Diode-Operating modes-types-i-v characteristics-advantages-disadvantages

INTRODUCTION
A general goal of this chapter is to develop the ability to use the piece wise linear model and approximation techniques in the hand analysis and design of various diode circuits. The chapter includes the following topics:

Introduction to diode
AC and DC analysis of diode.
Application of diodes to perform signal processing functions: rectification, clipping and clamping.
Zener diode, which operates in the reverse breakdown region
Application of Zener diode in voltage regulators

DIODE
Diode is a two terminal device with nonlinear i-v (current-voltage) characteristics.
Figure 1.1 shows the circuit symbol of a diode. In the diode symbol, the triangular head denoting the allowable direction of current flow and the vertical bar representing the blocking behaviour for currents in the opposite direction. The corresponding terminals are called the anode (or p -terminal) and the cathode (or n-terminal) respectively.

Operating Modes of a Diode
A diode operates in the following two modes:
1.Forward bias
2. Reverse bias
Forward Bias

In forward biasing, the negative terminal is connected to n-region which makes the electrons move towards p-region. Similarly, connecting the positive terminal to p-region makes the holes from p-region to move towards n-region.

Due to this movement of electrons and holes, neutral atoms are formed. Reduction in barrier potential reduces the width of the depletion region and with the increase in supply voltage more atoms are converted into neutral atoms and less charged ions are available in the region. The width of the depletion region is further reduced.

The conversion of atoms to neutral atoms continues till the depletion region collapses and hence large number of electrons and holes crosses the junction and the current flows from anode to cathode. The electrical resistance of a forward biased diode is very small and voltage drop is negligible across it. The forward voltage value for silicon diodes is about 0.7V.
Reverse Bias

When the diode is reverse biased i.e. when the n-region (cathode) is connected to positive terminal and p-region (anode) is connected to negative terminal, the negative terminal attracts the holes from the p-region and positive terminal attracts the electrons from the n-region.

The depletion region widens which opposes the current flow. The resistance of the diode is infinite and no current flows when it is reverse biased and small leakage current flows through the junction.

I-V Characteristics of PN Junction Diode

The I-V Characteristics of a PNJunction Diodes can be catagorized into two parts i.e. I-V Characteristics during Forward Bias and Reverse Bias.The figure below shows the Voltage-Current characteristics in diodes.

Current voltage characteristics of a diode

the theoretical relationship between the voltage and current in the PN junction is given by

....equation( 1.1)

Here, V_T = KT/q

I_D= Forward current

I_s= Saturation current

V_T= Forward voltage

K = 1.38×10^-23(Boltzmann’s constant)

T = Absolute Temperature

q = 1.6×10 ^-19(electronic charge)

n = a constant and has a value of 1 or 2 which depends on the material and physical structure of the diodes.

n=1 for Silicon and Germanium

n=2 for Gallium Arsenide.

Current voltage characteristics of an ideal diode
The ideal diode may be considered as the most fundamental non linear circuit element. Figure shows the current voltage characteristics of an ideal diode in the forward bias and reverse bias region respectively.

Ideal Diode Characteristics
Operation Mode	On(Forward biased)	Off (Reverse biased)
Current Through	I>0	I=0
Voltage Across	V=0	V<0
Diode looks like	Short circuit	Open circuit

combining the two graphs, we get the overall current voltage characteristics of an ideal diode(as opposed to the i-v characteristics of a diode given in equation (1.1)) as shown in Figure .

Types of Diode

The types of diode include:

Zener diode

PN junction diode

Tunnel diode

Varactor diode

Schottky diode

Photodiode

Pin diode

Laser diode

Avalanche diode

Light emitting diode

Advantages of Diode

The advantages of Diodes include:

Diodes are compact in size and compatible.
Designing electronic circuit is simple with certain diodes like Zener diodes.
Diodes help in controlling the current flow.
These diodes produce less unwanted noise.
Certain diodes like Schottky diodes can operate at high frequencies.
Light emitting diodes are highly efficient compared to other diodes and they can emit light of the expected colors.
Operates at high switching speed.

The disadvantages of Diodes include:

Power dissipation is more for Zener diodes and hence it is less efficient for heavy loads.
Diodes are highly sensitive to temperature.
Amplification is necessary in photodiode based circuits.
Light emitting diodes are expensive compared to other diodes.
Certain diodes like Schottky diodes have low maximum reverse voltage.
They have high reverse current and impedance.

Application:
Diode have many applications. here are a few of the typical applications of diodes include: