1 Rf Devices Basics

Basics of RF Devices I

OUTLINE• Introduction • Mainstream Electronics vs RF Electronics• Transistor Concepts I• RF Transistor Figures of Merit• Material Issues – Special Needs for RF• History and Evolution of RF Transistors

Frank Schwierz Technische Universität Ilmenau, Germany

TARGET Summer School, 13 September 2004Europa Beach, Crete 1

What is RF Electronics

RF electronicsRF: Radio Frequency, i.e. frequencies around and above 1 GHz

Synonymous: - microwave electronics- high-frequency electronics- high-speed electronics

RF electronics is one of the fastest growing fields in semiconductor(and electronics) industry

Traditionally: - defense related applications clearly dominatedCurrently: - large consumer markets for RF products

- defense related applications

2


OUTLINE• Introduction • Mainstream Electronics vs. RF Electronics• Transistor Concepts I• RF Transistor Figures of Merit• Material Issues – Special Needs for RF• History and Evolution of RF Transistors



Mainstream Electronics vs RF Electronics

Semiconductors• III-V compounds basedon GaAs and InP

• Si and SiGe• Wide bandgap materials(SiC and III-nitrides)

Transistor Types• MESFET - Metal Semiconductor FET• HEMT - High Electron Mobility Transistor• MOSFET - Metal Oxide Semiconductor FET • HBT - Heterojunction Bipolar Transistor• BJT - Bipolar Junction Transistor

Mainstream electronics (processors, ASICs, memories)

Semiconductors• Si

Transistor Types• MOSFETs• For a few applications BJTs

RF electronics

4

Mainstream Electronics vs RF Electronics

Mainstream electronics - Governed by Moore's Law- Increasing number of devices per chip- Scaling (decreasing device size)- CD < 1 µm around 1987 (commercially)- Si MOSFET dominates

1970 1980 1990 2000 2010 2020

1T

1G

1M

1k

Min

imum

feat

ure

size

, µm

Mem

ory

Bits

per

Chi

p

Year

0.01

0.1

1

10

100128G

0.001

DRAM

Processors7nm

18nm

ITRS 2003 targets

RF electronics - Moore's Law is not an issue- Integration is less important- Traditionally a submicron technology- Commercial devices

1980 0.5 - 2 µm1982 0.25 - 1 µm2004 0.05 - 0.5 µm

5

Intel MOSFET with 70 nm gate Evolution of Intel Processors

This device is in mass production!More than 40 Mio. transistors of this type are integrated on

a single Pentium 4 chip.

And even smaller transistors are already fabricated in the lab !

Mainstream Electronics

1970 1980 1990 2000 2010103

104

105

106

107

108

109

Itanium 2 ISSCC 2003410 mio trans.

ItaniumISSCC 2002

Pentium 4Pentium III

Pentium IIPentium

486

286386

8086

8080

80084004T

rans

isto

rs p

er c

hip

Year of introduction

6


Mainstream electronics is on the way to nanoelectronics

IEDM 2000Intel 30nm

DRC 2003Intel 10nm

IEDM 2002IBM 6nm

IEDM 2003NEC 5nm

7


Intel's new Tri-Gate MOSFET

- Smaller and smaller devices- New MOSFET concepts- But: Si still dominates

MOSFET still dominates

8

RF Electronics – Market Trends

• by 1980 Only military applications, RF is synonymous withmysterious. Philosophy: Performance at any price.

• 1980s-90s Increasing number of consumer applications

• Late 1990s Clear shift from military to consumer applications, explosion of the market for mobile communications.Now: reasonable performance at lowest price.

• 2001 Considerable turbulences, layoffs. There are no markets with unlimited growth!

• 2003/2004 Recovery. In the medium and long term, RF electronics will still grow dynamically. New applications will be introduced.

