Lecture 5

pn Junctions

pn junctions 1-1

Outline I-V characteristics

Bias voltage Forward and reverse currents Reverse breakdown

Special purpose diodes Light Emitting Diode (LED)

Diode circuit analysis approaches Graphical analysis (load-line method) Simplified analysis with ideal diode model Simplified analysis using constant voltage drop Simplified analysis using piecewise-linear

equivalent circuit

pn junctions 1-2

Test Yourself

The status where no external forces such as voltages, electric fields, magnetic fields, or temperature gradients are acting on pn junctions is called:

i. reverse bias ii. thermal equilibrium (zero bias) iii. forward bias

The charge condition in which semiconductor crystals exist when operating at thermal equilibrium is called:

i. negatively charged ii. positively charge iii. charge-neutrality

pn junctions 1-3

IV characteristics of pn juntions

Non-linear device

pn junction (Diode) current equation:

pn junctions 1-4

)1( e T

D

V

V

SD II

p n

VD

ID

Physics of forward bias:

pn junctions 1-5

• Potential drop across SCR reduced by VFB

• minority carrier injection in QNRs • Minority carrier diffusion through QNRs • Minority carrier recombination at contacts to the QNRs

(and surfaces) • Large supply of carriers injected into the QNRs

e T

D

V

V

DI

+ ve - ve

VFB

Physics of reverse bias

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• Potential drop across SCR increased by VRB

• minority carrier extraction from QNRs • Minority carrier diffusion through QNRs • Minority carrier generation at surfaces & contacts of

QNRs • Very small supply of carriers available for extraction I saturates to small value (scale of nanoamperes)

+ ve - ve

VRB

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pn junctions 1-8

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Temperature Effects

Reverse Breakdown

When a large reverse bias voltage is applied, breakdown occurs and an enormous current flows through the diode

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Diode Capacitance

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• Biasing a diode forms a volume of charges stored inside this diode and accordingly leading to have diode capacitance (no charge-neutrality)

• Changing diode biasing from reverse to forward or vice versa leads to change in the volume of charges stored in the diode and also the width of the depletion layer or SCR

• This results in a change in the diode capacitance

Depletion or Junction

Capacitance

Diffusion

Capacitance

Special purpose diodes

Light Emitting Diode (LED) In a forward-biased p-n junction,

recombination of the holes and electrons requires energy possessed by the unbounded free electrons

In Si and Ge, most of the energy is dissipated in the form of heat and photons (cause: indirect band gap)

But in other materials such as GaAs, the outcome of hole-electron recombination processes is light but it is invisible for the eye to see (infrared)

Other materials emit visible light during forward-bias operation

pn junctions 1-12

Color Construction Forward Voltage

Green

Orange

Red

GaP

GaAsP

GaAsP

2.2

2.0

1.8

Diode Circuit Analysis

Why ? Analyze operation of diodes

One common objective of diode circuit analysis is to find the quiescent operating point (Q-point), or bias point, for the diode

Enable knowing range of diode voltage and current due to input alternating signals

The Q-point consists of the dc current and bias voltage (ID, VD) that defines the point of operation on the diode’s IV characteristic curve

pn junctions 1-13

Practical Aspects of pn Junction

The left hand diagram shows reverse bias, with positive on the cathode and negative on the anode (via the lamp). No current flows

The other diagram shows forward bias, with positive on the anode and negative on the cathode. A current flows

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anode

cathode

Forward bias

Reversed bias Lamp off

Lamp on

Polarization of the pn Junction

Examples of forward biasing circuits

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Polarization of the pn Junction

Examples of reverse biasing circuits

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Graphical analysis (load-line method) Series diode circuit and

Characteristic curve

Connect a line between E / R

and E The overlap of the lines

becomes the Q-point of the

diode and IDQ and VDQ

will be obtained

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IRVVVE DRD

REI ,RIRI0E ,0VFor DDDD

EV ,VR)0(VE ,0IFor DDDD

Example Determine ID, VD & VR

pn junctions 1-18

RIVVVE DDRD

mA 24.2433.0

8

0 ,0

kREI

RIEV

D

DD

V 8

)0( ,0

EV

RVEI

D

DDR

E

E

Q-point

DQI

DQV

From the analysis: VDQ = VD ≈ 0.9 V IDQ = ID ≈ 21.5 mA For VR,

V 095.7)k33.0)(m5.21(RIV DR

Diode Approximation

Diode modeling circuits: Ideal equivalent circuit (ideal diode analysis)

Simplified equivalent circuit (constant voltage drop (CVD))

Piecewise-linear equivalent circuit

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Ideal Diode Characteristics

Ideal diode in Reverse Biased acts as an open-circuited (O/C) device and in Forward Biased acts as a short-circuited (S/C) device

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Simplified Equivalent Circuit (constant voltage drop)

Assume straight vertical line of ID at VD

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Piecewise-Linear Equivalent Circuit Real Diode is replaced by an ideal diode in

series with a battery (VD0) and resistance (rD)

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pn junctions 1-23

Lecture Summary Covered material Continue pn junctions

I-V characteristics Diode circuit analysis approaches

• Graphical analysis (load-line method) • Simplified analysis with ideal diode model • Simplified analysis using constant voltage drop • Simplified analysis using piecewise-linear equivalent circuit

Material to be covered next lecture

Examples on DC analysis of pn junctions Analysis of AC equivalent diode circuits