CBSE NOTES CLASS 12 PHYSICS
CHAPTER 14 SEMICONDUCTORS
A semiconductor diode is a p-n junction with metallic contacts provided at the ends for the application of an external voltage. It is a two terminal device.
Forward bias of p-n junction
The p -side is connected to positive terminal and n-side to negative terminal of a battery.
Forward current flows due to majority charge carriers and the width of depletion layer decreases.
The current under forward bias, first increases very slowly till the voltage across the diode crosses a certain value. After this, the diode current increases significantly (exponentially), even for a very small increase in the diode bias voltage. This voltage is called the threshold voltage or cut-in voltage
Reverse bias of a p-n junction
The p-side is connected to negative terminal and n-side to positive terminal of a battery.
Reverse current flows due to minority charge carriers and the width of depletion layer increases.
The current under reverse bias voltage is independent up to a critical reverse bias voltage, known as breakdown voltage (Vbr). When V = Vbr, the diode reverse current increases sharply. Even a slight increase in the bias voltage causes large change in the current. If the reverse current is not limited by an external circuit, the p-n junction will get destroyed.
V-I characteristics of a diode
A graph between V and I is plotted as shown in the figure. In forward bias measurement, a milliammeter is used, since the expected current is large (as explained in the earlier section) while a micrometer is used in reverse bias to measure the current.
In forward bias, the current first increases negligibly, till the voltage across the diode crosses a certain value. After the characteristic voltage, the diode current increases exponentially, even for a very small increase in the diode bias voltage. This voltage is called the threshold voltage or cut-in voltage (~0.2V for germanium diode and ~0.7 V for silicon diode).
For the diode in reverse bias, the current is very small (~μA) and almost remains constant with change in bias. It is called reverse saturation current. However, for special cases, at very high reverse bias (break down voltage), the current suddenly increases.
The p-n junction diode primarily allows the flow of current only in one direction (forward bias). The forward bias resistance is low as compared to the reverse bias resistance. This property is used for rectification of ac voltage.
The ratio of small change in voltage ΔV to a small change in current ΔI: