DIODE

DIODE is a active element which is a combination of two semiconductor material (n-type & p-type). When a bar of n-type material is jointed with a bar of p-type material then there is a flow of electrons and holes across the junction is take place till than a barrier is formed across the junction. This junction contains only donor ions and acceptor ions. It is known as p-n junction diode.

In actual, a diode is a two-terminal electronic component with asymmetric conductance, it has low (ideally zero) resistance to current flow in one direction, and high (ideally infinite) resistance in the other.

1: diode circuit symbol

Figure1: diode circuit symbol

BIASING :-

Two type of biasing:

1. 1. Forward bias (p is +ve and n is -ve)

2. 2. Reverse bias (p is –ve and n is + ve)

I-V CHARACTERSTICS OR DC ANALYSIS OF DIODE

It is the current voltage characteristics of diode. Ideally, a diode is a device that allows current to flow in one direction only. In practice, diodes allow large

amounts of forward current to flow when the positive voltage across them reaches a small threshold. They also have a small “saturation” current and a “breakdown” region in which a large amount of current will flow in the opposite direction when a large negative voltage is applied.

Here first is the ideal characteristic of diode but practical is different because in ideal characteristic potential barrier is assumed to be zero but in actual there is potential drop across junction so that characteristic is deformed.

Applications:-

Diode is used to design rectifiers,clippers,clampers and various electronic circuits.

Diode characteristic simulation through orcad :-

Orcad is a software which is used for designing simulation of circuits. To achieve our goal we design a circuit including diode, dc source and resistor. Then we will notice the voltage across the resistor. That will be the output of diode. By changing the polarity of source we can get the i-v characteristics of diode in both biasing (forward and reverse) .

Figure 2: circuit design in orcad capture

Run a simulation and create the i-v characteristic curve

o Set up a DC sweep from -15 to +15 volts in increments of 0.1 volt. (When you set up the DC

sweep analysis, be sure that you name your source “V1.”) You do not need to add any probes.

o Run the simulation.

You will get the waveform as shown in fig:-

Figure 3: forward characteristic of diode

Figure 4: Reverse characteristic of diode

Above characteristics represent the behavior of a diode in forward and reverse bias condition. It is the dc analysis of diode.

Forward Bias Condition:-

In forward bias condition as well as voltage across the diode increases the current also increase much lower rate as compared to voltage and after the knee voltage the current across the diode increase very rapidly. There is a potential barrier across the junction (.3V for Ge and .7V for Si). So this potential barrier reduces the overall voltage across diode. And when the voltage across diode is greater than the potential barrier then current comes into existence.

Reverse bias condition:-

In reverse bias condition ideally a diode is act as open circuit. But it is quite different from practical condition. Practically when reverse bias voltage across diode increase the current is due to only the minority carriers means in microampere . but after a value the current increase at very rapidly. In this state the diode will be destroyed. This region is called breakdown region.

AC ANALYSIS OF DIODE

Simply we say that diode is device which pass only the positive part of a waveform means it work as a rectifier. So we can use diode as half wave as well as

full wave rectifier.

HALF WAVE RECTIFIER: - It includes a diode, resistor AC source and ground connection. In HWR when a sine or square wave is applied to the diode then positive part of wave pass as it is and negative part will be zero. Its power and current ratings are typically much higher than those of diodes employed in other applications, such as computers and communication systems.

Figure 5: Half Wave Rectifier in Orcad

Above figure shows the circuit of half wave rectifier which is designed in cadence tool. A diode D1N4148 is used for this specific purpose. And a sine wave source is applied at the p- side of the diode. We can edit the value of the load resistance which is used to limit the output voltage. Two voltage probes are used to specify the input and output voltage.

Following waveform shows the behavior of diode under 5V ac supply:-

Figure 6: waveform of HWR

Full wave rectifier: -The dc level obtained from a sinusoidal input can be improved 100% using a process called full-wave rectification. The most familiar network for performing such a function appears in Fig with its four diodes in a bridge configuration. During the period t _ 0 to T/2 the polarity of the input is as shown in Fig . The resulting polarities across the ideal diodes are also shown in Fig to reveal that D2 and D3 are conducting while D1 and D4 are in the “off” state. The net result is the configuration of Fig. 2.54, with its indicated current and polarity across R. Since the diodes are ideal the load voltage is vo _ vi, as shown in the same figure.