9

RF Electronics – Applications

Taken from: ITRS 2003 Edition. Chapter Radio Frequency and Analog/Mixed-Signal Technologies for Wireless Communications

10

RF Electronics – Acronyms

• GSM Global System for Mobile Communications• CDMA Code-Division Multiple Access• ISM Industrial, Scientific and Medical (frequency bands)• PDC Personal digital cellular• GPS Global Positioning System (Satellite)• DCS Digital Communication System• PCS Personal Communications system• DECT Digital European Cordless Telephone/Telecommunications• WLAN Wireless Local Areal Network• UWB Ultra-Wideband

A comprehensive compilation of acronyms related to RF electronics can be found in: F. Bashore, Acronyms Used by the RF/Microwave Industry Pt. I: Microwave Journal, Feb. 1997, pp. 110-115Pt. II: Microwave Journal, Feb. 1997, pp. 114-122

11





Transistor Concepts I

Two basic transistor concepts

• FETs (Field-Effect Transistor)The output current (mainly a drift current) is controlled by a perpendicularfield.The conductivity of the channel is varied by changing the potential of a control electrode (gate).- MESFETs (Metal-Semiconductor FET)- HEMTs (High Elexctron Mobility Transistor)- MOSFETs (Metal-Oxide-Semiconductor FET)

• Bipolar transistorsThe output current (mainly a diffusion current) is controlled by the voltage across a pn junction.The carrier injection is varied by changing the voltage across the junction.- BJT (Bipolar Junction Transistor)- HBT (Heterojunction Bipolar Transistor).

13

Transistor Concepts I - FETs

Channelbulk-like

Channel 2DEG

S G D S G D

S.I. GaAsS.I. GaAs

EC

EF

EV

EC

EF

EV

n+ n+ n+ n+n AlGaAs

i GaAs

Channel 2DEG

S G D

p-type Si

EC

EF

EV

n+ n+

Inversion layer

MESFET HEMT MOSFET

Source Drain

Gate

VG

Lch n+ n+

Channel

A FET in general

FETs

14

RF FETs use n-channels.A more positive gate voltage increases the number of carriers (electrons) in the channel

Transistor Concepts I - Bipolars

A bipolar transistor in general

Emitter Collector

Base

n np

EB junctionforward biased

CB junctionreverse biased

n

x

more positive VBE

more negative VBE

Bipolar transistors• BJT

- same material for emitter and base- EB junction is a homojunction

• HBT- different materials for emitter and base- EB junction is a heterojunction- emitter: wider bandgap- base: more narrow bandgap

15

An RF transistor has to react as fast as possible on a variationof the input signalFET – gate voltage VG Bipolar Transistor – base voltage VB

(base current IB)

ØThe charge distribution in the active region of the transistor hasto be changed.

Ø To achieve this we have to consider- Transistor Design

Small active region of the transistor. Critical dimension – FET: gate length L

– Bipolar: base thickness (width) wB- Material Issues

Most important: Fast carriers

How to Make a Transistor Fast ?

16

How to Make an FET Fast ?

Source Drain

GateVG

n+ n+

Lch n+ n+

Design: • short channel (small Lch)• fast carriers (n-channel)

Material: • fast carriers

(high mobility, high velocity)

17

How to Make a Bipolar Transistor Fast ?

Emitter Collector

Base

VB

n n

wB

p

n n

p

large positive VBE

smaller positive VBE

Design: • narrow base (small wB)• fast carriers (npn)

Material: • fast carriers

- high mobility- high velocity

• high diffusivity 18





RF Transistor FOM (Figures of Merit)

The two most important features of a transistor are:- its ability to amplify (important for analog and RF electronics)- its ability to act as a switch (important for digital electronics)

Important Figures of Merit (FOMs)of RF transistors

RF power transistors RF low-noise transistors

- Gain (power gain, current gain) - Gain- Frequency limits fT and fmax - fT and fmax- Output power - Minimum noise figure- ... - ...

20

FOMs – Power Gains

Power GainGeneral definition:

Microwave electronics - several power gain definitions.Frequently used are:- Maximum stable gain MSG - Maximum available gain MAG- Unilateral power gain U - Associated gain Ga

Example: Definition of UPower gain of a two-port having no output-to-input feedback, with input and output conjugately impedance matched to signal source and load, respectively.

Power gains are frequently given in dB:

in

out

PP

G =

( ) ( ) ( ) ( )[ ]21122211

21221

yReyReyReyRe4

yyU

−−

=

[ ] ( )GainPowerlog10dBGainPower =21

Current GainGeneral definition:

Current gain in dB

Noise FigureNF describes the noise generated in the transistor. Should be as small as possible and is always above 0 dB in real transistors. For optimum matching and bias conditions, NF reaches a minimum - the minimum noise figure NFmin.

