Electrical Characteristics of MESFETs and HEMTs

INDEX

Chapter-1 Introduction

Chapter-2 Semiconductors
2.1 Brief Introduction to Semiconductors
2.2 Defects in Semiconductor Crystals
2.3 Need for Wide Bandgap Materials

Chapter-3 Study of Metal Effect Semiconductor Field Effect Transistor (MESFET)
3.1 Brief Introduction to MESFET 7
3.2 Theoretical Model of I-V Characteristics of MESFET
3.3 Material Selection for Substrates in MESFET
3.3.1. Advantages of Silicon Carbide (SiC) over Silicon (Si)
3.3.2. Advantages of Gallium Arsenide (GaAs) over Silicon (Si)
3.3.3. Applications and Benefits of SiC as Substrate
3.4. Comparative Study Analysis on MESFETs Using Different Substrates
3.4.1. I-V Characteristics of MESFET using Si, SiC & GaAs Substrates
3.4.2. I-V Characteristics of MESFET using 3C, 4H & 6H SiC Substrates

Chapter-4 Study of High Electron Mobility Transistor (HEMT)
4.1 Brief Introduction to HEMT
4.2 Material Selection for Substrates in HEMT
4.2.1. GaAs HEMT
4.2.2. GaN HEMT
4.3 Theoretical Model of I-V Characteristics of HEMT
4.4. Study Analysis on GaAs & GaN with respect to SiC HEMTs
4.4.1. I-V Characteristics of SiC -HEMT
4.4.2. I-V Characteristics of GaAs -HEMT
4.4.3. I-V Characteristics of GaN –HEMT

Chapter-5 Noise Analysis on High Frequency Devices-MESFET & HEMT
5.1 Noise in Semiconductor Devices
5.2 Low Frequency Noise Analysis
5.2.1. Flicker (1/f) Noise
5.2.2. Generation-Recombination (G-R) Noise
5.3 Noise Power Spectral Density Analysis on MESFET
5.3.1. Noise PSD vs. Vds characteristics for MESFET using Si, SiC & GaAs Substrates
5.3.2 Noise PSD vs. Vds characteristics for MESFET using 3C, 4H & 6H- SiC substrates
5.3.3 Noise PSD vs. frequency characteristics for MESFET using Si, SiC & GaAs substrates
5.3.4 Noise PSD vs. frequency characteristics for MESFET using 3C, 4H & 6H-SiC substrates
5.3.5 Relative Noise PSD vs. Temperature for MESFET using Si, SiC & GaAs substrates
5.3.6 Relative Noise PSD vs. Temperature for MESFET using 3C, 4H & 6H-SiC substrates
5.4 Noise Power Spectral Density Analysis on HEMT
5.4.1 Noise PSD vs. Vds characteristics for GaAs & GaN HEMTs
5.4.2 Relative Noise PSD vs. Temperature characteristics for GaAs & GaN HEMTs

Discussions

Proposed Work

Appendix-A

Appendix-B

Appendix-C

References

Certificate of Paper Acceptance in Conference

ACKNOWLEDGEMENT

I would like to extend my gratitude and my sincere thanks to my honorable, esteemed supervisor Head of the Department, Dr. Maitreyi Ray Kanjilal, for the ideas that led to this work, her timely comments, guidance, support and patience throughout the course of this work. I am very much thankful to Prof. Damayanti Ghosh, Asst. Professor, ECE department, for providing necessary guidance and her timely suggestions. Her trust and support inspired me in the most important moments of making right decisions.

I am also thankful to Prof. Sandhya Pattanayak, Asst. Professor, E.C.E department, for teaching me the basics of matlab software which had helped me a lot for completing the plots for this thesis in time. .

I would like to thank Narula Institute of Technology, for giving me this opportunity to pursue degree in Master of Technology in Electronics and Communication Engineering from their institute and I am also grateful to our Principal, Dean (Academic) and all the staff members of E.C.E department for their contributions throughout the whole course.

I wish to thank all my classmates for their time to time suggestions and cooperation At the last but not the least, I am grateful to Professor P.K. Banerjee and Professor A. K. Mallick for their encouragement and inspiration.

Moumita Bhoumik

M.Tech (Electronics & Communication)

2010-2012

Date: Email:

Place: Mobile:

ABSTRACT

Advanced developments that were made recently in the field of Silicon (Si) semiconductor technology have allowed it to approach the theoretical limits of the Si material. However there are latest power device requirements for many applications that cannot be handled by the present Si-based power devices. These requirements include such as higher blocking voltages, switching frequencies, efficiency, and reliability. And hence, new semiconductor materials for power device applications are needed to overcome these limitations.

