[Physics Class Notes] on Diode Pdf for Exam

What is a Diode? 

The semiconductor is one of the fascinating concepts in the world of electronics compared to the passive components like the capacitors, resistors, and inductors. A diode is a type of semiconductor electronic device which is most widely used.  

The semiconductor device, which acts as a one-way switch for current, is known as a diode. The device allows the easy flow of the current in one direction only; it puts a lot of restriction if the current tries to flow in the opposite direction.

The semiconductor diodes are also sometimes called rectifiers as the pulsating direct current (dc) is obtained by changing the alternating current (ac). The rating of the device is done based on its current and voltage capacity.

The polarity of the diode is determined by the anode (i.e., the positive lead) and the cathode (i.e., the negative lead). In most of the diodes, the current flow is allowed only when a positive voltage is applied to the anode. The configuration of a variety of diodes is displayed in the graphic below.

The current flow allowed by the diode is known as forward biased. When the diode does not permit the flow of the current, it is known as reverse biased, here the diode acts as an insulator.

Fun Fact: The arrow of the diode’s symbol points against the direction of the electron flow, the reason being the symbol was conceived by the engineers, and their schematic diagram shows the current flowing from anode to cathode, i.e., from positive voltage to negative voltage source.

Diode Symbol

Below, the symbol of the diode is shown. The arrowhead indicates the direction of the conventional current, which is in the forward biased condition. The literal meaning is that the cathode is connected to the n side, and the anode is connected to the p side.

V – I Characteristics of a Diode

The below circuit diagram shows the arrangement for the PN junction diode. The voltmeter is connected in parallel, and an ammeter is a connector in series to the diode, whereas the variable resistor controls the supply. The diode diagram is shown below:

When the circuit is operational, if the diode is in forward biased condition, then at some point of a particular voltage, the potential barrier will get eliminated. Such voltage is known as the Cut-off voltage or Knee Voltage. The forward current will rise exponentially if the forward voltages go beyond the limit, the continuous rise in voltage of this will cause the device to get damaged due to overheating.

The following graph shows the state of diode conduction in forward and reversed biased conditions.

In reverse bias, the minority current carriers produce reverse current when there is an increase in the reverse voltage. Due to a further increase in reverse voltage, the reverse current also increases, which will cause a sudden breakdown at a certain point, causing permanent damage to the junction.

What is the non-ideal diode equation?

I = I₀([e^{frac{qv}{kT}}] – 1)

I0 is directly proportional to recombination, and thus, inversely proportional to material quality.

Ideal Diodes

The below-shown diode equation provides you with the expression for current flowing through the diode as a function of voltage. The Ideal Diode Law is shown as:

I = I₀([e^{frac{qv}{kT}}] – 1)

Where:

I = through the diode the net current flowing;

I0 = the diode leakage current density in the absence of light, known as the dark saturation current

V = voltage applied across the terminals of the diode;

q = absolute value of electron charge;

k = Boltzmann’s constant; and

T = absolute temperature (K).

One of the critical parameters which differentiate the one diode from another is the dark saturation current (I0). I0 measures the recombination of the device. A diode will have a more considerable I0 value if the recombination of the diode is large.