IV

IV measurements are a process to determine devices characteristics and performance. Depending on the device in hands we will attain different information.

lets consider a ohmic device, that is, is obeys Ohm law, $V=RI$ then as the equation shows we would obtain a linear relation between voltage and current, as showed:

ohm law

The resistance we measure has a contribution from the sample, the wires and the contacts:

$R=R_{wires}+R_{contact}+R_{sample}$

4point probe

In order to minimise the contact and wires resistances the measurements are usually done with four probes;

We are initially considering diodes. Below is sketch an IV curve. from observing the curve we can determine if we have a good quality device, since the bigger $\Delta$ is, better will the device perform. The threshold voltage is taken from the extrapolation to V=0 of the IV curve in forward bias.

Another parameter that is possible to take is the Schoottky barrier height. A sketch of the band structure can be seen below:

SBH

In order to get the barrier height (φ_B) we need to consider the thermionic current voltage relationship:

$I= I_s \left(exp {\frac{qV}{nkT}}-1\right)$

where I_sis the saturation current and

$I_s=AA^*T^2exp{-\frac{q\phi_B)}{kT}}$

from this last equation we get

$\phi_B=\frac{kT}{q}ln{\left(\frac{AA^*T^2}{I_s}\right)$

with being the area of the diode and A*=120(m*/m). If we plot a semilog curve of I_svs. V and extrapolate the curve to V=0 we will get φ_Bfor zero bias.

Lets consider now a MOSFET. Below is a sketch of one:

where S is the source, D the drain and G the gate. One measure we can make is to measure the current at the drain and the source while we apply a constant voltage in the gate and a variable voltage along the drain and the source. What we will obtain is:

Mosfet IV

In this figure we can see two different regions: the linear region and the saturation region. In the linear region the conduction channel acts like a resistor. if we continue to increase the voltage the channel potential will reduce the current, translating in the lost of linearity, if we continue to increase the voltage we will get to a point where the inversion charge reaches zero and we no longer see a change in the current, we reach the saturation regime.

The threshold voltage is the gate bias at which the devices turn on. And, it can be determine from the transfer characteristic, for that we need to measure the drain current while we change the gate voltage and extrapolate the curve to V=0. If we then plot the same curve in a semi log scale we will get the subthreshold swing.

There are many parameters could be extracted from IV measurement such as thershold voltage ,subthershold slope and resistance which will discuss in next section.

Reference

1. D. K. Schroder, Semiconductor material and device characterization (3^rd Ed.) John Wiley & Sons.