hemt for low noise applications -

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Higher Institute for Applied Science and Technology

Department of Communication Engineering

HEMT for Low Noise Applications

Seminar

Massaken BarzehDamascus, Syria

Prepared byHasan Ahmad

Supervisor: Dr. Khaled YazbekCoordinators:Dr. Nizar Zarka & Mrs. Nada Mohanna

27 March 2013, Second Semester


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Choose a job you love, and you will never have to work.Confucius


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Preface

The real benefit of a seminar report comes when other Engineering students build upon it in a

certain related work. The benefit becomes greater in the absence of sufficient resources (Books,Articles and Theses). Taking this into account I tried to address a communication Engineering

student with Basic physics knowledge. I would be delighted to help any colleague making use ofthis report.

2/9/2013

Hasan Ahmad

Acknowledgement

I wish to express my appreciation to my supervisor, Professor Khaled Yazbek, for his invaluable

guidance during the research work and the writing of this report. I would also like to thank

Professor Nizar Zarka and Professor Nada Mohanna, seminar coordinators for their advices and

their hard work to ameliorate our skills.


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Abstract

HEMT excels in low noise applications at high frequencies. In this report we review theprinciples of HEMT, The development of different HEMT types and HEMT scaling. HEMTs

applications and the state of the art HEMT-based MMICs are demonstrated. The noise

characteristics of different HEMTs technologies are investigated. We also present the promising

advantages of GaN-HEMT.


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Table of Contents

List of Figure............................................................................................................................................... viiList of Table ................................................................................................................................................ viiList of Abbreviations ................................................................................................................................. viii

1 Introduction ................................................................................................................................................ 1

2 The High Electron Mobility Transistor ...................................................................................................... 1

2.1 History of HEMT: ................................................................................................................................... 12.2 Device Overview: ................................................................................................................................... 22.2.1 Indium Phosphide (InP) HEMT: .......................................................................................................... 2

2.2.2 Gallium Nitride (GaN) HEMT ............................................................................................................. 3

2.3 Important Concepts and Definitions: ...................................................................................................... 4

2.3.3 Conduction in semiconductors ............................................................................................................. 52.3.3.1 Electrons distribution: ....................................................................................................................... 5

2.3.3.2 Doping: ............................................................................................................................................. 52.3.4 Quantum Well and the formation of 2DEG: ........................................................................................ 5

...................................................................................................................................................................... 6

2.4 Device operation ..................................................................................................................................... 6

2.4.1 Small Signal Modulation: .................................................................................................................... 72.4.2 Cut-off frequency ft ............................................................................................................................. 22.4.3 Maximum oscillation frequency .......................................................................................................... 2

2.4.4 Summary of Device operation ............................................................................................................. 2

3 Device development and performance optimization .................................................................................. 4

3.1 HEMT scaling: ........................................................................................................................................ 4

3.2 Low noise optimization ......................................................................................................................... 113.2.1 Fukui formula ..................................................................................................................................... 113.2.2 Low noise bias conditions .................................................................................................................. 12

4 HEMTS Applications ............................................................................................................................... 12

4.1 Low Noise Amplifier MMICs............................................................................................................... 12

4.1.1 Noise figure vs. frequency ................................................................................................................. 13

4.1.2 Noise figure vs. HEMT type .............................................................................................................. 144.1.3 Noise figure vs. gate length................................................................................................................ 14

4.2 Radiometry ............................................................................................................................................ 14

4.2.1 Systems for millimeter-wave imaging sensor .................................................................................... 15

5 State of the art HEMT-based MMICs ...................................................................................................... 17

5.1 LNAs Millimeter-Waves MMICs: ........................................................................................................ 175.2 state of the art SMMICs: ....................................................................................................................... 17

5.2.1 LNA SMMICs ................................................................................................................................... 17

5.2.2 670-GHz Down- and Up-Converting HEMT-Based Mixers ............................................................. 18

6 GaN is the future ...................................................................................................................................... 18

6.1 GaN-HEMT advantages ........................................................................................................................ 18

6.2 GaN Amplifier ...................................................................................................................................... 18

