Steady state condition: qualitative description of current flow at a junction
One useful feature of a p-n junction is that current flows quite freely in the p to n direction when the p region has a positive external voltage bias relative to n (forward bias and forward current), whereas virtually no current flows when p is made negative relative to n (reverse bias and reverse current). This asymmetry of the current flow makes the p-n junction diode very useful as a rectifier.
Description of Current Flow at a Junction:
- We assume that an applied voltage bias V appears across the transition region of the junction rather than in the neutral n and p regions.
- Of course, there will be some voltage drop in the neutral material, if a current flows through it.
- But in most p-n junction devices, the length of each region is small compared with its area, and the doping is usually moderate to heavy thus the resistance is small in each neutral region, and only a small voltage drop can be maintained outside the space charge (transition) region.
- For almost all calculations it is valid to assume that an applied voltage appears entirely across the transition region.
- We shall take V to be positive when the external bias is positive on the p side relative to the n side.
Since an applied voltage changes the electrostatic potential barrier and thus the electric field within the transition region, we would expect changes
in the various components of current at the junction (Fig. given below).
- In addition, the separation of the energy bands is affected by the applied bias, along with the width of the depletion region.
- The electrostatic potential barrier at the junction is lowered by a forward bias Vf from the equilibrium contact potential V0 to the smaller value VQ - Vf.
- This lowering of the potential barrier occurs because a forward bias (p positive with respect to n) raises the electrostatic potential on the p side relative to the n side.
- For a reverse bias (V = -Vr) the opposite occurs; the electrostatic potential of the p side is depressed relative to the n side, and the potential barrier at the junction becomes larger (V0 VT).
- The electric field within the transition region can be deduced from the potential barrier.
- We notice that the field decreases with forward bias, since the applied electric field opposes the built-in field.
- With reverse bias the field at the junction is increased by the applied field, which is in the same direction as the equilibrium field.