Question 3
AIM: To determine the forward characteristics of a diode.
(a) Connect the circuit as shown in Fig. 4.
(b) Ask the supervisor to check the circuit connection.
(c) Adjust the power supply, Vs, to 2.0V.
(d) Close the switch S.
(e) Observe and record in Table 3 the values of current, I, and voltage VD.
(f) Repeat steps (c) and (e) for the other values of Vs in Table 3.
(g) Open switch S.
(h) Complete Table 3.
Table 3
Vs (V) |
I(mA) |
VD (V) |
VR = IR |
V = VR + VD |
2.0 |
|
|
|
|
4.0 |
|
|
|
|
6.0 |
|
|
|
|
8.0 |
|
|
|
|
10.0 |
|
|
|
|
12.0 |
|
|
|
|
(i) Plot a graph of I(mA) on the vertical axis against VD (V) on the horizontal axis.
(j) From your graph:
(i) determine the value of VD at I = 5 mA;
(ii) deduce the type of semiconductor material used to manufacture the diode.
(k) Give one reason for your answer in (j)(ii).
(l) State two precautions taken in carrying out the experiment.
Observation
(h) Table 3
Vs (V) |
I (mA) |
VD (V) |
VR (V) = IR |
V = VR + VD |
2.0 |
1.45 |
0.55 |
1.45 |
2.0 |
4.0 |
3.40 |
0.58 |
3.40 |
4.0 |
6.0 |
5.38 |
0.63 |
5.38 |
6.0 |
8.0 |
7.36 |
0.64 |
7.36 |
8.0 |
10.0 |
9.35 |
0.65 |
9.35 |
10.0 |
12.0 |
11.34 |
0.66 |
11.34 |
12.0 |
NOTE: Readings of I (mA) with accuracy of ±1;
Readings of V (V) with accuracy of ±0.05.
(i) A non-linear graph with the title, points, labelled axes and appropriate scale.
(j) (i) VD = 0.625V
(ii) Silicon
(k) Reason for the answer in (j)(ii)
- The potential barrier/knee voltage of the diode is approximately 0.5V
- The voltage started reading at 0.5V
(l) Precautions taken in carrying out the experiment
- Ensure proper polarity of the diode
- Ensure tight connections
- Avoid error due to parallax
- Switch off the power supply when not in use
Question 3 required candidates to connect a circuit, take readings, compute values and plot a graph. It was reported that majority of the candidates did not obtain the correct graph. They were reportedly unable to obtain appropriate scales as well as other features that make a good graph.