6.3 Noise figure ........................................................................................................................................... 196.5 GaN speed ............................................................................................................................................. 20

6.5 summary ................................................................................................................................................ 20

7 Conclusion ............................................................................................................................................... 20

8 Future works: ........................................................................................................................................... 21Glossary ...................................................................................................................................................... 21Bibliography ............................................................................................................................................... 22

Appendix A: Spectrum Chart ............................................................................................................... 24


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Appendix B: Noise figure vs. frequency references ............................................................................. 24

List of Figure

Figure 1: Typical InP-HEMT structure. ........................................................................................................ 2

Figure 2: Typical GaN-HEMT. ..................................................................................................................... 4Figure 3: Lattice-mismatching ...................................................................................................................... 4Figure 4: Heterojunction Bands .................................................................................................................... 6

Figure 5:Idealised HEMT I-V characteristics ............................................................................................... 6

Figure 6:I-V characteristics showing Kink Effect. ....................................................................................... 6

Figure 7: Extrinsic equivalent circuit, including the parasitic resistances, inductances and capacitances

arising from the contacts ............................................................................................................................... 7

Figure 8: Gate length vs time. [30] ............................................................................................................... 4

Figure 9: ft vs. frequency [14] .................................................................................................................... 11Figure 10: Schematic diagram of 1 stage cascode amplifier ....................................................................... 13Figure 11: NF vs. frequency ....................................................................................................................... 13

Figure 12: Block diagram of Fujitsu imaging system ................................................................................. 16

Figure 13:Passive millimeter-wave image of a concealed metal object shown next to a photo ............... 16Figure 14: Chip photograph of four-stage 460 GHz mHEMT amplifier .................................................... 17 Figure 15:Future of GaN [8] ....................................................................................................................... 20

List of Table

Table 1: The effects of Circuit elements . ..................................................................................................... 3Table 2:Comparison of 100 nm HEMTs noise figures at 94 GHz .............................................................. 14

Table 3: NF vs. gate length ......................................................................................................................... 14

Table 4:Radiometry applications [19] ......................................................................................................... 15

Table 5: State of the art LNA amplifiers..................................................................................................... 17

Table 6:Comparison of Physical properties of GaN with other materials [ 8]. ............................................ 18

Table 7:Advantages of GaN Amplifier [8]. ................................................................................................ 19Table 8: Comparison between noise figures of published GaAs and GaN LNAs [22]............................... 19
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List of Abbreviations

FET: Field Effect Transistor

MOSFET: Metal Oxide Semiconductor Field Effect Transistor

MESFET: Metal Semiconductor Field Effect Transistor.

HEMT: High Electron Mobility transistor

pHEMT: pseudomorphic High Electron Mobility transistor .

mHEMT: metamorphic High Electron Mobility transistor.

InP-HEMT: Indium phosphide High Electron Mobility transistor

LNA: low noise amplifiers

NF: Noise figure

MMIC: Microwave monolithic Integrated Circuit

SMMIC: sub-Millimetre wave monolithic integrated circuit

2DEG: Two-Dimension Electron Gas

MOCVD: Metal Organic Vapour phase epitaxy

PMMW: Passive millimeter-wave image


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1 Introduction

In communication systems there is an ever increasing need for higher power and higher speeds. Thegrowth of multi layers of different semiconductors (hetrostructures), resulted in the first simultaneously

high power and high speed structures. Among hetrostructure devices HEMTs show the best noisecharacteristics [1]. These features Makes HEMTs the optimal candidate for sensitive application such asreceivers. In this report we try to relate the parameters of HEMT with it characteristics in a way clearenough to understand the main methodologies adopted in the development of HEMTs. We willdemonstrate different applications of HEMTs with focus on low noise applications and finally anoverlook on the future of HEMTs.

2 The High Electron Mobility Transistor

In this section, a review of HEMT basics and development is presented.

