RF ELECTRONICS APPLICATIONS

PRESENTATION #1

RF TRANSISTOR STRUCTURES

MUSA ÜŞÜMÜŞ 040070363

AYDIN GÖKHAN KAHRAMAN 040070313

ALİCAN UYSAL 040080337

In high frequency applications controlling of the transistors are not easy then at low

frequencies. RF Transistors are very complicated and it has a lot of differet types .Here we

will discuss some usable transistor types in high frequency RF applications.

RF SMALL SIGNAL TRANSISTOR MESFET

MESFET is short name of metal semiconductor field effect transistor or Schottky gate FET

(Schottky is the special name of metal semicunductor). And it has big affinities with JFET.

MESFET and JFET are classified as p+n junction FET with p

+→∞. Because of electron

mobility is bigger than hole mobility, all types of MESFET are classified as n-type channel.

The isolated gate JFETs have a isolator under the gater over the active switching region but

the MESFETs haven’t. That is important difference between to seperate them. One of the

most significant properties in design is the gate metal extent over the switching region.

MESFET is used in different areas such as mobile phones and microwawe devices. The most

used material is GaAs in MESFET and GaAs has high level electron mobility. (8000 cm2/VS)

MESFETs are known as voltage controlled like JFETs. If we want to do a compare between

MESFET and BJT, we can see this result.

MESFET is generally generated in complex semiconductor technologies lacking high

quality surface passivation for example; GaAs, InP or SiC. These technologies are faster than

silicon based MOSFET and JFET technologies but MOSFET and JFET are cheaper than

MESFETs. Manufacturing of MESFET is usually done at 45 GHz. If the working frequency

is higher than 2 GHz, GaAs is preferred. On the other hand if the working frequency is less

than 2 GHz, Si transistors are used in this technology.

The deficit of MESFET structure is generating from Schottky Metal Gate. Schottky Metal

Gate restricts the forward bias voltage on the gate to the turn-on voltage of the Schottky

diode.

RF SMALL SIGNAL TRANSISTOR HEMT

HEMT is the short writing of High Electron Mobility Transistor. It is known as

Heterostructure Field Effect Transistor(HFET) or Modulation Doped Field Effect

Transistor(MODFET).Generally we can identify HEMTs as field effect transistors what the

current goes among two resistant parts drain and source and the control part which name is

gate instead of doped region. Schottky gate is usually used for controlling. While the

MESFETs are processing ion transporting, the HEMTs to supported by different band gaps Eg

with grown layers. AlGaAs and GaAs are used in this transistor generally. And GaSb,

InP,InAlP,AlAsP,GaP are the different types. The HEMT invented by Takashi Miura from

Japan but, the team where worked in Bell Laboratories which managed by Ray Dingle have a

lot of important reseraches about invention of HEMT in America. The patent of HEMT

possess by Daniel Delagebeaudeuf and Trong Linh Nuyen in Europe. If there is lacking of

Gallium Nitride(GaN) compound, it necessitates heteroepitaxy on compatible substrates, with

sapphire and SiC. On the other hand, AlN, Silicon and compound oxides can be used. The

depositing temperature is low level approximately 600C to nucleation layer forms from GaN

and AlN when the sapphire is used for material. After this process, the heat is increased for

main layer. The capable temperature for AlGaN and GaN layers is 1000C while the growth

rate is 1m/hr. There are HEMTs two different versions: pHEMT and mHEMT. In p type two

different materials is used in transistor to have same lattice constant and in m type the lattice

constants are different and that situation is provided by placing a buffer between them. p is the

capital letter of pseudomorphic and m is the short writing of metamorphic. In m, the used

buffer is generally generated from AlInAs substract. HEMT is used different areas such as

radar, imaging, microwawe, radio astronomy etc. While the frequency is bigger than 600 GHz

HEMT has current gain and the frequency is bigger than 1 THz HEMT has power gain.