For high power requirements, wide bandgap semiconductors like Silicon Carbide (SiC) and Gallium Nitride (GaN) and Gallium Arsenide (GaAs), which are having superior electrical properties, are likely to replace Si in the near future. This Study thesis compares the electrical characteristics of wide-bandgap semiconductors with respect to Silicon (Si) to verify their superior utility for power applications and predicts the future of power device semiconductor materials.

This thesis also includes the study that has been performed regarding the electrical characteristics of high frequency semiconductor devices in terms of I-V characteristics and Noise Power Spectral Density (PSD) Analysis with respect to drain current fluctuation in the semiconductor devices. The semiconductor devices that are used for this particular thesis are – Metal Effect Semiconductor Field Effect Transistors (MESFETs) and High Electron Mobility Transistors (HEMTs).

Chapter-1 Introduction and scope of Thesis

1.1 Recent Trends in Semiconductor Technology

The need for higher frequency of operation in microwave and millimeter-wave applications and for very high speed digital circuits has created a great deal of interest in high-speed devices that are used in semiconductor technology [1],[2].

In Semiconductor technology, Silicon (Si) is the most commonly used electronic semiconductor material and dominated in this field for past many years [3]. As an established semiconductor material, Si continues to define the frontier for advanced fabrication of very small devices and is suitable for operating in low frequency devices i, e up to few Hz or KHz range. But with the advancement in latest technologies and needs in many established semiconductor industries, new semiconductor materials such as- SiC (wide bandgap) and GaAs (direct bandgap) [5], [6] are emerging as possible additional capabilities for higher speed, lower power, and other advantages in semiconductor technology [4].

In recent days, electronic devices must be able to operate under harsh conditions, for example under high temperature. The maximum junction temperature limit for most Silicon (Si) electronics devices is 150°C [3] and therefore there is a limitation in operating temperature of the Si chips and power devices to remain under this particular value. However, increasing the effectiveness of Silicon (Si) to meet the needs of the Semiconductor Industry is not viable because it has reached its theoretical limits [3]. But the wide bandgap semiconductor-based power devices surpass Silicon’s theoretical limits and can achieve high frequency performances.

This thesis deals with the detail study of electrical characteristics of semiconductor devices - Metal Effect Semiconductor Field Effect Transistor (MESFET) & High Electron Mobility Transistor (HEMT) using different substrates, which includes the drain current-drain source voltage i, e. Id-Vds characteristics and Noise Power Spectral Density Analysis due to drain current fluctuation in these devices.

1.2 Substrate Materials for Comparative Analysis

For high power, high-voltage, high-frequency and high-temperature applications Silicon carbide (SiC) is an attractive wide band-gap semiconductor material due to its superior properties, such as the wide bandgap, high critical electric field, high thermal conductivity and high electron generation [4].

Silicon carbide (SiC) is a very promising material for use in high performance semiconductor devices [2]. Among the most important transport parameters for electronic devices are the mobility, saturation velocity, breakdown electric field, and thermal conductivity. The mobility describes the mean velocity that the electrons travel with on the application of electric field. Due to its superior material characteristics, SiC has developed into one of the leading contenders among the wide bandgap semiconductors.

In this thesis, we have selected the substrate materials as – Silicon (Si), Gallium Arsenide (GaAs) and Silicon Carbide Polytypes (3C, 4H & 6H SiC) for MESFET and hence the analysis is being done regarding the I-V characteristics and noise analysis for each individual substrate materials to generate a comparative study report. The main aim of this comparative study is to determine the superiority of SiC [7] material over the other materials.

Another high frequency device is HEMT [8], which is a form of FET (Field Effect Transistor) which is used to provide very high level of performance at Microwave frequencies. Mainly the semiconductor materials with high mobility values are used for this device so as to ensure generation of high mobility electrons.

Recent advances in the growth technology of III-V semiconductor heterojunctions have spawned a new generation of electronic devices that depend on heterojunctions for their operation. Hence the most probable materials for HEMT structures are-Gallium Arsenide (GaAs) & Gallium Nitride (GaN).

Chapter-2 Semiconductor Devices

2.1 Brief Introduction to Semiconductors

A semiconductor material has a resistivity lying between a conductor and an insulator. The ‘Energy Gap’ fundamentally impacts the mechanisms through which electrons associated with the crystal's atoms of the material can become free and serves as conduction electrons.