2.1 History of HEMT:

Two years after the invention of FETs (Field Effect Transistors ) in 1960, The MOSFET (MetalOxide Semiconductor Field Effect Transistor) was invented by engineers Steven Hofstein and

Frederic Heiman at RCA's research laboratory in Princeton [2]. Although slower than a bipolar

junction transistor, a MOSFET was smaller and cheaper and uses less power. The efficiency ofoperation and ease of fabrication of FETs enhanced the development of a faster Transistor, the

MESFET( MEtal Semiconductor Field Effect Transistor). The insulated oxide gate of the

MOSFET was replaced by a Schottky gate increasing the transconductance and hence allowing

the MESFET to operate at higher frequencies (up to 40 GHz).

The genesis of the HEMT in 1979 was in many ways accidental. At the time, Mimura, theinventor of HEMT, was working on GaAs MOSFET development for high-speed logic. The

transport velocity limitations of the MOSFET prevented Mimura from making a progress in his

work, so he began working on the Modulation-doped heterostructures which was earlierpresented in 1978 by Bell Labs. The first structures to simultaneously exhibit high electron

density and high mobility. As a result, the first HEMT logic circuits were reported in 1981, and

the first low-noise amplifiers entered commercial production in 1985. [3].

The primitive designs used Gallium Arsenide AlGaAs/GaAs structures. Later on wider band

gaps and higher carrier mobilities accompanied the use of indium compositions AlGaAs/InGaAs

pseudomorphic HEMTs (pHEMTs), AlInAs/ InGaAs/InP HEMTs (ordered by increasing ft).

However, HEMTs mainly found military and space applications. Only in the 90s the technologyentered the consumer market in satellite receivers and emerging mobile phone systems [4].

The 90s witnessed the emerge of new HEMT technologies, the metamorphic growth of

InGaAs/GaAs which enabled the production of MHEMT (Metamorphic HEMT), and the new

methods for deposition of GaN on sapphire by MOCVD(Metal Organic Vapour phase epitaxy)[5] which enabled the production of AlGaN/GaN-based HEMTs.


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In The last decade, the frequency performance of GaN HEMTs has increased steadily an fTof

153 GHz and an fmax of 230 GHz in HEMTs with Lg= 100 nm was reported [6].

Technical issues such as drain current collapse prevented the mass production of GaN HEMTs

[7].Whilst the Optimization of InP-Based HEMT resulted in a cut-off frequency of 644 GHz in

2010(MIT) and subsequently 660 GHz in 2011(Fraunhofer Institute) using high indium contentmetamorphic HEMT technology.

Recently, Fujitsu developed a new technology for GaN HEMT that provided suppression of gate

leakage and current collapse and proved the ability of mass-production of GaN HEMT [8].

2.2 Device Overview:

HEMT consists of a multi-layer stack of semiconductor materials. Like all FETs, HEMT relies

on the application of a voltage difference between source and drain to create a current flow in a

channel region. For a given source-drain voltage, the electron population of this channel, andhence the current flow, is then controlled by the application of a gate voltage.

To meet the desirable material properties, four structures were mainly introduced throughout thedevelopment of HEMT, Gallium Arsenide (GaAs-HEMTs), Indium Phosphide (InP-HEMTs),

high indium content metamorphic HEMTs and Gallium Nitride based GaN-HEMTs. The

development of channel growth techniques enabled the robust operation of the higher electronmobility InP-based HEMTs. However, mHEMTs offers the possibility of combining the advantages oflow-cost and manufacturability of GaAs substrates and the high performance of InP-based devices. Thatexplains why mHEMTs received the focus of researchers and industrial groups. The GaN-HEMT havesimilar noise figures to other HEMTs but with higher power characteristics which nominates it to be thechoice of the future.

2.2.1 Indium Phosphide (InP) HEMT:

Figure 1: Typical InP-HEMT structure.


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Figure 1 shows typical InP-HEMT structure .The cap layer is highly-doped and allows the

formation of low-resistance source and drain contacts for higher cutoff and maximum oscillation

frequencies (section 2.4). The gate is defined on the surface of the undoped barrier layer. Therole of the thin spacer (few nm) layer is to enhance electron mobility and confinement in the

channel.

