Semiconductor Electronics: Materials, Devices And Simple Circuits 14.1 Introduction | Class 12 Physics Notes

14.6 SEMICONDUCTOR DIODE

A semiconductor diode is essentially a p-n junction with metallic contacts at both ends, forming a two-terminal device. It allows current to flow primarily in one direction, making it useful for rectification and switching applications.

The diode symbol shows an arrow indicating the conventional current direction when forward biased.

The diode behavior depends on the applied external voltage (bias) across its terminals, which modifies the built-in barrier potential of the p-n junction.

14.6.1 p-n junction diode under forward bias:

When the p-side is connected to the positive terminal and the n-side to the negative terminal of a battery, the diode is forward biased. The applied voltage opposes the built-in potential, reducing the barrier height and narrowing the depletion region.

At small applied voltages, only a few carriers have enough energy to cross the junction, resulting in small current. As the voltage increases beyond a threshold (cut-in voltage ~0.7 V for silicon, ~0.2 V for germanium), more carriers cross, and current increases exponentially.

Electrons from the n-side cross into the p-side, and holes from the p-side cross into the n-side, a process called minority carrier injection. This increases minority carrier concentration near the junction, causing diffusion currents on both sides.

The total forward current is the sum of electron and hole diffusion currents, typically in milliamperes.

14.6.2 p-n junction diode under reverse bias:

When the polarity is reversed (n-side positive, p-side negative), the diode is reverse biased. The applied voltage adds to the barrier potential, increasing the depletion width and barrier height.

This suppresses diffusion current drastically. However, a small drift current due to minority carriers exists, typically in microamperes, and is almost independent of applied voltage.

At a critical reverse voltage called breakdown voltage, the reverse current increases sharply, potentially damaging the diode if current is not limited.

The diode's current-voltage (V-I) characteristics show negligible current under reverse bias and exponential increase under forward bias beyond threshold voltage.

Dynamic resistance of the diode is defined as the ratio of small changes in voltage to current (r_d = ΔV / ΔI).

This section explains the fundamental operation of the p-n junction diode and its asymmetric conduction properties.

📊 Diagram: Figure 14.12(a) shows a semiconductor diode with metallic contacts; (b) shows the diode symbol. Figure 14.13(a) depicts forward bias connection; (b) shows barrier potential changes with applied voltage. Figure 14.14 illustrates minority carrier injection under forward bias. Figure 14.15(a) shows reverse bias connection; (b) shows increased barrier potential under reverse bias. Figure 14.16(a) and (b) show experimental setups for V-I characteristics in forward and reverse bias; (c) shows typical V-I characteristic curve of a silicon diode.

🧪 Activity: Experimental study of V-I characteristics of a p-n junction diode using variable voltage source and measuring current with milliammeter and microammeter.

🔗 Connection: Leads to practical applications of diodes such as rectification in power supplies.