3C25 Solid State Physics diodes

Solid State Physics Lab 4 Semiconductor Diode and Carriers’ Charge

Purpose of the experiment •

Determine the main carriers’ charge of a semiconductor diode

The Lab Measurements of the I-V curve of a pn Junction A simple experiment that demonstrates the properties of a pn junction is discussed below. One simply measures the current as a function of (positive and negative) voltage across a diode. Additional properties can be demonstrated by varying the temperature of the diode, which changes the number of carriers in the conduction band. That is, the carriers (be they electrons or holes) will lead to a current density of the form J e , h = ( J e ,h ) 0 exp(

eV ), kT

where V is the bias voltage across the diode. The minority carriers will cancel this current exactly when there is no bias voltage applied, so the net current through an ideal diode has the form eV

I = I 0 (e kT − 1)

The experiment is quite simple and it is easy to figure out how to create it. Basically, one connects in series a diode, a voltage supply, a current meter, and a resistor (~1 kOhm) as a load. A voltmeter should be connected to the diode to measure the voltage drop on it. The diode should be placed into a hotstage with controllable temperature.

The first goal is to measure I(V) at different temperatures for both the direct and reverse switching of the diode. To analyze the data we must appreciate that the diode does not obey the ideal diode equation but operates in the so called recombination regime, where I = I 0 (e

eV 2 kT

− 1) ≅ I 0 e

eV 2 kT

, eV

and the last approximation is justified because the term e 2 kT >> 1. Therefore, we present the V-I curves for V > 0, on a semi-log plot (Figure 1). From the fit we find the slopes: T = 24°C =

297K

e 2kT

= 21.3V-1

Semiconductor Diode and Carriers’ Charge - 1

319K

e 2kT

= 19.6V-1

342K

e 2kT

= l8.9V-1.

Note the onset of saturation for biases V ~ 0.5 V and also the different intercepts at V = 0.

We observe that the measured slopes do indeed scale with temperature as expected and if we average the three results we obtain e 2k

=(6.28Âą0.l9)*103 K/V

Figure 1: Measurement of the current through a diode as a function of bias voltage, for different temperatures. (a) is for positive bias, plotted on a semi-logarithmic scale. Exponential fit s are indicated. (b) is for negative bias voltage, plotted on a linear scale.

Thus, using the value of the Boltzman constant k=1.38 x 10-23 J/K we find that e=(1.73 Âą 0.05)*10-19 C in good agreement with the value of the electron charge.

Semiconductor Diode and Carriers’ Charge - 2

The different intercepts are an indication of the variation of I0 with temperature. Of course at V = 0, I = 0 but this point cannot be reached on the logarithmic plot. A better way of determining I0 is by applying negative bias, but it does not work for every diode. From the classical negative bias data (Fig. 1b) we find that T= 297K I0 = 3.9 pA 303K 4.4 pA 310K 6.7 pA

Semiconductor Diode and Carriers’ Charge - 3

1N914 silicone diode

Current, mA (ln scale)

3

e2 e1 e0 e-1 e-2 e-3 e-4 e-5 e-6 e-7 e-8 e-9 e-10 e-11 e-12 e-13 e-14 e-15 e-16 e-17 e-18 e 0.0

25 40 60 Linear Fit of direct25_25

-1

The slope is 23.24 V

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Voltage, V (linear scale) e 2kT

25 0C 40 0C 60 0C

0.0

e=1.91E-19 C

23.81 22.6

e=1.81E-19 C

25 40 60

-0.5

Current, ÂľA

23.24 V-1

-1.0 -1.5 -2.0 -20

-15

-10

-5

0

Voltage, V

Semiconductor Diode and Carriers’ Charge - 4