The Silicon -doping layer is highly doped to provide the high densities of accumulation electronin the channel, resulting in higher current densities and higher cutoff frequency , thus InP-HEMT

have Excellent power performance at high frequency.

The source and drain contacts are metallic, the gate is also metallic, but forms a Schottky contact

to the barrier.

The buffer layer has the dual purpose of providing a high-quality surface with a lattice constant

convenient for the channel growth, and to provide electron confinement in the channel,

preventing real space transfer of electrons from the channel.

The technique used for the growth of the channel on the buffer determines the device type. Thechannel can be pseudomorphically grown when its lattice-mismatched with the buffer

(Pseudomorphic HEMT), or Metamorphically grown When the buffer is gradually changing

lattice to match the channel (Metamorphic HEMT).

The T-gate structure reduces noticeably the gate resistance and the external electrostatic

capacitances [9], yielding a higher fmax (Maximum Oscillation frequency). A dielectric

passivation layer is grown on the transistor; this provides electrical stability by isolating the

transistor surface from electrical and chemical conditions in the environment.

Its good to note that The GaAs-HEMT is structured in the same way of an InP-HEMT.

2.2.2 Gallium Nitride (GaN) HEMT

As discussed in section 1.3.4, the 2DEG (Two-Dimension Electron Gas) is formed due to the built-in

piezoelectric field and thus there is no doping in the AlGaN layer (). Higher 2DEG concentrations areachievable due to the very large conduction band discontinuity. [10]


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Figure 2: Typical GaN-HEMT.

2.3 Important Concepts and Definitions:

To understand the magic of HEMT, some basic knowledge about its material physics is required.We try to simply demonstrate the basic concepts in this section.

2.3.1 Heterostructurs:

The development of epitaxial growth techniques throughout the last two decades of the

twentieth century brought the capacity to grow multiple layers of dissimilar semiconductors,

known as heterojunctions or heterostructures, allowing the vertical engineering of electronic

devices with atomic-level precision; a capability on which devices such as the HEMT are

founded.

The Lattice-mismatching ( Figure 3) between consecutive semiconductors layers results in a strain on the

structure. The strain can cause poor surface morphology hindering device realization.

Figure 3: Lattice-mismatching


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It is important to note that the strain can be managed to improve the electron transport.

2.3.3 Conduction in semiconductors

2.3.3.1 Electrons distribution: Electrons are fermions therefore, the probability of electron

filling an Energy state follows Fermi Dirac distribution [11] . Fermi level, Ef , describes the energy atwhich this probability is one half. The Fermi energy, given by Equation 1 , isusually positioned in the center of the bandgap (band comprised between conduction and valence bands)

in intrinsic semiconductors:

Equation 1: Femi Energy.

Where and are the conduction and valence band minimum and maximum, respectively. K is theBoltzmann constant (1.3806503 10-23 m2 kg s-2), T is temperature, and Nc and Nv are the Conductionand valence band densities of states. [3].

2.3.3.2 Doping: In un-doped semiconductor, electrons fill lower Energy levels leaving theconduction band empty. Doping increases the charge carriers densities, either holes p-type

doping or electrons n-type doping. Electrons are faster than holes thus, HEMTs are n-type toobtain higher carrier mobility.

2.3.4 Quantum Well and the formation of 2DEG:

The simplest way to detect the formation of the quantum well is to begin with the constancy of

the Fermi level of a system in equilibrium.

Electrons are transferred from the higher-lying conduction band to the lower conduction band ,till the bending of the bands due to this transfer aligns the Fermi levels of the two constituents.

Such alignment is illustrated inFigure 4. [12]

In other words, a quantum well of dimensions similar to the wave length formed at the interface

confine the electrons in high densities.

The confinement is perpendicular to the interface thus, electrons are at liberty to move in only intwo dimensions .And a Two Dimensional Electron Gas 2DEG is formed .

In InP-HEMT the accumulation electrons in the channel comes from The Silicon -doping layer.

While in GaN-HEMT the strain results in a piezoelectric field which accumulates the electrons indensities higher than the densities in InP-HEMT.


