ELECTRICAL MEASUREMENTS

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Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial* framework**

Consortium leader

PETER PAZMANY CATHOLIC UNIVERSITY

Consortium members

SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER

The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund ***

**Molekuláris bionika és Infobionika Szakok tananyagának komplex fejlesztése konzorciumi keretben

***A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával valósul meg.

PETER PAZMANY CATHOLIC UNIVERSITY

SEMMELWEIS UNIVERSITY

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Peter Pazmany Catholic University Faculty of Information Technology

ELECTRICAL MEASUREMENTS

Semiconductors basics: diodes and transistors

www.itk.ppke.hu

(Elektronikai alapmérések)

Félvezető alapismeretek: a dióda és a tranzisztor

Dr. Oláh András

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Electrical measurements: Semiconductors basics: diodes and transistors

Lecture 6 review

• Introduction to Circuit Theory

• Defnitions of corresponding quantities

• History of Circuit Theory

• Definition of elements

• The Kirchhoff laws

• Classificition of elements

• Linear resistive circuits

• Thevenin and Norton equivalent circuits

• System and Networks

• Linear dynamic circuits

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Outline

• Nonlinear resitive components

• Diode: p-n junction

• Actuel diode characterestics and used models

• Nonlinear element in linear resistive network

• Load-line analysis (graphical method)

• Types and applications of diodes

• Field Effect Transistor (FET)

• JFET and MOSFET operating characteristics

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Nonlinear components

i=Φ

_i

{u}=F

_i

(u) u=Φ

_u

{i}=F

_u

(i)

Question:

How can it be implemented?

Answer:

Semiconductive devices

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• The electrical characteristics of silicon and germanium are improved by adding materials in a process called doping.

• There are just two types of doped semiconductor materials:

– n-type materials contain an excess of conduction band electrons. (eg:

Phosphor)

– p-type materials contain an excess of valence band holes.(eg.: Boron)

Diode: Si crystal and doping

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Diode: p-n junction

• One end of a silicon crystal can be doped as a p-type material and the other end as an n-type material. The result is a p-n junction.

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Diode production: planar structure

200μm

p

SiO

₂

n

Metal bearing

Materials commonly used in the development of semiconductor devices:

Silicon (Si)

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• The excess conduction-band electrons on the n-type side are attracted to the valence-band holes on the p-type side.

• The electrons in the n-type material migrate across the junction to the p- type material (electron flow).

• The electron migration results in a negative charge on the p-type side of the junction and a positive charge on the n-type side of the junction.

• The result is the formation of a depletion region around the junction.

Diode: p-n junction (cont’)

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• External voltage is applied, a diode has three operating conditions:

– No bias: u_D = 0V → i_D = 0 A

– Reverse bias: external voltage across the p-n junction in the opposite polarity of the p- and n-type materials. u_D<0V → i_D=0A – Forward bias: external voltage across the

p-n junction in the same polarity as the p- and n-type materials. u_D>0V → i_D>0A

Diode: p-n junction (cont’)

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Actuel diode characterestics

{ }

0 ^T 1

u U

i i u I e⎛ ⎞

= Φ = ⎜⎜⎝ − ⎟⎟⎠

{ }

T ln ⁰ 1

i I

u u i U ⎛e ⎞

= Φ = ⎜⎜⎝ + ⎟⎟⎠

B

T k T 26mV

U = q ≈ I₀ is the reverse bias saturation current, U_T is the thermal voltage.

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• The Zener region is in the diode’s reverse-bias region. At some point the reverse bias voltage is so large the diode breaks down and the reverse current increases dramatically.

• The maximum reverse voltage that won’t take a diode into the zener region is called the peak inverse voltage or peak reverse voltage.

• The voltage that causes a diode to enter the zener region of operation is called the zener voltage (u ).

Zener range (or breakdown range)

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Basic linear elements:

– Resistor, R, [Ω] (Ohms)

– Nonlinear resistor (eg. diodes)

Nonlinear resistive networks (circuits)

Thevenin equivalent

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Nonlinear resistive circuits (or networks)

KVL and KVC:

s

0

D R

R D

u u u

i i

− + + =

− =

Charateristics of elements:

The system function:

Nonlinear equation

0

1 .