FOMs – Current Gain and Noise Figure

11

21

1

221 y

yii

h ==

[ ] 211021 hlog20dBh =

[ ]NoSo

NiSi

PPPP

log10dBNF =22

FOMs – The Characteristic Frequencies fT and fmax

0.1 1 10 1000

10

20

30

40

50

fmax

=138 GHz

fT=108 GHz

U fit (-20 dB/dec)

h21

fit (-20 dB/dec)

Gai

n, d

B

measured h21

measured U

Frequency, GHz

h21 and U roll off at higher frequencies at a slope of –20 dB/dec.

Cutoff Frequency fTFrequency, at which the magnitude of the short circuit current gain h21 rolls off to 1 (0 dB).

Maximum Frequency of Oscillation fmaxFrequency, at which the unilateral power gain U rolls off to 1 (0 dB).

Attention: Frequently fmax is NOT extrapolated from measured U, but from measured MAG or MSG ! Check before working with published fmax values !

Measured h21 and U of a GaAs MESFETAfter K. Onodera et al., IEEE Trans. ED 38, p. 429.

23

FOMs – The Characteristic Frequencies fT and fmax

Measured h21, MSG, MAG, and U of a SiGe HBTRef.: J.-S. Rieh et al., IEDM 2002.

24

FOMs – The Importance of fT and fmax

A frequently asked question:Is the extrapolation of h21 and U with –20dB/dec actually useful?

The answer is: definitely YES !

1. P. Greiling (1984)"For those of us associated with this technology, this measurement problem always seems to exist. We are in a catch 22 situation in which we are developing circuits for instruments that are needed to measure the circuits we are developing."

2. If we know the extrapolated fmax we find the power gain U at anyfrequency according to:

3. Using fT and fmax we can compare the RF potential of differenttransistors reported somewhere in the literature.

( ) maxflog20flog20fU +−=

25

FOMs – fT and fmax vs fop

Rule of thumb

fT ~ n × fop, fT ~ fmax

- Low-noise transistors: n ~ 10 (conservative) , i.e., fT should be around 10 × the operating frequency fop of the RF system in which the transistoris to be used.

- Power transistors: n ~ 5.

What does this mean? If n = 10 and fT = fmax, we have 20 dB unilateral power gain U at fop. Note that Uis the upper limit for the power gain a transistor can achieve. The actual gain in a realistic circuit environment, e.g. Ga for minimum noise, will be several dB lower.

Examples: GaAs MESFET: fmax = 70 GHz U @ 12 GHz = 15.3 dB, Ga @ 12 GHz = 11 dBAlGaAs/GaAs HEMT: fmax = 50 GHz U @ 12 GHz = 12.4 dB, Ga @ 12 GHz = 9 dBAlGaAs/GaAs HEMT: fmax = 120 GHz U @ 18 GHz = 16.5 dB, Ga @ 18 GHz = 11.6 dB

26

FOMs – Output Power Amount of RF power in Watt (W) that can be deliveredfrom a transistor to the load.

General definition: ∫=T

LLout iRT

P0

21

27

I

Imax

Imax /2

Load line

Class A operating point

Vk VVmVm+Vk

2

A

B

( )8

max kmout

VVIP

−=

Class A amplifier

FOMs – Output Power

[ ] [ ][ ] [ ] 10/10

log10dBmP

out

outout

outmWP

mWPdBmP

=

=

Examples:1 mW = 0 dBm -10 dBm = 0.1 mW

10 mW = 10 dBm 30 dBm = 1000 mW = 1 W20 mW = 13 dBm 40 dBm = 10 W

100 mW = 20 dBm 50 dBm = 100 W

The output power is frequently given in dBm:

Ref.: A. W. Scott, Understanding Microwaves, J. Wiley 1993

A FOM related to the output power is theoutput power density PDout:- PDout in W/mm gate width (FETs)- PDout im mW/µm2 emitter area (bipolar transistors)

28





Material IssuesMost Important for high speed (high fT and fmax) and for low noise:Fast carriers, i.e. - high low-field mobility (µ0)

- high peak and/or saturation velocity (vpeak , vsat)

Important for high output power:- high breakdown voltage, i.e. wide bandgap- high thermal conductivity