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2.4 Device operation

At zero gate bias, the channel is populated with a high-density 2DEG resulting from the channel

doping. . A positive gate bias will increase channel population While it becomes completely

depleted(Pinched-off) for some given negative threshold vth .Figure 5 shows the Theoretical Ids-Vds curve. However, In The real operation , An unpredictable drain current increase appear For

a certain drain-voltage such phenomenon is called The kink effect (Figure 6) .

Figure 4: Heterojunction Bands

Figure 5:Idealised HEMT I-Vcharacteristics

Figure 6:I-V characteristics showing Kink Effect.


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To avoid the non-linearity introduced by Kink effect, we are interested in low-field condition

corresponding to small signal input. In which the electron velocity varies linearly with electric

field until saturation.

2.4.1 Small Signal Modulation:

In order to represent the real operation of a HEMT the intrinsic Theoretical Schematic isinsufficient. The lumped elements model should take into consideration the parasitic elements

which appear at high frequencies and the electrostatic capacitances.

The resulting Modulation circuit is shown in Figure 7.

The theoretical limits between the intrinsic and the extrinsic model end when the parasitic source

resistance Rs appears in the intrinsic transconductance relation.

gmo= ()Equation 2:Intrinsic Transconductance.

Cs is the 2DEG-gate capacitance defined by the gate-channel separation.

Equation 2 Clarify how the reduction of the channel gate separation and the source resistance

would boost the transconductance. [3].

Figure 7: Extrinsic equivalent circuit, including the parasitic resistances, inductances and

capacitances arising from the contacts


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2.4.2 Cut-off frequency ft

At the cutoff frequency the current gain falls to unity. the extrinsic ft is defined as [3] :

Equation 3: Extrinsic Cutoff frequency

Equation 3 clearly shows that Electrostatic capacitances and parasitic elements decreases cutoff

frequency. It also demonstrates the importance of increasing the intrinsic transconductance gm0.

An Equivalent Equation offt demonstrate the general need to increase electron velocity and

reduce gate length

Equation 4: ft vs gate length. vsatis the saturation velocity, is the doping efficiency percentage.

2.4.3 Maximum oscillation frequency

The Expression of the maximum frequency of oscillation as given in [3] is maximized by

increasing output resistance, cutoff frequency and minimizing gate resistance

(Equation 5). However, the two latter requirements contradicts at some point (section 2)

fmax=

Equation 5: Maximum oscillating frequency

2.4.4 Summary of Device operation

The effect of each circuit element is shown in Table 1. Red color signifies that decreasing the

element value optimize the performance. While green color signifies the contrary.


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Table 1: The effects of Circuit elements .

Circuit elements High Power High Frequency

Intrinsic

Transconductance

CutOfffrequency

Maximumoscillationfrequency

Intrinsic

Elements

Cgd

Cgs

Cds

Cs

Ri

Rds

Extrinsic

Elements

Lg

Rg

Rd

Rs


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3 Device development and performance optimization

One way of optimizing HEMT was the use of different materials which resulted in the types

discussed previously. Further discussion of the material properties like doping and alloying

percentage can give better comprehension of the operation of HEMT. However, The essential

optimization technique adopted in the history of HEMT is HEMT scaling .

3.1 HEMT scaling:

Shorter gate lengths have the dual advantage of achieving higher cut-off frequency and higher

integrity .Unfortunately that accommodate decrease in gate resistance slowing down fmax . The

most common solution is the use of T-shaped gate.

Channel separation discussed in section 1.4 is no longer an optional optimization. For a given

gate length, only one gate-channel separation is optimal for the device. Therefore a verticalscaling of the HEMT including the barrier, spacer and channel thicknesses is required and The

HEMT must be fully scaled. T-shaped gate length reduction in the last decade is shown in Figure

8 .

Using HEMT scaling a maximum oscillating frequency of 1.1 THz [13]. And a cut-off frequency

of 688 GHz was achieved. Figure 9 shows the development of cut off frequency till 2011 [14].

