Diode Curve Tracer Using Digilent Analog Discovery … · Diode Curve Tracer ... Diode Curve Tracer...

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Diode Curve Tracer

Rochester Institute of Technology

Microelectronic Engineering

ROCHESTER INSTITUTE OF TECHNOLOGY ELECTRICAL & MICROELECTRONIC ENGINEERING

Diode Curve Tracer Using Digilent Analog Discovery Module

Dr. Lynn Fuller, Adam Wardas Webpage: http://people.rit.edu/lffeee

Electrical and Microelectronic Engineering Rochester Institute of Technology

82 Lomb Memorial Drive Rochester, NY 14623-5604

Tel (585) 475-2035 Email: Lynn.Fuller@rit.edu

8-10-2015 DiodeCurveTracer.ppt

Diode Curve Tracer

INTRODUCTION

The Analog Discovery module is used with your computer and the “free” WaveForms software to turn your computer into a two channel oscilloscope, curve tracer, 16 channel logic analyzer, arbitrary waveform generator, 16 channel digital pattern generator, power supplies and voltmeters, network analyzer, spectrum analyzer and more. The module connects to your USB port. Find more information on line at www.digilentinc.com

Academic price ~$159

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ANALOG DISCOVERY PIN-OUT

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THEORY

The equations on the next two pages should be familiar to you from your basic course in semiconductor devices. The ideal diode equation used to model the current-voltage characteristics of a diode in forward and reverse bias. The second page gives the equations for the space charge layer within a uniformly doped pn junction. Including build in voltage, width of space charge layer, maximum electric field, and junction capacitance. These equations are use in the Excel spread sheet that follows. You can download this Excel file from Dr. Fuller’s web page.

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THEORY - CURRENTS IN PN JUNCTIONS

Vbi = turn on voltage

~ 0.7 volts for Si

Id VRB = reverse

breakdown voltage

+ VD -

Forward Bias

Reverse Bias

Id = Is [EXP (q VD/KT) -1]

Ideal diode equation

Is = qA (Dp/(LpNd) +Dn/(LnNa))ni2

Is = CT2 exp (-qEg/KT)

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THEORY - UNIFORMLY DOPED PN JUNCTION

WW1W2= [ (2q+VR) (1/NA 1/ND)]1/2

W1= W [ND/(NA ND)] W2= W [NA/(NA ND)]

= - [(2q/+VR) (NA ND/(NA ND))]1/2

Cj’rW= r[(2q+VR) (1/NA 1/ND)]1/2

= KT/q ln (NA ND /ni2)

ni = 1.45E10 cm-3

Built in Voltage:

Width of Space Charge Layer, W: with reverse bias of VR volts

Junction Capacitance per unit area:

Maximum Electric Field:

o r = 8.85E-12 (11.7) F/m

= 8.85E-14 (11.7) F/cm

W1 width on p-side W2 width on n-side

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THEORY – pn JUNCTION CALCULATOR

To use this spread sheet change the values in the white boxes. Results are given in the purple boxes and the I-V plot is updated. You can investigate effects of doping, temperature and photocurrent, determine reverse breakdown and junction capacitance. Enjoy!

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COMPONENTS USED FOR THIS LAB

100 ohm resistor 1N4448 Diode Vishay BPW46 Photodiode LED any color 9 Volt Battery and connector 47 ohm ¼ watt resistor (used as heater) Protoboard

1N4448

Resistors Photodiode LEDs

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SCREEN CAPTURE OF DIGILENT SETUP AND DATA

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CIRCUIT SCHEMATIC FOR DIODE I-V CURVE

Scope ch1+

Scope ch2-

Scope ch2+

Scope ch1-

4 volt peak

triangle wave

100 hZ

I = VR/R

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PHOTO OF SET UP FOR DIODE I-V CHARACTERISTICS

100 ohm resistor in

series with 1N4448

diode.

I = VR/R

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WAVEFORM GENERATOR SETUP

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OSCILLOSCOPE SET UP

R=100 ohm

I = VR/R

or 10mA/volt

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SCREEN CAPTURE OF DIGILENT SETUP AND DATA

R=100 ohm

I = VR/R

or 10mA/volt

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DATA IS ALSO AVAILABLE

10,000 data points are measured. Some are shown here.

~ -2 V ~ 0 V ~ +0.2 V ~ +0.6 V ~ +0.67 V ~ +0.75 V

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I R VR

Scope ch1+

Scope ch2-

Scope ch2+

Scope ch1-

4 volt peak

triangle wave

100 hZ

I = VR/R

V+ = 9V

On/Off 47 ohm heater

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PROTOBOARD FOR TEMPERATURE EFFECT

A 47 ohm resistor under the diode is used as a heater and is connected to 9V battery. I will get hot and the diode curve will shift to the left. Theory predicts a ~ -2.2 mV/°C Caution: don’t let the 9V wires touch wires going to Analog Discovery module…

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DIODE I-V AT ROOM TEMPERATURE

I = ~ -200 uAat Room T ~25°C

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DIODE I-V HEATED

Shift ~.05 volts

Temperature ~100°C above ambient

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I R VR

Scope ch1+

Scope ch2-

Scope ch2+

Scope ch1-

4 volt peak

triangle wave

100 hZ

I = VR/R

120 ohm V+ = 5V

On/Off LED

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PROTOBOARD FOR PHOTODIODE I-V

Triangle wave from the

waveform generator and 100

ohm resistor in series with

1N4448 diode. Plus 5 volt

power supply and 120 ohm

resistor in series with LED.

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LED IS OFF – V+ OFF

I = ~ 0

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LED IS ON – V+ IS ON

I = ~ -200 uA

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SUMMARY

We were able to investigate diode I-V characteristics. We observed the effect of

light on the I-V characteristics. We observed the effect of heat on the I-V

characteristics. The diode I-V characteristics were compared to theoretical

calculations.

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HOMEWORK – DIODE CURVE TRACER

1. Use the excel spread sheet to investigate diode I-V characteristics.

2. What happens to the I-V characteristic when a diode is heated?

3. What happens to the I-V characteristic when a diode is exposed to light?

Bonus: Modify the excel spreadsheet to include ideality factor (n) and internal

series resistance (RS).

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