{\mathrm{kT}}}<<1$$\mathrm{I} \cong-\mathrm{I}_{0}$

    Here $\mathrm{I}_{\mathrm{r}}$ = reverse current

    $\mathrm{I}_{0}$ = reverse saturation current

    V = applied voltage

    q = charge of electron

    T = temperature in kelvin
Special Point
  • The diffusion current in the p-n junction is greater than the drift current in magnitude if the junction is forward biased.
  • A hole diffuses from the p-side to the n-side in a p-n junction. This means that a bond is broken on the n-side and the electron freed from the bond jumps to a broken bond on the p-side to complete it.

So, that’s all from this article. I hope you get the idea about what is P-N junction. If you enjoyed this article then please leave your thoughts in the comments section down below.

Also, read

What is Fermi energy level in semiconductors

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