PN JUNCTION DIODE

PN Junction Formation

Depletion Region Formation

At the instant of junction formation

Energy Diagram of PN Junction

At Equilibrium

Energy Diagram of PN Junction

Width of the depletion layer of an open circuited PN junction

Barrier Potential of an open circuited PN junction

Type of the semiconductor material

Intrinsic concentration of Si or Ge before doping

Amount of Doping (on P & N sides)

Temperature

Factors deciding the Barrier Potential Value

Depletion Width Penetration as a function of Doping Concentration

Depletion Width Penetration as a function of Doping Concentration

Depletion Width Penetration as a function of Doping Concentration

PN Junction Diode

Diode under Forward Bias

Diode under Forward Bias

Diode under Reverse Bias

Diode under Reverse Bias

Reverse Current

It occurs for high reverse bias voltage

Impact IonizationAvalanche EffectReverse Breakdown Voltage

Reverse Breakdown

VI Characteristics Curve of a Diode

Resistance Levels

DC or Static Resistance AC or Dynamic

Resistance

VI Characteristics of Si & Ge diodes

Effect of Temperature on VI characteristics of a Diode

Diode Equation

The derivative of a function at a point is equal to the slope of the tangent line drawn at that point

Dynamic Resistance

Dynamic Resistance

It is defined as the flow of electric current due to the motion of the charge carriers under the influence of an external electric field

Drift Current

Equation for Drift Current Density

It is defined as the movement of charge carriers taking place from higher concentration region to lower concentration region of the same type of charge carriers under the presence of concentration gradient

Diffusion Current

Diffusion Current

Equation for Diffusion Current Density

It is the sum of drift current & diffusion current

Total Current in a Semiconductor

Einstein Relationship for Semiconductor

Depletion width (d) will increase with increased Reverse-bias potential, the resulting transition capacitance CTwill decrease

Transition or Depletion Region or Space charge Capacitance (CT)

CD exists in the forward bias region

CD is defined as rate of change of injected charge with applied voltage

Diffusion or Storage Capacitance(CD)

Transition & Diffusion Capacitance

Ideal Diode Model

Practical Diode Model

Complete Diode Model

The concentration of NA & ND are constant throughout the p and n sides

The doping density changes abruptly from p-type to n-type

Step Graded Junction

Step Graded Junction

Current Components in PN Junction Diode

Current Components in PN Junction Diode (fig 1)

Two diffusion currents as a function of distance x from the junction can be defined as

1. Inp(x) = Diffusion current due to electrons on the P-side as a function of x.

2. Ipn(x) = Diffusion current due to holes on the N-side as a function of x.

Inp(x) & Ipn(x) are two minority currents

Current Components in PN Junction Diode

A. Currents at the junction (x=0)I = Inp(0) + Ipn(0)

Diffusion currents due to electrons & holes at the junction (for x=o) will be in the same direction

I = the current at the junction, that is, the total current.

Current Components in PN Junction Diode

B. But, the total current should remain constant

These diffusion currents Inp(x) & Ipn(x) decrease exponentially with increase in x

This means that on both sides, there must be some other components of current present which can actually maintain the current I constant

Current Components in PN Junction Diode

C. On P-sideInp(x) + Ipp(x) = I

Ipp(x) = majority carrier current due to holes

Current Components in PN Junction Diode

D. On N-sideIpn(x) + Inn(x) = I

Inn(x) = majority carrier current due to electrons

Current Components in PN Junction Diode

The figure 1 is drawn for an unsymmetrically doped diode so that

Ipn ≠ Inp

Current Components in PN Junction Diode

Rectifier CircuitsClipping circuitsClamping circuitsDemodulation circuitsSwitch in digital logic circuits used in computers

Application of PN Junction Diode

Electronic devices by Floyd Electronic devices & circuits by

Salivahanan Electronic devices & circuits by Millman

& Halkias

Reference