1.22.90.050.51.3κ, W/cm-K

1.5...220.70.81vsat, 107cm/s

2.522.5...321vpeak, 107cm/s

83061070004700710µ0, cm2/Vs

443346.45.7EBR, 105 V/cm

3.443.20.741.421.12EG, eV

GaN4H SiCIn0.47Ga0.53AsGaAsSi Data for n-typematerial withND = 1017 cm-3

30

Low-Field Mobility and Drift Veloctity

31

Ele

ctro

n v

elo

city

Electric field

The slope at low fields corresponds to the low-field mobility µ0

Peak velocity vpeak

Saturation velocity vsat

Stationary velocity-field characteristics of GaAs

Low-Field Mobility

0 1 2 3 4102

103

104

105

GaN

4H SiC

6H SiC

Al0.3

Ga0.7

As

In0.52

Al0.48

As

GaAs

InP

Si

Ge

In0.53

Ga0.47

As

InAs

Ele

ctro

n Lo

w-F

ield

Mob

ility

, cm

2 / V

s

Bandgap, eV

In any semiconductor theelectron mobility is larger

than the hole mobility.

Thus, for RF applications transistors where electrons carry the main

current are preferred:

n-channel FETs npn bipolars

Carrier velocity at low electric field:v = µ0 × E

low-field mobility of electrons(undoped material, T = 300K)

32

Si and III-V semiconductors Wide bandgap semiconductors

Drift Veloctity

Stationary velocity-field characteristics (v-E)

0 50 100 150 2000

1

2

3

GaAs In

0.53Ga

0.47As

InP Si

Vel

ocity

, 107 c

m/s

Electric Field, kV/cm0 100 200 300 400

0

1

2

3

GaN 4H SiC GaAs

Vel

ocity

, 107 c

m/s

Electric Field, kV/cm

33

Breakdown Voltage

A large bandgap results in a large breakdown voltage.For power transistors a high breakdown voltage is important.

LARGE POTENTIAL FOR WIDE BANDGAP MATERIALS!

Breakdown voltage of abrupt one-sided pn junctions as a

function of doping (at the low-doped side)

The breakdown voltage is closelyrelated to the breakdown field.

Bandgap increases

4H SiC EG = 3.20 eVGaAs EG = 1.42 eVGe EG = 0.66 eV

34

1014 1015 1016 1017 1018100

101

102

103

104

105

4H SiC6H SiC

GaPGaAsSiGe

Bre

akdo

wn

volta

ge, V

Background doping, cm-3

Heterostructures

Heterostructures Ø are semiconductor structures consisting of at least

two different semiconductors

Ø are uncommon in mainstream electronics

Ø are frequently used in RF transistors- FETs: HEMTs- Bipolars: HBTs

Ø The RF transistors showing- the highest fT and fmax- the highest output power densities- the lowest noiseare heterostructure transistors.

Therefore it is useful to discuss some aspects ofheterostructures in the following.

35

Heterostructures

EF

EV

EC

EF

EV

EC

AlGaAs GaAs

EF

EV

EC

2DEG

AlGaAs/GaAs

∆ EC

∆ EV

EF

EV

EC

Barrier for electrons

Barrier for holes

Emitter Base Collectorn AlxGa1-x As n GaAsp+ GaAs

The physics exploited in RF heterostructure transistorsMost important: band offsets ∆EC and ∆EV

HEMTs(here: AlGaAs/GaAs HEMT)

HBTs(here: AlGaAs/GaAs HBT)

36

Heterostructures

Semiconductor pairs frequently used in heterostructures

37

Al0.3Ga0.7As GaAs

1.80 1.422

Si0.85Gea0.15Si

0.9951.12

In0.52Al0.48As

In0.53Ga0.47As

Al0.3Ga0.7N GaN

1.451 0.737 4.04 3.44

∆ EC = 0.219∆ EV = 0.159

∆ EC = 0.02∆ EV = 0.105

∆ EC = 0.52∆ EV = 0.194

∆ EC = 0.42∆ EV = 0.18

Bandgap vs Lattice Constant

HETEROSTRUCTURES:

Lattice matched- AlGaAs/GaAs- In0.52Al0.48As/In0.53Ga0.47As/InP

Pseudomorphic (strained)- AlGaAs/InxGa1-xAs/GaAs- In0.52Al0.48As/InxGa1-xAs/InP- Si/Si1-xGex- AlxGa1-xN/GaN

5.4 5.5 5.6 5.7 5.8 5.9 6 6.1

Lattice Constant, A

Ban

dgap

, eV

Si

Ge

GaAs

GaP

AlP

AlAs

InAs

InP10

2030

4050

6070

8090 a = 5.6533 A

Matched to GaAs

°

a = 5.8697 A Matched to InP

°°

2.5

2

1.5

1

0.5

0

38

Pseudomorphics Heterostructures

AlGaAs

InGaAs

Grown pseudomorphic AlGaAs/InGaAs/GaAs heterostructure

GaAs

The InGaAs layer is STRAINED (provided the critical thickness is not exceeded) 39

