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Optoelectronics Seminar 2011Metal Semiconductor Field Effect Transistor

MESFET :Structure and Operating Principles

Presented by:Mridula Sharma

(30104730)

04/13/2023 MESFET: Structure and Operating Principles 2

Outline

Introduction

Basic Structure

Operating Principle

Applications

Summary


Introduction

• In 1945, Shockley: idea for making a solid state device out of semiconductors

• Reason: a strong electrical field could cause the flow of electricity within a nearby semiconductor

• 1948: Brattain & Bardeen built the first working transistor: the germanium point-contact transistor, designed as the junction (sandwich) transistor

•1960: Bell scientist John Atalla developed a new design based on Shockley's original field-effect theories

•By the late 1960s, manufacturers converted from junction type integrated circuits to field effect devices


Introduction

• Field effect devices are those in which current is controlled by the action of an electron field, rather than carrier injection

• Field-effect transistors are so named because a weak electrical signal coming in through one electrode creates an electrical field through the rest of the transistor

• The FET was known as a “unipolar” transistor • current is transported by carriers of one polarity (majority), whereas in

the conventional bipolar transistor carriers of both polarities (majority and minority) are involved


Introduction

MESFET: Metal Semiconductor Field Effect Transistor


Introduction

• First proposed in 1966

• Until the late 1980s, almost all microwave integrated circuits

used GaAs MESFETs

• Most popular: GaAsMESFET

• Dominant active device for power amplifiers and switching

circuits in the microwave spectrum


Why GaAs?

Fig 1: Comparison of Electron Mobility GaAS v/s Si


Why GaAs?• Significant advantages over Si

– Higher room temperature mobility (more than 5 times )• saturation velocity about twice that in silicon

– fabrication of semi-insulating (SI) GaAs substrates possible • Eliminates absorption of microwave power due to free carrier

absorption

• f > 2 GHz: GaAs transistors• f < 2 GHz: Si transistors


MOSFET• Metal Oxide Semiconductor Field Effect Transistor : NMOSFET and PMOSFET

• Main choice of semiconductor : Si, however SiGe is used by some chip manufacturers

• Gate terminal is composed a of a layer of polysilicon with a thin layer of silicon dioxide which acts as an insulator between the gate and the conducting channel


MOSFET

Fig 2: MOSFET Schematic


MOSFET

•Operation: potential is applied between the source and gate, generating an electric field through the oxide layer, creating an inversion channel in the conducting channel, also known as a depletion region

•The inversion channel is of the same type as the source and drain, creating a channel in which current can pass through

•By varying the potential between the gate and body, this channel in which current flows can be altered to allow more or less or current to flow through, depending on its size


Basic Structure• Conducting channel positioned between source and drain contact channel

• Schottky Metal Gate

Fig 3: MESFET Schematic


Basic Structure

• Basic Material: GaAs substrate

• Buffer layer: epitaxial growth over the substrate

• Conducting layer (channel): epitaxial growth over buffer layer

– N-type semiconducting material

– High electron mobility

– Microwave transistor


Basic Structure

• Low-resistance Ohmic Contacts

• Fabrication aided by highly doped (n+) layer grown on the surface

• Alternative: ion implantation

• Two ohmic contacts: source and drain

• Provide access to external circuit

• Schottky contact between ohmic contactsTypical:• Schottky contact: Ti–Pt–Au• Ohmic Contacts: Au–Ge


Principle of Operation• Operation similar to JFET (Junction gate Field-Effect transistor)• p-n junction gate replaced by Schottky barrier• the lower contact and p-n junction are eliminated as the lightly doped p-

type substrate is replaced by a semi-insulating substrate

Fig 4: MESFET


Principle of Operation

• Current flows between the two contacts when a small voltage is applied between the source and drain

• Ohmic resistances: RS and RD

• Channel resistance: RDS

• Current linearly increases with an associated resistance: – RDS +RS+RD

(1)



Fig 5: Schematic and I-V characteristics for an ungated MESFET


(2)


• Further increase in voltage: applied electric field becomes greater than electric field required for electron velocity saturation