Figure 8: Gate length vs time. [30]


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3.2 Low noise optimization

The noise of an amplifier is characterized by the noise figure Fwhich is defined as the signal-to-noiseratio at the amplifier input divided by the signal to noise ratio at the output.

3.2.1 Fukui formula

An approximation of the minimum noise figure Fmin as a function of HEMTs parameters can be obtainedfrom Fukui formula (Equation 1).

Equation 6: Minimum noise figure.

KG is a fitting factor which takes into account the properties of channel transport. The physical

effects which make one kind of HEMT of a certain dimensions superior to a HEMT withanalogous dimensions of another type are contained in this factor.

Figure 9: ft vs. frequency [14]


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3.2.2 Low noise bias conditions

While higher frequencies requires small Cgs values (Equation 1). Fukui formula shows that the

minimum noise figure corresponds to the minimum CGS values. This contradiction makes the

choice of proper voltage bias which corresponds to an appropriate capacitance value important.

So we have a tradeoff between frequency gain and noise.

4 HEMTS Applications

HEMTs applications map the Microwave and the terahertz frequency ranges. HEMT is a crucialcomponent in different MMICs for Radar, Satellite and telecommunication applications. Currently themost developed HEMT-based circuits are for millimeter-waves applications, HEMT based low noise

amplifiers are the key components in the millimeter-waves sensing systems which requires low noise andhigh sensitivity.

In the recent years terahertz frequencies (300GHz-3Hz) are of a growing interest for atmospheric scienceand astronomy applications which is mainly about remotely analyzing the atmosphere and surfacecomposition of planets or their moons. These applications usually require converting the terahertz signalto a lower IF(intermediate frequency) for processing. Vice versa, transmitting generally requires up-conversion from IF to the RF. The current state-of-the-art device for performing the frequency conversion

is based on Schottky diode mixers. Schottky diode technology requires separate MMICs for Amplifierson contrary to HEMT amplifiers, hence using all-HEMT circuits, would crucially simplify circuitstopology.

One of the mostattractive features of millimeter and sub millimeter waves is the transparency of

the atmosphere at the atmospheric windows, 94, 140, 220, 340, 410,480, and 670 GHz. This

explains why a lot of research groups design their MMICs at these frequencies.

In this section: HEMT low noise amplifiers, Noise characteristics and we focus on radiometryapplications.

4.1 Low Noise Amplifier MMICs

Amplifiers are essential components in the different parts of communication systems. Low noise

amplification is particularly required in Receivers to handle the weak received signals due toattenuation in the free space or the transmission line.

A Cascode configuration is usually utilized in HEMT-LNAs since it demonstrates a superior

gain performance compared to conventional HEMTs in common source configuration.Figure 10

Shows a schematic diagram of a single cascade amplifier.


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4.1.1 Noise figure vs. frequency

Figure 11 shows the increment of noise figure of different HEMT types for increasing frequencies. Thiscan be intuitively predicted as a result of the increase in scattering due to collisions at higher speeds. Theincrement in the noise figure is due to the smaller values of C gs at higher frequencies. Appendix Bcontains the references of data used in the figure.

We can predict from Figure 11that GaAs-mHEMTs have the best noise characteristics followed by Inp-HEMTs. But the plotted data corresponds to transistors with different gate lengths and it needs furtherinspection.

Figure 11: NF vs. frequency

0

1

2

3

4

5

6

7

0 50 100 150 200 250 300

NF

frequency

GaAs-pHEMT

GaAs-mHEMT

Inp-HEMT

Figure 10: Schematic diagram of 1 stage cascode amplifier


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4.1.2 Noise figure vs. HEMT type

To investigate the HEMT type with the best noise characteristics we compare the noise figures of GaAs-pHEMT mHEMTs and Inp-HEMTs for a fixed gate length of 100 nm at a fixed frequency

Table 2:Comparison of 100 nm HEMTs noise figures at 94 GHzFrequency(GHz) NF(dB) Gain(dB) Gate Length(nm) HEMT REF

