What is a PN Junction and PN junction diode?

PN junction forms by connecting P-type and N-type semiconductor material together. In PN junction, the electrons and holes will recombine at the junction which forms a depletion layer. The biased PN junction is called diode and it acts a one way gate to flow of charge carriers. Practically, a PN junction is formed by adding N-type and P-type impurities to one semiconductor bar in a measured quantity. The P-side is called anode and N-side is called cathode.

diode symbolWhat is a Junction capacitance of a diode?

The capacitance between the positive charge on the P-side of the diode junction and the negative charge on the N-side is called Junction capacitance.  It is denoted by CJ.

                                     {color{Magenta} C_{T}= frac{Area * k varepsilon _{0}}{W}}

Where,        Area = is the PN Junction area

                   K = Dielectric constant (Silicon=11.7)

                   ε0 = Permittivity of empty space (8.85 * 10-14 F/cm2)

                   W= Separation between positive barrier and negative barrier

Note: The difference between ordinary capacitance and junction capacitance is that the W (Separation) varies with applied voltage. The larger reverse bias voltage increases the separation which results in decrease of junction capacitance. In other hand during forward bias the separation decreases and capacitance increases.

What is diffusion capacitance or Transit time capacitance?

The change in charge storage from the diode junction with applied forward bias voltage is called diffusion capacitance. This is also called as transit time capacitance (CT).

In detail, during forward biasing of a diode, the current (ID) flows through the diode. This current implies that the majority carrier transportation in unit time i.e. ID = Q/T.  Where Q is the charge transported and T is the time taken or transit time of the charge.  This charge transportation varies with the applied voltage at Q point.

{color{Blue} C_{T}=frac{partial Q_{DQ}}{partial V_{D}}}

{color{Blue} C_{T}=frac{I_{DQ}T}{V_{T}}}

Where VT =  Thermal Voltage = 26mv at room temperature

Note:  By the above equation it is illustrated that Ct depends on the current flowing through the diode. So in frequency applications of diode this transit capacitance effect should be considered for calculation.

What is a diode dynamic resistance?

Dynamic resistance is a small signal AC resistance of a diode which mathematically represented as the slope of the V-I output curve of the diode at a particular operating point. It is denoted as rd.

{color{Blue} r_{d}= frac{partial v_{d}}{partial i_{d}}}

{color{Blue} r_{d}= frac{eta V_{T}}{I_{D}}}

It changes dynamically with the change in current for a change in voltage so that it is called as dynamic resistance.

The dynamic resistance is used in determining the voltage drop across the diode for an input AC voltage source.

What is Reverse Break down (VBD) voltage of a diode?

In reverse bias condition of a diode the reverse current is constant and independent of reverse bias voltage. But at a voltage the diode break down occurs and the reverse current increases rapidly. This voltage is called reverse break down voltage.

The break down occurs due to high field intensity at the junction which accelerates the minority carriers. The minority carriers acquire energy which is sufficient to break the covalent bonds, so an anonymous current flows through the diode.

What are Reverse break down mechanisms in a diode?

There are two types of reverse break down mechanism of diode

  • Avalanche Break down
  • Zener Break down

Avalanche Break down

Avalanche break down of a diode is due to high field intensity applied to the diode. The high electric field accelerates the minority carries across the junction to such a high kinetic energy which can break the covalent bonds in the lattice. Therefore a new hole-electron pair will be created which again acquires energy from the field and participate in the breaking the bonds. This results avalanche multiplications of charge carriers which results in high reverse current through the diode.

If the reverse current is not limited, the excessive developed in the device will breaks the device permanently. If it is limited before the damage, the property of the device can be reverted.

Zener Break Down

This is the second type of diode break down mechanism and widely used technique in all the applications. The depletion layer width is the barrier for current flow in reverse bias condition. The width ‘W’ depends on the reverse bias voltage as well as the impurity concentrations in the semiconductors P and N. I.e. higher doping density decreases the width of the depletion region (very narrow), and a very high field intensity exists across the junction for a small reverse bias voltage (E=dV/dX). This high electric field accelerates the charge carries which breaks the covalent bonds and creates an anonymous current flow in the device. The Zener break down occurs at less reverse voltage compared to avalanche break down. Zener breakdown is also reversible provided thermal limits are not exceeded, i.e. provided power dissipation is limited to avoid overheating the device.

Differences between Avalanche and Zener break Down?

Avalanche Breakdown Zener Break Down
Avalanche effect causes the diode break down above 5V of reverse bias voltage Zener effect causes the diode break down below 5V of reverse bias Voltage
Avalanche breakdown occurs due to external electric field in lightly doped junction Zener breakdown occurs due to internal electric field due to heavily doped junctions
Wide depletion layer Narrow depletion layer
With increase in junction temperature Avalanche break down voltage increases With increase in junction temperature Zener break down voltage decreases
positive temperature coefficient. negative temperature coefficient

What are high frequency diodes?

The diodes which are used at high frequencies are called high frequency diodes.

High frequency diodes:

  1. Tunnel diode
  2. PIN Diode
  3. Varactor Diode
  4. Step recovery Diode
  5. Schottky Diode

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