• Under large bias conditions: alternative expression for ID

• parasitic resistances omitted– RS and RD

• ID saturates with v(x): IDSS

Z: width of the channelb(x): effective channel depth q: electron chargen(x): electron densityv(x): electron velocity


Fig 6: Schematic Diagram for a Gated MESFET



• Consider: gate electrode placed over channel without any gate bias– VG = 0

• Depletion region formed reduces effective channel depth, b(x)– Increase in resistance to current flow

• Depletion region depth depends on voltage drop across Schottky junction

• Larger voltage drop across the drain than at the source• Result: depletion region depth increases on the drain side of the

channel



• Two effects of non-uniform channel depth – Accumulation of electrons on the

source– depletion of electrons on drain of the

depletion region– feedback capacitance between the

drain and the channel: CDC

– Electric field due to the dipole adds to the applied electric field; saturation conditions occur at a lower VD

Fig 7: Schematic showing non-uniform channel depth


Principle of Operation• Applying bias to the gate junction

– the depletion depth and hence resistance of current flow between the source and drain and saturation current can be controlled

• Applying large enough negative gate bias : depletion region depth equals channel depth– Channel is pinched off

• pinch-off voltage

• Under pinch-off conditions: drain current drops to a very small value.– transistor can act as a voltage-controlled resistor or a switch

(3)



Fig 8: Normalized I-V characteristics of GaAs MESFET


(5)

(4)

Principle of Operation• Transconductance

• Using short-channel approximations,

• IS :maximum current that can flow if the channel were fully un-depleted under saturated velocity conditions.

• IS proportional to the channel depth, a• VP is proportional to the square of the channel depth• gm is inversely proportional to the channel depth. • For large IS and gm, the parasitic resistances RS and RD must be

minimized


(6)

(7)

Principle of Operation• Gain-Bandwidth Product: most commonly used figure of merit• frequency where the unilateral power gain of the device is equal to one, ft

• maximum frequency of oscillation, fmax

• lower limit on L at approximately 0.1 mm• L/a > 1

– channel depth on the order of 0.05 to 0.3 mm for most GaAs MESFETs– carrier concentration in the channel be as high as possible


Key Features

• Advantage: higher mobility of the carriers in the channel as compared to

the MOSFET

- superior microwave amplifier or circuit

• Buried channel: better noise performance


Key Features

• Disadvantage: presence of the Schottky metal gate

– Limits the forward bias voltage on the gate to the turn-on voltage of

the Schottky diode (typically 0.7 V for GaAs Schottky diodes)

– The threshold voltage therefore must be lower than this turn-on

voltage

– Difficulty in fabricating circuits containing a large number of

enhancement-mode MESFET

– But this limitation by the diode turn-on is easily tolerated


Key Features• May be used to increase the power level of a microwave signal

– Provides gain• Can be modeled as a voltage-controlled current source

– Drain current can be varied greatly with small variations in gate potential

• High transit frequency useful for microwave circuits. • Typically depletion-mode devices are used

– provide a larger current and larger transconductance – circuits contain only a few transistors, so that threshold control

is not a limiting factor


Fig 9: Photographs of Modern MESFETs [5]


Applications

• Microwave circuits

• High frequency devices

• Cellular phones

• Satellite receivers

• Radar

• Military Communications

• Commercial Optoelectronics


Summary

• FET

• Conducting channel, Source, Drain, Gate

• Why GaAs?

• MOSFET

• Linear and Saturation regions

• Pinch-off

• Key Features: advantages; disadvantages

• Wide-spread applications


References• [1] "Electronic Devices and Circuit Theory," Prentice Hall, Boylestad, R and

Nashelsky, L. 9th ed. 2005• [2] “The electrical engineering handbook“, Wai-Kai Chen, Academic Press,

2005• [3] Radio-Electronics.com URL:

http://www.radio-electronics.com/info/data/semicond/fet-field-effect-transistor/gaasfet-mesfet-basics.php

• [4] „Principles of Semiconductor Devices“, B. Van Zeghbroeck, 2007• [5] „Microelectronics Technology”, Prof. E. F. Schubert (Lecture Notes)• [6] Semiconductor Devices. Physics and Technology, S. M. Sze, J. Wiley Inc.

2002, 2nd edition




Thank You !!!