Optoelectronics seminar: MESFET

seminar on MESFET

1. Presented by: Mridula Sharma (30104730)

2. 2/4/2011 2 MESFET: Structure and Operating Principles

3. 2/4/2011 MESFET: Structure and Operating Principles 3 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

4. 2/4/2011 MESFET: Structure and Operating Principles 4 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

5. 2/4/2011 MESFET: Structure and Operating Principles 5 MESFET : Me tal S emiconductor F ield E ffect T ransistor

6. First proposed in 1966 Until the late 1980s, almost all microwave integrated circuits used GaAs MESFETs Most popular: GaAs MESFET Dominant active device for power amplifiers and switching circuits in the microwave spectrum 2/4/2011 6 MESFET: Structure and Operating Principles

7. 2/4/2011 7 MESFET: Structure and Operating Principles Fig 1: Comparison of Electron Mobility GaAS v/s Si

8. 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 2/4/2011 8 MESFET: Structure and Operating Principles

9. 2/4/2011 9 MESFET: Structure and Operating Principles M etal O xide S emiconductor F ield E ffect T ransistor : N MOSFET and P MOSFET 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

10. 2/4/2011 10 MESFET: Structure and Operating Principles Fig 2: MOSFET Schematic

11. 2/4/2011 11 MESFET: Structure and Operating Principles 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

12. Conducting channel positioned between source and drain contact channel Schottky Metal Gate 2/4/2011 MESFET: Structure and Operating Principles 12 Fig 3: MESFET Schematic

13. 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 2/4/2011 13 MESFET: Structure and Operating Principles

14. 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 contacts Typical: Schottky contact: Ti–Pt–Au Ohmic Contacts: Au– Ge 2/4/2011 14 MESFET: Structure and Operating Principles

15. 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 2/4/2011 15 MESFET: Structure and Operating Principles

16. C urrent flows between the two contacts when a small voltage is applied between the source and drain O hmic resistances: R S and R D C hannel resistance: R DS Current linearly increases with an associated resistance: R DS + R S +R D (1) 2/4/2011 16 MESFET: Structure and Operating Principles

17. Fig 5: Schematic and I-V characteristics for an ungated MESFET 2/4/2011 17 MESFET: Structure and Operating Principles

18. (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 I D parasitic resistances omitted R S and R D I D saturates with v ( x ) : I DSS Z : width of the channel b ( x ): effective channel depth q : electron charge n ( x ): electron density v ( x ): electron velocity 2/4/2011 18 MESFET: Structure and Operating Principles

19. 2/4/2011 MESFET: Structure and Operating Principles 19 Fig 6: Schematic Diagram for a Gated MESFET

20. Consider: gate electrode placed over channel without any gate bias V G = 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 2/4/2011 20 MESFET: Structure and Operating Principles

21. 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: C DC Electric field due to the dipole adds to the applied electric field; saturation conditions occur at a lower V D 2/4/2011 21 MESFET: Structure and Operating Principles Fig 7: Schematic showing non-uniform channel depth

22. 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) 2/4/2011 22 MESFET: Structure and Operating Principles

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24. Fig 8: Normalized I-V characteristics of GaAs MESFET 2/4/2011 24 MESFET: Structure and Operating Principles

25. (5) (4) Transconductance Using short-channel approximations, I S :maximum current that can flow if the channel were fully un-depleted under saturated velocity conditions. I S proportional to the channel depth, a V P is proportional to the square of the channel depth g m is inversely proportional to the channel depth. For large I S and g m , the parasitic resistances R S and R D must be minimized 2/4/2011 25 MESFET: Structure and Operating Principles

26. (6) (7) Gain-Bandwidth Product : most commonly used figure of merit frequency where the unilateral power gain of the device is equal to one, f t maximum frequency of oscillation, f max lower limit on L at approximately 0.1 m m L / a > 1 channel depth on the order of 0.05 to 0.3 m m for most GaAs MESFETs carrier concentration in the channel be as high as possible 2/4/2011 MESFET: Structure and Operating Principles 26

27. Advantage: higher mobility of the carriers in the channel as compared to the MOSFET superior microwave amplifier or circuit Buried channel : better noise performance 2/4/2011 MESFET: Structure and Operating Principles 27

28. 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 2/4/2011 MESFET: Structure and Operating Principles 28

29. 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 2/4/2011 MESFET: Structure and Operating Principles 29

30. Fig 9: Photographs of Modern MESFETs [5] 2/4/2011 30 MESFET: Structure and Operating Principles

31. Microwave circuits High frequency devices Cellular phones Satellite receivers Radar Military Communications Commercial Optoelectronics 2/4/2011 31 MESFET: Structure and Operating Principles

32. FET Conducting channel, Source, Drain, Gate Why GaAs? MOSFET Linear and Saturation regions Pinch-off Key Features: advantages; disadvantages Wide-spread applications 2/4/2011 MESFET: Structure and Operating Principles 32

33. [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 2/4/2011 MESFET: Structure and Operating Principles 33

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