94 1 7.5 100 Inp_HEMT [15]94 3.2 16 100 InP-HEMT [16]94 4.3 19 100 InP-HEMT [16]94 4.8 14 100 GaAs-mHEMT [16]94 5.5 13.3 100 GaAs-pHEMT [16]94 4.7 14.8 100 GaAs-mHEMT [16]94 2.5 22 100 GaAs-mHEMT [17]

GaAs-pHEMT have the highest noise figure as predicted. While the data seems ambiguous and cant tellthe best among GaAs-mHEMT and Inp-HEMTs. Although Fujitsu Inp-HEMT shows the lowest noise

figure of 1 dB. It doesnt offer the best gain-NF tradeoff. The Gain 22 dB of fraunhofers GaAs-mHEMTcorresponding to NF=2.5dB can be more convenient for certain applications. This ambiguity makes sense

because Table 2 doesnt take into consideration the percentage of indium content in the GaAs-mHEMTschannel, which should be calibrated to obtain the best noise figure [18].

4.1.3 Noise figure vs. gate length

Shorter gate lengths yields smaller gate resistance Rg , and consequently smaller noise figures. Table 3

Shows smaller noise figures of a 50nm GaAs-mHEMT comparing to 100nm GaAs-mHEMT.

Table 3: NF vs. gate length

Frequency [GHz] NF(dB),Lg=100 nm NF(dB),Lg=50 nm

94 2.5 1.9

210 7.4 4.8

4.2 Radiometry

Unlike in active sensing (such as Radar) where we transmit signals and receive echo from thetarget to obtain information about it. In Radiometry the sensor receives waves emitted from theobject. The amplitude of the radiation depends on the objects emissivity and temperature. Since

Radiometric sensors do not need a source, the system block is simple when compared with active

sensing and since there is no transmitting.Table 4resumes the numerous applications of

Radiometry.


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Figure 12: Block diagram of Fujitsu imaging system

Figure 13:Passive millimeter-wave image of a concealed metal object shown next to a photoand IR image


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5 State of the art HEMT-based MMICs

In this section we present the most advanced millimeter MMICs and sub millimeter MMICs.

5.1 LNAs Millimeter-Waves MMICs:

Monolithic Millimeter-Waves integrated Circuits operate at frequency range 30-300 GHz. At

these very high frequencies The RF interconnects are of special importance.

Because of its good isolation and small dimensions, grounded coplanar waveguides (GCPW) areusually used as transmission lines within the MMICs, enabling successful suppression of

unwanted substrate modes.

Faurnhofer institute has reported the state of the art amplifiers. Although they couldnt

measure the noise figure due to the lack of noise source, the simulated noise figure along with

measured gain at Millimeter wave frequencies are promising for the future use in active and

passive high-resolution Imaging applications [20] [21].

Table 5: State of the art LNA amplifiers50 nm 35 nm

Gain(dB) 15 20

Frequency Range(GHz) 240-320 220-320

Simulated NF(dB) 7.3 ______

5.2 state of the art SMMICs:

5.2.1 LNA SMMICs

Using common source configuration a four stage 460 GHz amplifier circuitwas designed (Figure14) revealing the high-frequency performance of the 35 nmgate length mHEMTs. Reasonable

bandwidth and highSmall-signal gain in the WR-2.2 waveguide band (325 to 500 GHz) Were

achieved. [20]

Figure 14: Chip photograph of four-stage 460 GHz mHEMT amplifier


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5.2.2 670-GHz Down- and Up-Converting HEMT-Based Mixers

As mentioned in section 4 Using all-HEMT circuits instead of Schottky-based circuits, would result much

more ease of integration .NASAs Jet Propulsion Laboratory, reported the most advanced pHEMTbased Mixers operating at the atmospheric window 670-GHz.

6 GaN is the future

To demonstrate that GaN is the future we discuss power, frequency and noise characteristic of GaN-

HEMT.

6.1 GaN-HEMT advantages

The wide band gab of GaN allows the realization of high voltage amplifiers up to 50 v GaN-HEMTs.

GaNs physical properties (Table 6) also make it suitable for high speed operation .

Table 6:Comparison of Physical properties of GaN with other materials [8].