Transistor Structure

Cross section of a GaAs/AlGaAs/InGaAs pHEMT

Band Gap and Lattice Constant Graphic of 3 and 4 Group Semiconductors

Band structure in GaAs/AlGaAs heterojunction based HEMT

HEMT with delta doped upped barrier

EF: Fermi Level

EC : The conduction band of the channel relative to the Fermi level EF

ND : Doping Level

dGC : the gate to channel separation

qB: the barrier height of the Schottky contact

qVGS: The applied voltage on gate

Basic Structure of HEMT

RF SMALL SIGNAL TRANSISTOR HBT

The HBT is well known short name of heterojunction bipolar transistor and it is one of the

different types of BJTs. The HBT was invented by Hiroshi Fujioka in Tokyo,Japan in 1951.

Although it is a type of BJT, it has at least two semiconductors in its structure. And that

structure changes the all parameters of emitter, base and collector also. HBTs are usually

producted from AlGaAs and GaAs semiconductors. And they are used in analog and digital

microwave areas,modern ultrafast circuits,RF systems and required high power applicatios

etc. The cause of the BJTs are faster than silicon bipolar transistors that the BJTs decrease

base resistance and collector-to-substrate capacitance. When they are compared with GaAs

FETs, they need less lithography and result of that they have easier producting capacitance in

fabriques. And if we continue the this comparing, we can see the HBT technology has higher

breakdown voltage and simplier broad-band impedance. If we compare the HBTs with Si

BJTs, HBTs are more practical than Si BJT for emitter injection efficiency, base resistance,

base-emitter capacitance, cut-off frequency . In this figure below, you can see the this

comparison.

In these graphics below we can see the flatband energy band diagram and energy band

diagram under forward active bias. The HBTs production method is based on using

semiconductor GaAs material layers and this method can be divided two different ways

according to doing style. First, metal organic chemical vapor deposition (MOCVD) and

second molecular beam epitaxy (MBE).

From the figure above, we can see the structure of HBTs and in the n+ GaAs Collector

Contact is doped extremely(4x1018

cm-3

). The collector doping density 3x1016

cm-3

. In base

layer which p+ GaAs Base is highly doped(5x10

18cm

-3) , Berillium and Carbone are used in

doping the base layer. The layer which under of emitter n+ GaAs causes the low-resistance

ohmic contancts. If we add In material to this semiconductor and make a new alloy (InGaAs),

this new alloy provide stable, low-resistance emitter contacts. The HBTs increase to highest

level the compositonal grading base-emitter junction in two ways and increase the peak point

the electron injection efficiency and suppress hole injection to emitter from base. In base

contact metallizaion the AuBe/Pd/Au are used and AuGe/Ni/Au are used in emitter and

collector contanct metallization.

Flatband energy band diagram Energy band diagram under forward active bias

LATERAL-DİFFUSED MOSFET(LDMOSFET)

The Laterally Diffused Metax Oxide Semiconductor Field Effect Transistor(LDMOS) is an

asymetric power MOSFET .It is designed to form high blocking voltage and to reduce the

resistance.To form these features,there must be low-doped n type drain region in a diffused p

region.The charge in low-doped n region and the length from the drain to gate terminal must

be optimized to maximize the breakdown voltage. Charge of about 1 x is found

that it is optimal to get high breakdown voltage.

When positive bias voltage comes to the gate,the drain current flows.When a positive drain

voltage occurs,from source to the drain it provides a path for electron transport.The current

flows from the drain terminal to the source top metal.Then the current flows to the P+

Substrate region via P+ sinker.The resistance of the LDMOS is calculated by the following

equation..

Ron,sp = RCH,Sp + RD,sp + RSUBS,Sp

RSUBS,Sp:Substrate resistance

RD,sp:Drift region resistance

RCH,Sp :Channel resistance

LDMOS Structure

LDMOSFET CHARACTERISTICS

When a positive bias occurs to the gate of the LDMOS,an inversion layer occured on the p

region surface.This helps to the create a path for electrons between the drain and source

terminals.The current after the source use the P+sinker region to reach the P substrate.Also

from gate to the P-Epitaxial Layer there is current flowing.

