Pn Junction Diode - what happens to carrier profiles when voltage biases are applied?

The pn Diode Circuit Symbol and Terminal Characteristics(6.1)

There are 3 regions of operations of the pn diode

  1. Zero Bias- diode current \(I_D>0\) with the diode voltage \(V_D\approx 0.7 v\)

  2. Forward bias- diode current \(I_D\approx 0\)

  3. Backward Bias- diode current \(I_D<0\) and the diode voltage \(V_D< V_{BD}\) where \(V_{BD}\) is the diode’s breakdown voltage



The transport of holes from the p side across the depletion region into the n side under a forward bias is called hole injection

The transport of electrons from the n side to p side of the junction under a forward bias is called electron injection

Let us see what is happening in the different regions



Thermal equilibrium. Recap that the right side is the n side while the left is the p side. S-shape increasing graph will be the electrons while the S-shape decreasing graph will be the holes. No net hole or electron current density. All these covered in the 1st section if you recall

We have \[|J_\text{drift}| = |J_\text{diff}|\]



Forward bias: i.e. net E field pointing right. Expect net movement holes right and electrons left

We have \[|J_\text{drift}| < |J_\text{diff}|\] This means we have a net diffusion current in the space charge region or into the QNRs

  • Net diffusion hole current \(J_h\) to the right

  • Net diffusion electron current \(J_e\) to the left

Both of which in the direction of the SCR towards the ohmic contacts



Backward bias: i.e. net E field pointing left. Expect net movement holes left and electrons right

We have \[|J_\text{drift}|>|J_\text{diff}|\] This means we have a net drift current in the space charge region or from the QNRs

  • Net drift hole current \(J_h\) to the left

  • Net drift electron current \(J_e\) to the right

Both of which in the direction of away from ohmic contacts into the SCR

To sum up

  • In forward bias: injected minority carriers diffuse through QNR and recombine at semiconductor surface

  • In reverse bias, minority carriers generated at the semiconductor surface diffuse through the QNR and extracted by SCR

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