# V_n equaion question on PN junction

1. May 22, 2010

### nhrock3

the formula for Vn should be
V_n=(E_f-E_i)/q

field divided by charge in the potential
like i was taught in high school
but why
the write it
V_n=(E_f-E_i)/KT

k in the boltsman constant
t is the temprature

there multiplication cannot be the charge

?

2. May 22, 2010

### mohammed.omar

Hello nhrock3,

There is a small misconception here.

When we write $$V_n=\frac{E_f-E_i}{kT}$$, E for energy; specifically the energy level in the energy band diagram. In such sense the term V_n is dimensionless.

If I'm getting you wrong, could you please reference your equation or state the full equation so I can get a better understanding.

Regards

3. May 22, 2010

### uart

No that does not have dimensions of voltage. You would have to show the context in which it's used but I would assume it is the dimensionless ratio of $V/V_T$, where $V_T$ is the temperature dependant "constant" $kT/q_e$ (approx 26 mV at room temperature).

4. May 29, 2010

### fullstop

I am using a keithley system for my four probe measurements.I am getting confused with th

5. May 29, 2010

### fullstop

I am using a keithley system for my four probe measurements.I am getting confused with the curre

6. May 29, 2010

### fullstop

I am using a keithley system for my four probe measurements.I am getting confused with the current from outer two probes .Since we require a constant current source for that.My problem is that as we are forcing current from outer two probes the system is asking for voltage input.How can it be possible to set any voltage as we are targeting to force current and want to measure from inner two probes.Also if we enter different voltages in that coloumn we are different sheet resistivity.But to the best of my knowledge sheet resisitivity of a material is fixed.