D T

R R

u U D

u Ri i i e

=

⎛ ⎞

= ⎜ ⎜ ⎝ − ⎟ ⎟ ⎠

1 0

uD

u Ri e ⎛

U

⎞ u

− + − + =

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• The load line plots all possible combinations of diode current (i_D) and voltage (u_D) for a given circuit. The maximum i_D equals u_s/R, and the maximum u_D equals u_s.

• The point where the load line and the characteristic curve intersect is the Q- point (equlibrium, or operation point set by the linear networks), which identifies i_D and u_D for a particular diode in a given circuit.

Load-line analysis (graphical method)

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Newton-Raphson iterative method ( ) ( )

1

n

n n

n

x x f x

+

= − f x

′

( ) ( )

( )

^D( )

1

D D

D n

n n

n

u u f u

f u

+

= −

′

( )

_D _s ₀ ^U^u^D^T

¹

_D

f u u R I e ⎛ ⎞ u

= − + ⋅ ⎜ ⎜ − + ⎟ ⎟

⎝ ⎠

( ) ⁰

f x =

( )

_D ⁰ ^D^T

T

1

u

R I

U

f u e

U

′ = + ⋅

x_n

x_n+1 x

f(x)

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Some useful models of diode characteristics

1. Piecewise linear model:

2. Series loss resistor model:

{ }

_T

^ln

^Iⁱ⁰

¹

_loss

u

i U ⎛ e ⎞ R i

= Φ = ⎜ ⎜ + + ⎟ ⎟ ⋅

⎝ ⎠

( )

^D ⁰

D

D 0 D 0

0 u u

i G u u u u

⎧ ≤

= ⎨ ⎩ − ≥

G = ∞

u₀ = 0

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• Semiconductors react differently to DC and AC currents.

• There are two types of resistance:

– DC (static) resistance – AC (dynamic) resistance

For a specific applied DC voltage u

_D

, the diode has a specific current i

_D

, and a specific resistance R

_D

.

Resistance levels: static resistance

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• Semiconductors react differently to DC and AC currents.

• There are two types of resistance:

– DC (static) resistance – AC (dynamic) resistance:

In the forward bias region:

In the reverse bias region:

Resistance levels: dynamic resistance

D D

D u i_{Q Q},

r du

= di

D loss

D

r 26mV R

= i +

r = ∞

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• Zener diode

• Light-emitting diode

• Diode arrays

• Schottky diode

• Varactor diode

• Power diodes

• Tunnel diode

• Photodiode

• Photoconductive cells

• IR emitters

• Liquid crystal displays

• Solar cells

Types of diodes: Zener diode

A Zener is a diode operated in reverse bias at the Zener voltage (u_Z). Common Zener voltages are between 1.8 V and 200 V.

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• Zener diode

• Diode arrays

• Schottky diode

• Varactor diode

• Power diodes

• Tunnel diode

• Photodiode

• IR emitters

• Solar cells

• Thermistors

Types of diodes: LED

An LED emits photons when it is forward biased.These can be in the infrared or visible spectrum. The forward bias voltage is usually in the range of 2 V to 3 V.

Applications:

–Instrumentation circuits as a sensor –Alarm system sensor

–Detection of objects on a conveyor belt

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• Zener diode

• Diode arrays

• Schottky diode

• Varactor diode

• Power diodes

• Tunnel diode

• Photodiode

• IR emitters

• Solar cells

Types of diodes: Schottky diode

Characteristics:

– Lower forward voltage drop (0.2-.63V) – Higher forward current (up to 75A) – Significantly lower voltage drop – Higher reverse current

– Faster switching rate

• Applications

– High frequency switching applications – Low-voltage high-current applications – AC-to-DC converters

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• Zener diode

• Diode arrays

• Schottky diode

• Varactor diode

• Power diodes

• Tunnel diode

• Photodiode

• IR emitters

• Solar cells

• Thermistors

Types of diodes: Tunnel diode

• The characteristics of the tunnel diode indicate the negative resistance region.

Note that this is only a small region of the characteristic curve. If the forward bias voltage is in the negative resistance region then the diode can be used as an oscillator.

• Applications:

– Oscillators

– Switching networks Pulse generators

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The diode only conducts when it is forward biased, therefore only half of the AC cycle passes through the diode to the output.

The DC output voltage is 0.318U_p, where U_p is the peak AC voltage.

Application of regular diodes: rectification

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Four diodes are connected in a bridge configuration.