Elemental semiconductors consist of crystals composed of only a single atomic element from group IV of the periodic chart, i.e., germanium (Ge), silicon (Si), carbon (C), and tin (Sn).

Silicon (Si) is the most commonly used electronic semiconductor material [3], and is also the most common element on earth. In addition to crystals composed of only a single group IV atomic species, one can also create semiconductor crystals consisting of two or more atoms, all from group IV. For example, silicon carbide (SiC) has been investigated for high temperature applications [4].

In addition, III-V semiconductors have a potential for higher speed operation than silicon semiconductors in electronics applications, with particular importance for areas such as wireless communications. The compound semiconductors have a crystal lattice constructed from atomic elements in different groups of the periodic chart.

The bonding in III-V semiconductors is largely covalent, though the shift of valence charge from the Group V atoms to the Group III atoms induces a component of ionic bonding to the crystal (in contrast to the elemental semiconductors which have purely covalent bonds). Example of such III-V compound semiconductors are (Gallium Phosphate) GaP, (Gallium Arsenide) GaAs, (Indium Phosphate) InP and Gallium Nitride (GaN) etc.

GaAs is probably the most familiar example of III-V compound semiconductors, used for high speed electronics devices such as MESFET and HEMT.

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Fig. 2.1: Velocity versus electric field for silicon and GaAs semiconductors [5],[ 6]

Figure 2.1 shows velocity vs. electric field characteristics for electrons in silicon (Si) and in GaAs. The linear dependence at small fields illustrates the low field mobility, with GaAs having larger low field mobility than silicon. However, the saturated velocities vsat do not exhibit such a large difference between Si and GaAs. Also, the saturated velocities do not exhibit as strong a temperature dependence as the low field mobilities, since the saturated velocities are induced by large electric fields, rather than being perturbations on thermal velocities.

Figure 2.1 also exhibits an interesting feature in the case of GaAs [5],[6] i, e. with increasing electric field, the velocity initially increases beyond the saturated velocity value, falling at higher electric fields to the saturated velocity. This negative differential mobility is a potential means for achieving device speeds higher than would be obtained with fully saturated velocities.

In particular, although higher low field velocities generally lead to higher device speeds under low field conditions, the saturated velocities give similar speed performance at high electric fields.

2.2 Defects in Semiconductor Crystals

A variety of defect known as crystalline defects [9] occurs in semiconductor crystals, which lead to degradations in performance due to generation of noise which is either in form of current or voltage fluctuation and hence require careful growth and fabrication conditions to minimize their occurrence to maintain high frequency and output performance.

There is another defect known as line defects that are generally important factors in the electrical behavior of semiconductor devices since their influence (as trapping centers, scattering sites, etc.) extends over distances substantially larger than atomic spacings.

In addition to their impact on the electrical performance, such defects can compromise the mechanical strength of the crystal. Later in this study thesis, we will observe the effect of crystal defects that will lead to generation of low frequency noises [9] inside the semiconductor devices and hence gives rise to drain current fluctuation in the devices –MESFET & HEMT.

2.3 Need for Wide Bandgap Materials

Wide bandgap semiconductor power devices, with their superior characteristics, offers great performance improvements and can work in harsh environments where Si power devices cannot function properly.

Some of the advantages of wide bandgap (WBG) based power devices as compared with Si based power devices are as follows [3]:

i. WBG semiconductor-based power devices have higher breakdown voltages because of their higher electric breakdown field, while Si Schottky diodes are commercially available typically at voltages lower than 300 V, the first commercial SiC Schottky diodes are already rated at 600 V [3].
ii. WBG devices have a higher thermal conductivity. Therefore, WBG-based power devices have a lower junction-to case thermal resistance . This means heat is more easily transferred out of the device, and thus the device temperature increase is slower. GaN is an exception in this case.
iii. WBG semiconductor-based power devices can operate at high temperatures. The literature notes operation of SiC devices up to 600°C. Si devices, on the other hand, can operate at a maximum junction temperature of only 150°C [3].
iv. Forward and reverse characteristics of WBG semiconductor-based power devices vary only slightly with temperature and time, hence they are more reliable.

Some of these characteristics are tabulated for the most popular wide bandgap semiconductors and Si [10] in Table 2.1.

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Table.2.1: Physical Characteristics of Si and main wide bandgap semiconductors [10].

