Optoelectronics seminar: MESFET

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seminar on MESFET

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Optoelectronics Seminar 2011 Metal Semiconductor Field Effect Transistor MESFET : Structure and Operating Principles

Presented by:Mridula Sharma(30104730)

2/4/20112MESFET: Structure and Operating Principles2/4/2011MESFET: Structure and Operating Principles3 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

2/4/2011MESFET: Structure and Operating Principles4 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 involved2/4/2011MESFET: Structure and Operating Principles5

MESFET: Metal Semiconductor Field Effect Transistor

First proposed in 1966Until the late 1980s, almost all microwave integrated circuits used GaAs MESFETsMost popular: GaAsMESFETDominant active device for power amplifiers and switching circuits in the microwave spectrum2/4/20116MESFET: Structure and Operating Principles2/4/20117MESFET: Structure and Operating Principles

Fig 1: Comparison of Electron Mobility GaAS v/s SiSignificant advantages over SiHigher room temperature mobility (more than 5 times )saturation velocity about twice that in siliconfabrication 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 transistors2/4/20118MESFET: Structure and Operating Principles

2/4/20119MESFET: Structure and Operating Principles 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

2/4/201110MESFET: Structure and Operating PrinciplesFig 2: MOSFET Schematic

2/4/201111MESFET: Structure and Operating PrinciplesOperation: 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

Conducting channel positioned between source and drain contact channelSchottky Metal Gate

2/4/2011MESFET: Structure and Operating Principles12Fig 3: MESFET Schematic

Basic Material: GaAs substrateBuffer layer: epitaxial growth over the substrate Conducting layer (channel): epitaxial growth over buffer layerN-type semiconducting materialHigh electron mobilityMicrowave transistor

2/4/201113MESFET: Structure and Operating Principles

Low-resistance Ohmic Contacts Fabrication aided by highly doped (n+) layer grown on the surfaceAlternative: ion implantation Two ohmic contacts: source and drainProvide access to external circuitSchottky contact between ohmic contactsTypical:Schottky contact: TiPtAuOhmic Contacts: AuGe2/4/201114MESFET: Structure and Operating PrinciplesOperation similar to JFET (Junction gate Field-Effect transistor)p-n junction gate replaced by Schottky barrierthe lower contact and p-n junction are eliminated as the lightly doped p-type substrate is replaced by a semi-insulating substrateFig 4: MESFET2/4/201115MESFET: Structure and Operating Principles

Current flows between the two contacts when a small voltage is applied between the source and drainOhmic resistances: RS and RDChannel resistance: RDS Current linearly increases with an associated resistance: RDS +RS+RD


2/4/201116MESFET: Structure and Operating Principles

Fig 5: Schematic and I-V characteristics for an ungated MESFET2/4/201117MESFET: Structure and Operating Principles

(2)Further increase in voltage: applied electric field becomes greater than electric field required for electron velocity saturationUnder large bias conditions: alternative expression for ID

parasitic resistances omitted RS and RDID saturates with v(x): IDSS

Z: width of the channelb(x): effective channel depth q: electron chargen(x): electron densityv(x): electron velocity2/4/201118MESFET: Structure and Operating Principles2/4/2011MESFET: Structure and Operating Principles19

Fig 6: Schematic Diagram for a Gated MESFET

Consider: gate electrode placed over channel without any gate bias VG = 0Depletion region formed reduces effective channel depth, b(x)Increase in resistance to current flowDepletion region depth depends on voltage drop across Schottky junctionLarger voltage drop across the drain than at the sourceResult: depletion region depth increases on the drain side of the channel

2/4/201120MESFET: Structure and Operating Principles

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

2/4/201121MESFET: Structure and Operating PrinciplesFig 7: Schematic showing non-uniform channel depthApplying bias to the gate junction the depletion depth and hence resistance of current flow between the source and drain and saturation current can be controlledApplying large enough negative gate bias : depletion region depth equals channel depthChannel is pinched offpinch-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)2/4/201122MESFET: Structure and Operating Principles

2/4/2011MESFET: Structure and Operating Principles23

Fig 8: Normalized I-V characteristics of GaAs MESFET2/4/201124MESFET: Structure and Operating Principles



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, aVP 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 minimized2/4/201125MESFET: Structure and Operating Principles


(7)Gain-Bandwidth Product: most commonly used figure of meritfrequency 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 mmL/a > 1channel depth on the order of 0.05 to 0.3 mm for most GaAs MESFETscarrier concentration in the channel be as high as possible

2/4/2011MESFET: Structure and Operating Principles26If short gate length approximations are made, ft can be related to the transit time of the electrons through the channel, t, by the expression

Since vsat is approximately 6 x1010 mm/s for GaAs with doping levels typically used in the channel, the gate length must be less than 1 mm for ft to be greater than 10 GHz26Advantage: higher mobility of the carriers in the channel as compared to the MOSFETsuperior microwave amplifier or circuitBuried channel: better noise performance

2/4/2011MESFET: Structure and Operating Principles27Disadvantage: presence of the Schottky metal gateLimits 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 voltageDifficulty in fabricating circuits containing a large number of enhancement-mode MESFETBut this limitation by the diode turn-on is easily tolerated

2/4/2011MESFET: Structure and Operating Principles28May be used to increase the power level of a microwave signalProvides gainCan be modeled as a voltage-controlled current sourceDrain current can be varied greatly with small variations in gate potentialHigh 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 factor2/4/2011MESFET: Structure and Operating Principles29

Fig 9: Photographs of Modern MESFETs [5]2/4/201130MESFET: Structure and Operating PrinciplesMicrowave circuitsHigh frequency devicesCellular phonesSatellite receiversRadarMilitary CommunicationsCommercial Optoelectronics2/4/201131MESFET: Structure and Operating Principles

The possibility of using MESFETs for high speed photo-detection and switching is being actively investigated by various workers at present. In this paper an attempt is made to theoretically evaluate the light generated voltage due to the optical radiation (0.87 m) falling o