LDMOSFET OUTPUT CHARACTERISTICS

The output characteristics of LDMOSFET is shown that below .The gate voltage is

ranging between 3.7 to 7.5V.If we look to the below,when the drain voltage increases,the

current is increasing so the output resistance is decreasing.

Output Characteristics

LDMOSFER RF PERFORMANCE

To get the RF performance of LDMOS an ac source is connected to the gate.The input RF

signal is below.Then the drain,output of the LDMOS shows exponential transient response

below.

RF İnput Gate Signal

RF Output Drain Signal

From 0.1 GHz to 16 GHz we see the current and power gain.Increasing of the frequency

decreases the gains.To increase the output power ,larger ac gate drive can be used.

Current and Power Gain

RF Output Power

VERTİCAL-DİFFUSED MOSFET(VDMOSFET)

From the figure below ,we see that N-Epitaxial Layer grown on a N+ Substrate.To attain

the desired breakdown voltage N-Epitaxial Layer concentration and its thickness must be

chosen.We see that the Drain Terminal of the VDMOSFET is at the bottom of the N+

Substrate.

Current flow starts while the positive bias occurs to the Gate Terminal.This results an

inversion layer on the P-Base under the Gate Terminal.This inversion layer create transport

of the electrons when Drain voltage occurs positive.The channel length must be long to

avoid reach-through of the depletion layer in the P-base region.The current from drain, reachs

source terminal through the N+ Substrate,N-Epitaxial Layer(Drift Region),P regions(JFET

Region,between P Regions) and inversion layer.The resistance of this ,is calculated by this

Formula below.

Ron,sp = RSUBS,SP + RD,sp + RJFEJ,sp + RCH,sP

RCH,sP:Channel Resistance

RJFEJ,sp :Resistance of JFET Region

RD,sp:Drift Region Resistance

RSUBS,SP:Substrate Resistance.

Vertical Diffused MOSFET Structure

VD MOSFET CHARACTERISTICS

When o positive bias occurs on the gate terminal,which is bigger than threshold voltage,the

inversion layer is formed at the surface of P- base region.This provides a path for current

flowing from Drain to Source terminal.The JFET Region(between P regions) presses the

source current.This results an on-resistance.

VD MOSFET OUTPUT CHARACTERISTICS

Gate bias voltages ranging from 3.5 V to 10 V, we see that the drain curret increases

nonlinear from the schematic below.Increasing of the gate voltage results increasing of the

transconductance.From the figure increasing of the temperature makes the decreasing of the

current.This feature is similar like LDMOSFET.Therefore this feature shows the resistance is

increasing with the increasing temperature.This is detrimental to RF Performance

Output Characteristics of the VDMOSFET

Transconductance of the VDMOSFET

VD MOSFET RF PERFORMANCE

To measure the RF performance of VD MOSFET ,the load line was chosen to get the

maximum drain voltage and drain current to get the maximum output power. The

power gain at 1 GHz is 13.5 dB.The reduction of the output power with the increasing

frequency can be seen from the figure below.At lower frequencies output power of the

VDMOSFET is similar to the LDMOSFET.The drop-off in power with frequency is more

gradual in VDMOSFET.Comparing of the output characteristics of the LDMOSFET and

VDMOSFET shows the VDMOSFET is much greater.In addition VDMOSFET is more

rugged from the avalanche energy.At lower frequencies these features made the VDMOSFET

structure attractive for pulse power and avionics applications.

Power Gain & Current Gain

RF Output Power

CHARGE-COUPLED VERTICAL MOSFET(CCMOSFET)

As shown in the figure below,trench electrode and gate electrode are shorted.This is

convenient because they are too close.This event increases the input capacitance.The device is

fabricated by starting a N-Eritaxial Layer grown on N+ Substrate.The channel is formed by

using P- base and its above N+ diffusion cycles.To obtain optimum charging ,the doping

concentration of the N-Epitaxial region and width of the trenches must be chosen.The drain

contact is located below the N+ Substrate.When positive biar voltage occurs on gate

electrode,the drain current flows.This results an inversion layer at the surface of the P- Base

region.When positive bias voltage occurs on drain terminal,this inversion layer results a path

for electron transport between the source terminal and drain terminal. The channel length in

must be sufficiently long to avoid reach-through of the depletion layer in the P-base

region.Simulations show that CCMOSFET Structure enable to support voltage within N-Drift

region by creating low electric field.This results reduction channel lenght,thus this reduces

resistance and makes high transconductance which is beneficial for high frequency

applications.The current follows the drain terminal,N+Substrate,Drift N Region, inversion

layer and then the top of the source terminal.The resistance is calculated by summing of these

ways.

