Question

(a) Draw the V-I characteristics of silicon diode.

(b) Explain the following terms:

(i) minority carrier injection in forward bias
(ii) breakdown voltage in reverse bias

Hint:

Describe the V-I curve with its knee at about 0.7 V and the reverse breakdown. Then define injection of minority carriers in forward bias and reverse breakdown voltage.

Answer 1

Step 1: (a) V-I Characteristics of a Silicon Diode:
Draw two perpendicular axes. The horizontal axis shows voltage \(V\), with forward bias to the right and reverse bias to the left. The vertical axis shows current \(I\), with forward current in mA upward and reverse current in \(\mu\)A downward.
Forward bias (first quadrant): the current is almost zero up to a threshold (knee) voltage of about 0.7 V for silicon. Beyond that, the curve rises very steeply.
Reverse bias (third quadrant): a very small reverse saturation current of a few \(\mu\)A flows, almost constant, as the reverse voltage grows.
At a large reverse voltage, the curve drops sharply downward. This point is the breakdown voltage. Label the axes, the knee at 0.7 V and the breakdown region.

Step 2: (b)(i) Minority Carrier Injection in Forward Bias:
In forward bias, the applied voltage opposes the barrier potential and lowers the barrier height. The depletion region becomes narrow.
Electrons from the n-side cross into the p-side, where they are minority carriers. Holes from the p-side cross into the n-side in the same way.
This crossing of carriers into the region where they are the minority is called minority carrier injection. The excess carriers diffuse and recombine, and this flow makes up the forward current.

Step 3: (b)(ii) Breakdown Voltage in Reverse Bias:
In reverse bias, a small reverse current flows. When the reverse voltage reaches a particular value, the current rises suddenly and sharply. This voltage is the breakdown voltage.
There are two causes. In Zener breakdown (heavily doped diode, low voltage), the strong field breaks covalent bonds directly. In avalanche breakdown (higher voltage), fast carriers collide with atoms and free more carriers in a chain.

Final Answer:
The characteristics curve has a knee at about 0.7 V in forward bias and a sharp rise at the breakdown voltage in reverse bias. Minority carrier injection makes the forward current, and breakdown is the sudden rise of reverse current.