Example: InGaAs/GaAs

Critical Thickness

0.0 0.2 0.4 0.6 0.8 1.01

10

100

1000

Empirical upper limit (AlGaN on GaN)

Empirical upper limit (InGaAs on GaAs)

Exp. AlGaN Exp. AlGaN Exp. InGaAs

Crit

ical

thic

knes

s, n

m

Composition x40

Properties of 2DEGs

1.3 x 1013

3.0 x 1012

2.2 x 1012

1.4 x 1012

nS, cm-2

0.420.61 400Al0.3Ga0.7N/GaN

0.520.7110 000In0.52Al0.48As/In0.53Ga0.47As

0.410.586400Al0.3Ga0.7As/In0.2Ga0.8As

0.220.385400Al0.3Ga0.7As/GaAs

∆EC, eV∆EG, eVµ0, cm2/VsHeterojunction Type

Common III-V heterostructures:- more In in the channel layer leads to higher mobility µ0- larger ∆EC causes a higher higher sheet concentration nS

Interesting for AlGaN/GaN:- lower mobility than III-V's- rather moderate ∆EC but extremely high nS - WHY ?- reason: polarization effects 41





1 9 5 0

1 9 6 0

1 9 7 0

1 9 8 0

1990

2000

Invent ion ofthe BJT Bas ic H B T P a t e n t

F i r s t G a A s M E S F E T

B a s i c H E M T P a t e n tF i rs t HEMT

R F I I I -V HBTsF i rs t pHEMT

F i rs t S iGe HBTF i r s t m H E M T

II I -V HBT withfm a x > 8 0 0 G H z

First Ge BJT with GHz Operation

RF Wide BandgapFETs (SiC, GaN)

1980 Only two types of RFtransistors available:Si BJTs (fop up to 4 GHz)GaAs MESFETs (fop 4-18 GHz)

2004 Many different types of RF transistors available:Bipolar: Si BJTs, SiGe HBTs,

III-V HBTsFET: GaAs MESFETs,

III-V HEMTs,Wide Bandgap HEMTsSi MOSFETs

Frequency LimitsIII-V FETs: fmax > 600 GHzIII-V HBTs: fmax 1.1 THz

History and Evolution of RF Transistors

43

Important Trends:

• Continuous improvement of transistor performance - Continuous increase of the frequency limits fT and fmax

- Increase of the output power at a given frequency- Decrease of the minimum noise figure at a given frequency

• During the last 10 years: Shift of the applications of RF systems from defense and space applications to commercial mass markets- RF is becoming mainstream- Most commercial applications are in the lower GHz range

• Development of low-cost RF transistors for mass consumer markets.- Growing role of of Si-based RF transistors - For mass markets, cost is an extremely important issue – and Si technology is less expensive that any other semiconductor technology.

Trends in the Evolution of RF Transistors

44

Continuous increase of the frequency fT and fmax .


45

1960 1970 1980 1990 20001

10

100

1000

InP HBT

*

*

Transferred substratefT

fmax

AlGaAs/GaAs HEMT

InP HEMT

Si BJT

AlGaAs/GaAs HEMT

GaAs pHEMTInP HEMT

InP HBT

GaAs MESFET

Ge BJT

f max

, f T

, G

Hz

Year

Evolution of the frequency limits of RF transistors


46

Output power of RF transistors vs frequency1970: Only Si power BJTs for RF available 2003: Si BJTs, different HBT types, and different FETs types available

1 10 10010-3

10-2

10-1

100

101

102

103

104

Si BJT 1970 Si BJT 2003 FETs 2003 HBTs 2003

Out

put p

ower

, W

Frequency, GHz


47

Minimum noise figure of low-noise RF transistors vs frequency1970: Only Si power BJTs for RF available1980: GaAs MESFET least noisy 2003: InP HEMTs least noisy

1 10 1000

1

2

3

4

5

6

2003

1980

1970

1970 (Si BJTs) 1980 (GaAs MESFETs) 2003 (Lower Limit

InP HEMTs)

Min

imum

Noi

se F

igur

e, d

B

Frequency, GHz

Documents

1 Rf Devices Basics