Ron,sp = RSUBS,sp + RD,sp + RCH ,sp

RSUBS,sp =Specific Substrate Resistance

RD,sp=Specifif Drift Region Resistance

RCH ,sp =Specific Channel Resistance

CCMOSFET Structure

CCMOSFET CHARACTERISTICS

When a positive bias occured at the gate terminal above the threshold voltage, the

inversion layer is formed at the P-Base region surface.This provides a path between source

and drain terminals.

CCMOSFET OUTPUT CHARACTERISTICS

Output characteristics of CCMOSFET is shown below.Gate voltage is ranging from 1.5 V

to 9.5 V. We see from the figure, all current reaches saturation region approximately at the

same drain voltage.Thus the output resistance degrades.This shows this structure has poor

output resistance.From the figures the characteristics are more linear than

VDMOS,LDMOS.Howevet the increasing of the bias voltage makes this linearity bad.The

current decreases while the temperature increases.This feature is similar to VDMOS and

LDMOS.Reduction of the transconductance and resistance are detrimental to RF

performance.

Output Characteristics of CCMOSFET

Transfer Characteristics

Transconductance of CCMOSFET

CCMOSFET RF PERFORMANCE

At lower frequencies the output power obtained from the CCMOS is similar to

LDMOS.The good output power and RF gain show that this structure is used in RF Power

Amplifiers between 0.8 to 2.2 GHz.These features are also available for pulse power and

avionics applications.The output power is greater than LDMOS structure.

Power Gaion & Current Gain of CCMOSFET

Output Power of CCMOSFET

This structure enables to do super linear position by reducing the voltage developed across

the channel.This device shows that high RF gain and output power can be delivered by this

structure.

Comparision of Electrical Paramaters

We see that CCMOS Structure has the minimum resistance and transconductance as we

expect.We can guess the relationship maximum operating frequency between the

structures.CCMOS has the minimum output capacitance so its maximum operating frequency

is greater than the others.High compression current results high output power .

Comparision of MOSFET RF Performance

GaAs TRANSISTOR STRUCTURE

GaAs (galium arsenide) has been developed after silicon technology. Silicon and

Germaium are the most used materials in electronics due to their one type element structure.

Especially Si (silicon) is the easiest material to be found in nature used in electronics, which

makes its cost very low. However, when high frequency or low noise applications are needed,

Si or Ge becomes useless. Electron mobility, the major factor when reaching high

frequencies, must be high enough when dealing with RF waves. Mobility values at 25 degree

is given in table below.

Semiconductor material Si Ge GaAs

Electron mobility [m2/(V·s)]@ 25

°C

0.39 0.14 0.85

“Cited from http://en.wikipedia.org/wiki/Transistor”

As shown above, GaAs material has a very high advantage over other two. Electron

velocity vs electric field is shown below.

Cited from “http://www.ecse.rpi.edu/~schubert/Course-ECSE-6290%20SDM-

2/1%20JFETs%20and%20MESFETs.pdf”

Aside from its mobility, GaAs has also another advantagein noise characteristics which is

also important at some applications.

On the other hand, GaAs has some disadvantages over Si. Main problems about GaAs can

be summerized as below.

- Multi elemantal structure causes more process steps and complex production, which

makes the material more expensive.

- GaAs is weaker against environmental effects such as electrostatic discharge, heat etc.

rather than silicon. Those effects can easily break down electrical behaviour of device.