The above mentioned substrate materials are used for obtaining the electrical characteristics of high frequency devices such as MESFET [11] & HEMT [12]. The detail descriptions of these devices are mentioned in the next two chapters (Chapter-3 & 4).

Chapter-3 Study of Metal Effect Semiconductor Field Effect Transistor (MESFET)

3.1 Brief Introduction to MESFET

The Metal-Semiconductor-Field-Effect-Transistor (MESFET) consists of a conducting channel generated in between a source and drain contact region [11] as shown in the Figure 3.1. The carriers flow from source to drain is controlled by a Schottky metal gate. The channel is controlled by varying the depletion layer width under the metal contact which modulates the thickness of the conducting channel and thus the current between source and drain is modulated.

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Fig.3.1: Structure of a MESFET with gate length, L, and channel thickness a [11]

The MESFET is having more advantages as compared to the MOSFET due to the higher mobility of the carriers in the channel. This higher mobility results into a higher current, trans-conductance and transit frequency of the device. However the disadvantage of the MESFET structure is the presence of the Schottky metal gate since it limits the forward bias voltage on the gate to turn-on the Schottky diode.

The threshold voltage therefore must be lower than this turn-on voltage. As a result it is more difficult to fabricate circuits containing a large number of enhancement-mode MESFET. Hence, we are mostly using depletion mode MESFETs in our thesis.

The higher transit frequency of the MESFET makes it most preferable device for using in microwave circuits. The advantage of the MESFET provides a superior microwave amplifier or circuit and the limitation of diode turn-on is easily tolerated. Typically depletion mode devices are used since they provide a larger current and larger trans-conductance and the circuits involved contain only a few transistors, so that controlling the threshold voltage is not a limiting factor.

The gate contact in MESFET generates a layer in the semiconductor that is completely depleted of free-carrier electrons. This depletion layer created acts like an insulating region and constricts the channel available for current flow in the n layer between the source and drain contacts. The width of the depletion region depends on the voltage applied between the semiconductor and the gate.

When a negative voltage is applied to the gate (i,e. Vg<0) as shown in Figure 3.3 [7], the gate-to channel junction is reverse biased, and the depletion region grows wider. For small values of Vds, the channel will act as a linear resistor, but its resistance will be larger due to a narrower cross section available for current flow. As Vds is increased, the critical field is reached at a lower drain current than in the Vg=0 case as shown in Figure 1.2, due to the larger channel resistance. For a further increase in Vds, the current remains saturated. The MESFET consists of a semiconducting channel whose thickness can be varied by widening the depletion region under the metal-to-semiconductor junction.

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Fig.3.2: Vds-Ids Representation with respect to Vgs=0 V of a MESFET [11]

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Fig.3.3: Vds-Ids Representation with respect to Vgs<0 V of a MESFET [11]

As shown in Fig.3.3, when reverse bias is applied to the gate, the depletion layer penetrates deeper into the active channel. These further reductions in channel region result in further reduction of current. Then the gate voltage Vgs acts as a means for limiting the maximum amount of source-drain current Ids that can flow. When enough reverse bias is applied, the depletion region will extend across the entire active channel and allow no current to flow through it. That gate to source potential is termed as the Pinch-off voltage, denoted by Vp.

3.2. Theoretical Model of I-V Characteristics of MESFET

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Fig.3.4: Basic I-V characteristics of MESFET with linear region & saturation region [11]

The current in the channel Ids in the linear region is given by [4]

Id(Vds,Vgs) = KnAbbildung in dieser Leseprobe nicht enthalten} for Vds≤ Vgs-Vp.. (Eq.3.1)

Similarly the current in the channel Ids in the saturation region is given by [4]

Idsat (Vds,Vgs) = Kn Abbildung in dieser Leseprobe nicht enthalten} for Vds> Vgs-Vp . (Eq.3.2)

Where,

Kn is the pinch-off current and given by [4]

Kn = Abbildung in dieser Leseprobe nicht enthalten (Eq.3.3)

Vbi is the built-in potential and given by [4]

Vbi = Abbildung in dieser Leseprobe nicht enthalten… (Eq.3.4)

Vp is pinch-off voltage and given by [4]

Vp=Vbi-Vp0… (Eq.3.5)

Where, Vp0 is the internal pinch-off voltage given by [4]

Vp0= Abbildung in dieser Leseprobe nicht enthalten… (Eq.3.6)

ηi is the intrinsic carrier concentration is given by [ 4]

