Conceptual difference between potential barrier & potential difference

[dexterdev]

Hi PF,

What is the conceptual difference between potential barrier and potential difference? both have an electric field associated with it. so is potential barrier a sort of potential difference itself and will it cause current flow in a closed circuit like diode shorted. Also why cannot we measure potential barrier using voltmeter (even if it is ideal).

-Devanand T

 

[Simon Bridge]

The term "potential barrier" refers to the appearance of the potential function when you graph it in comparison with the (kinetic) energy level. It looks like a wall. The place where the energy level meets the barrier is the classical limit of motion. Classically, the particle goes no further ... hence: "barrier".

The height of the barrier is a potential difference - though you are usually more interested in the difference between the particle kinetic energy far from the barrier and the potential at the top.

To understand the potential barrier in a solid state device like a p-n junction, you need to look more closely at how the potential function comes about.

 

[dexterdev]

The height of the barrier is a potential difference

I thought that that the potential difference (due to barrier) across the PN can cause current flow if the circuit is closed.

What about measuring barrier using voltmeter (even if ideal)

 

[Simon Bridge]

That was an answer to the general question about the relationship between the concept of a potential barrier and the potential difference.

To understand the potential barrier in a solid state device like a p-n junction, you need to look more closely at how the potential function comes about.

 

[dexterdev]

Ok. Can you suggest some good reference book or links etc...

 

[Simon Bridge]

What level do you need?
I'd suggest college-level solid-state textbooks... those would be overkill.

I think the main concept for you to grasp is the idea of the solid-state potential being due to an inderlying configuration - it's used as an abstraction. "Free" charges in the solid move with the potential acting as a landscape. Linking a high potential zone to a low one with some good conductor would be like linking the top of a hill with it's bottom with a smooth slide ... nothing moves on the slide because there is nothing to move ... to get a current of children (say) you have to add children - to the top if the hill.

That should help the concept ... but I want to stress that it is just an analogy.
The potential barrier in the diode is formed across the junction ... the characteristic charge of the junction comes from the crystal lattice. Mobile charges are attracted across the junction creating the depletion zone.

Connecting the far ends with an ideal (R=0) wire would create a new junction at the other end ... it's the same as putting the p-n material in a circle. So you'd get a small temporary current as a new depletion layer forms... so it's a circuit of two diodes head-to-tail.

 

[dexterdev]

Thankyou sir.