FET (Field Effect Transistor)

FETs (Field Effect Transistors) are transistors in which drain current is controlled by gate

voltage. Applying voltage to gate causes electric field from gate to substrate which pushes

away positive charges and causes a channel that current pass through. An NMOS structure is

shown below.

As it can be seen there are two n type area and one p type substrate. Gate is isolated from

substrate by an oxide layer. When gate voltage is about VTn a channel occurs between two n

type areas and current becomes able to flow. The current flows on channel depends on drain

and gate voltage.

MESFET

MESFET (Metal Semiconductor Field Effect Transistor) is a structure like MOSFET

(Metal Oxide Semiconductor Field Effect Transistor) with a bit difference. In MESFET,unlike

MOSFET, gate is connected to substrate. A cross-section of a MESFET is shown below.

Main advantage of MESFET structure is low gate capacitance because of the

semiconducting layer between gate and substrate. This advantage is mostly required for high

frequency applications.

GaAs (Gallium Arsenide) FET

GaAs FET is a type of MESFET in which substrate is made from GaAs (gallium arsenide).

The advantage of GaAs is, as mentioned above, high electron mobility and low noise.

Cited from “http://www.radio-electronics.com/info/data/semicond/fet-field-effect-

transistor/gaasfet-mesfet-basics.php”

GaN (Gallium Nitride) Structure

GaAs is very useful at high frequency and low noise applications. However it is fragile and

is not much suitable for high power applications. GaN is a hard material that handles power

problem. Despite being expensive, GaN provides an extremely crucial need required in power

stages. GaN has high current and voltage capability which makes it more difficult to break

against high power. A power transistor cross section is shown below.

Cited from “http://powerelectronics.com/images/enhancement-fig-1.jpg”

SiC (Silicon Carbide) Structure

SiC bipolar junction transistors (BJTs) are also used in power electronics applications at

RF frequencies. An abstract about a SiC BJT device from internet explains benefits of SiC:

"Astralux, Inc. proposes to develop a new high-power SiC bipolar junction transistor

(BJT) operating at ten times higher power density and five times higher efficiency compared

to conventional power transistors. This new technology will enable truebroadband (3-10GHz)

ultra-linear power amplifiers and superior T/R modules for use in radar, satellite

communication and other wireless systems. The BJTs will not only outperform existing high-

power semiconductor components and vacuum tubes, but also besmaller and more robust.

Furthermore, these powerful components will reduce overall systems costs by simplifying the

amplifier architecture and facilitating waste heat rejection.The SiC BJT combines high

power, high power density, high efficiency, low noise and high linearity, which will result in

more powerful, compact, and less costly X-band BMD class radar systems with enhanced

resolution and improved discrimination. Theultimate benefit is enhanced national security

through early warning, target identification and tracking of hostile targets. SiC BJTs are

identified as potential high-power RF devices and would greatly benefit the development of

high-power amplifiers and T/R modules." (Cited from http://sbir.gov/sbirsearch/detail/98880)

References

http://www.ausairpower.net/AC-0700.html

http://www.ee.ui.ac.id/~astha/courses/ts/teksem/mesfet.htm

http://en.wikipedia.org/wiki/MESFET

http://www.ecse.rpi.edu/~schubert/Course-ECSE-6290%20SDM-

2/1%20JFETs%20and%20MESFETs.pdf

http://my.ece.ucsb.edu/mishra/classfiles/overview.pdf

http://www.iue.tuwien.ac.at/phd/brech/ch_2.htm

http://en.wikipedia.org/wiki/High_electron_mobility_transistor#pHEMT

http://parts.jpl.nasa.gov/mmic/3-V.PDF

http://ecee.colorado.edu/~bart/book/book/chapter5/pdf/ch5_7.pdf

http://www.google.com.tr/patents?hl=tr&lr=&vid=USPAT5198689&id=RdEbAAAAEBAJ&

oi=fnd&dq=heterojunction+bipolar+transistor&printsec=abstract#v=onepage&q&f=false

Jayant BALIGA "Silicon RF Power MOSFETS", 2005.

http://www.microwaves101.com/encyclopedia/LDMOS.cfm