U

_DC

= 0.636U

_p

Application of simple diodes: rectification (cont’)

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• There are two types of transistors:

– pnp – npn

• The terminals are labeled:

– E - Emitter – B - Base

– C - Collector

Bipolar transistor construction

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Bipolar transistor operation

• With the external sources, V

_EE

(or U

_EE

) and V

_CC

(or U

_CC

), connected as shown:

– The emitter-base junction is forward biased – The base-collector junction is reverse biased

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Bipolar transistor configuration: common-base

• The base is common to both input (emitter–base) and output (collector–

base) of the transistor.

Operation mode B-E junction B-C junction Cutoff Close (V_BE<0) Close(V_CB>0) Normal active Open(V_BE>0) Close

I_E = I_C+I_B (Kirchhoff current law) I_C = AI_E (transitor equation) IC A

= = B

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Bipolar transistor configuration: common-collector

• The input is on the base and the output is on the emitter.

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Bipolar transistor configuration: common-emitter

• The emitter is common to both input (base-emitter) and output (collector-emitter).

• The input is on the base and the output is on the collector.

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Common-emitter characteristics

Collector Characteristics Base Characteristics

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Bipolar transistor Modeling

• A model is an equivalent circuit that represents the AC characteristics of the transistor.

• A model uses circuit elements that approximate the behavior of the transistor.

• There are two models commonly used in small signal AC analysis of a transistor:

– r_e model: the bipolar transistor is basically current-controlled device;

therefore the r_e model uses a diode and a current source to duplicate the behavior of the transistor.

– Hybrid equivalent model (→simplified equivalent model)

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Simplified equivalent model:

Ideal transistor = current controlled current sources

BE B

D

i U

= r

C B

i = ⋅β i

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Field Effect Transistor (FET)

• Similarities:

– Amplifiers

– Switching devices

– Impedance matching circuits

• Differences:

– FETs are voltage controlled devices. Bipolar transistors are current controlled devices.

– FETs have a higher input impedance. Bipolar transistors have higher gains.

– FETs are less sensitive to temperature variations and are more easily integrated on ICs.

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• JFET: Junction FET

• MOSFET: Metal–Oxide–

Semiconductor FET

– D-MOSFET: Depletion MOSFET – E-MOSFET: Enhancement

MOSFET

FET types

There are three terminals:

– Drain (D) and Source (S) are connected to the n-channel

– Gate (G) is connected to the p- type material

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JFET operating characterictics

There are three basic operating conditions for a JFET:

V_GS = 0, V_DS increasing to some

positive value

Pinch Off V_GS < 0, V_DS at some positive value

Voltage-controlled resistor:

At the pinch-off point any further increase in U_GS does not produce any increase in

I_D. V_GS at pinch-off is denoted as V_poff. I_D is at saturation or maximum. It is

referred to as I_DSS.

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JFET transfer characteristics

In a JFET, the relationship of V_GS (input) and I_D (output) is a little more complicated:

GS P

2 D DSS 1

V

I I

^⎛^⎜_⎜ ⁻V ^⎞^⎟_⎟

⎝ ⎠

=

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• JFET: Junction FET

• MOSFET: Metal–Oxide–

Semiconductor FET

– D-MOSFET: Depletion MOSFET – E-MOSFET: Enhancement

MOSFET

FET types

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MOSFET operating characterictics

A depletion-type MOSFET can operate in two modes:

•Depletion mode:

–When V_GS = 0 V, I_D = I_DSS –When V_GS < 0 V, I_D < I_DSS –The formula used to plot the transfer curve still applies:

•Enhancement mode:

2 GS

D DSS

Poff

1 V

I I

V

⎛ ⎞

= ⎜ − ⎟

⎝ ⎠

These devices are off at zero gate–source voltage U_GS, and can be turned on by pulling the gate voltage in the direction of the drain voltage.

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Summary

• Diodes are two-terminal devices that conduct current easily in one direction, but not in the other.

• The ideal diode model is a short circuit for forward currents and an open circuit for reverse voltages.

• Zener diodes are intended to operate in the breakdown region.

• Transistors are three-terminal devices.

• Circuits containing a nonlinear device can be analyzed using a graphical technique called a load-line analysis.

• The analysis of nonlinear electronic circuits is often accomplished in two steps: First, the dc operating point is determined, and a linear small-signal equivalent circuit is found; second, the equivalent circuit is analyzed.

Next lecture: Nonlinear resistive networks