Since the Si devices operates at 28 V which is lower than the base station system power of 48V,An

immediate application of GaN-HEMT is to replace the si devices at base stations, eliminating theconversion power and achieving smaller Base stations with lower power consumption.

6.2 GaN Amplifier

Although The GaN Amplifier is theoretically superior to Inp-HEMT Amplifiers, it cant replacethe Inp-HEMT yet, its frequency range is still much lower than Inp-HEMTs, and only recently

noise figures competitive to GaAs-mHEMTs were achieved [22].


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Advantages of GaN Amplifier are shown in Table 7

Table 7:Advantages of GaN Amplifier [8].

6.3 Noise figure

Noise figures of GaN-HEMT LNAs are similar to other HEMT types. Although the numbers achieved inGaAs HEMT are better, GaN HEMT is still under development and the emerging reports are proving its

good noise characteristics table 8 shows different noise figures of published GaAs and GaN LNAs andparticularly a noise figure of 0.5 was achieved [22] which is competitive to GaAs-HEMTs LNAs.

Table 8: Comparison between noise figures of published GaAs and GaN LNAs [22].


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6.5 GaN speed

In 2011, HRL laboratories reported a cutoff frequency of 200 GHz and a maximum oscillating frequencyof 300 GHz. In 2012 a cut-off frequency of 370 GHZ was reported [ 23].The speed of GaN is steadily

improving

6.5 summary

From the previous discussion. The GaN-HEMT RF characteristics are improving to meet Inp-HEMTcharactersitics and taking into account its better power performance it is expected to be the denominating

7 Conclusion

We have reviewed the development of HEMT demonstrated its applications. The state of the art cut-offfrequency of 688 GHz, 460 GHz low noise amplifier, 670 GHz Mixers are presented. The results

comparison of published data is consistent with Fukui formula. The noise figure increases with frequency,and decreases with gate length. GaAs-mHEMT and Inp-HEMT have comparable noise figures and they

both show better noise characteristics than pHEMTs. The published data also indicates that GaN-HEMTwill dominate in the future.

Figure 15:Future of GaN [8]


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8 Future works:

1- HEMTs Gain and power consumption.2- Cryogenically cooling for lower noise figures.3- GaN HEMTs Development.

Glossary

Lattice constant: Semiconductors have crystalline Structure in which atoms are arranged in unit cells(smallboxes). Lattice constant is the distance between these unit cells.

Real-space transfer: describes the process in which electrons in a narrow semiconductor layer,accelerated by an electric field parallel to the layer, acquire a high average energy (become hot) andthen spill over an energy barrier into the adjacent layer.

Transconductance: the ratio of the current change at the output port to the voltage change at the input

port. It is written as gm.

Atmospheric window: A range of electromagnetic wavelengths to which Earth's atmosphere is largely orpartially transparent.
http://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Voltage


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Appendix A: Spectrum ChartSuper high Frequencies (SHF)

(Microwave)3 GHz to 30.0 GHz

C-band 3600 MHz to 7025 MHz

X-band: 7.25 GHz to 8.4 GHz

Ku-band 10.7 GHz to 14.5 GHz

Ka-band 17.3 GHz to 31.0 GHz

Extremely High Frequencies (EHF)

(Millimeter Wave Signals)30.0 GHz to 300 GHz

Additional Fixed Satellite 38.6 GHz to 275 GHz

Infrared Radiation 300 GHz to 430 THz

Visible Light 430 THz to 750 THz

Appendix B: Noise figure vs. frequency referencesFrequency[GHz] NF(dB) Ref

GaAs-pHEMT GaAs-mHEMT Inp-HEMT

0.9 0.7 [24]

1.9 0.4 [24]

6 0.5 [24]

8 1.2 [25]

9 0.85 [24]

20 1.5 [24]22 1.1 [24]

34 1.5 [24]

35 2.3 [24]

14 1.5 [24]

5 1 [24]

90 2.8 [24]

44.5 3.2 [26]

85 3.5 [27]

94 1.9 [17]

130 4.5 [24]210 4.8 [21]

215 5.2 [28]

243 6.1 [29]

hemt for low noise applications -

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