ηi = Abbildung in dieser Leseprobe nicht enthalten…. (Eq.3.7)

Where,

Nd is the channel doping concentration

µ is the mobility of electron in substrate material

єs is the permittivity of the substrate material used

Nc is the density of state in conduction band

Nv is the density of state in valence band

Eg is the energy band gap

k is Boltzmann constant

3.3 Material Selection for Substrates in MESFET

In MESFET, we can use different substrates for generating conduction electrons. Based on the electronic properties of various substrates, we can easily choose the suitable semiconductor material to use it as a substrate for MESFET. Initially, the most common material used was Silicon (Si) [3] that is commercially available and is employed in most of the commercial solid-state electronics. It is a low bandgap material with bandgap energy ‘Eg’ as 1.12 eV. While comparing it with other higher bandgap materials, it is considered as the standard against which other semiconductor materials (Si, GaAs, and SiC etc.) are evaluated [12].

In this thesis, we have selected the substrate materials as – Silicon (Si), Gallium Arsenide (GaAs) and Silicon Carbide Polytypes (3C, 4H & 6H SiC) for MESFET and hence performed the analysis regarding the I-V characteristics and noise analysis for each individual substrate materials to generate a comparative study report.

Polytypes of SiC:

Silicon carbide occurs in many different crystal structures, called polytypes. More information about SiC crystallography and polytypes can be found in Reference [13]. There are over 100 known polytypes of SiC, only a few are commonly grown in a reproducible form acceptable for use as an electronic semiconductor. The most common polytypes of SiC presently being developed for electronics are 3C-SiC, 4H-SiC, and 6H-SiC.

For applications operating in microwave ranges, out of the three polytypes of SiC (as mentioned above), the 4H-SiC polytype is preferable because it has a larger bandgap and higher electron mobility than 6H-SiC. 4H-SiC is a wide band gap material with bandgap energy ‘Eg’ of 3.25 eV, as compared to 1.12 eV for Si and 1.45 eV for GaAs, that gives SiC its major benefit for high power devices. This wide bandgap gives rise to a breakdown electric field that is times higher than in GaAs or Si.

The most beneficial inherent material superiorities of SiC over other materials (listed in Table 3.1 below) are its exceptionally high breakdown electric field, wide bandgap energy, high thermal conductivity, and high carrier saturation velocity [12].

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Table 3.1: Comparison of Selected Important Semiconductor Electronic Properties of Major SiC Polytypes (3C, 4H & 6H) with Si and GaAs at 300 K [13],[14]

3.3.1 Advantages of Silicon Carbide (SiC) over Silicon (Si)

Silicon carbide (SiC)-based semiconductor electronic devices and circuits are presently being developed for use in high-temperature, high-power, and high-radiation conditions under which conventional semiconductors cannot adequately perform. Silicon carbide’s ability to function under such extreme conditions is expected to enable significant improvements to a far-ranging variety of applications and systems. These range from greatly improved high-voltage switching for energy savings in public electric power distribution and electric motor drives to more powerful microwave electronics for radar and communications to sensors and controls for cleaner-burning more fuel-efficient jet aircraft and automobile engines [15–21].

In particular area of power devices, theoretical appraisals have indicated that SiC power devices and diode rectifiers operates over higher voltage and temperature ranges, and have superior switching characteristics, and yet have die sizes nearly 20 times smaller than correspondingly rated silicon-based devices [22].

A summary of the most important properties of SiC polytypes in comparison to Si is shown below and the conclusions has been made on the basis of table 3.3 (listed above):

Wide Energy Bandgap (eV):

Electronic devices formed in SiC can operate at extremely high temperatures [2] without suffering from intrinsic conduction effects because of the wide energy bandgap. Also, this property allows SiC to emit and detect short wavelength light which makes the fabrication of blue light emitting diodes and nearly solar blind UV photo detectors possible.

High Breakdown Electric Field (V/cm):

SiC can withstand a voltage gradient (or electric field) over eight times greater than Si without undergoing avalanche breakdown. This high breakdown electric field enables the fabrication of very high-voltage, high-power devices such as diodes, power transistors, power thyristors and surge suppressors, as well as high power microwave devices. Additionally, it allows the devices to be placed very close together, providing high device packing density for integrated circuits.

High Thermal Conductivity (W/cm-K):

SiC is an excellent thermal conductor. Heat will flow more readily through SiC than other semiconductor materials. In fact, at room temperature, SiC has a higher thermal conductivity than any metal. This property enables SiC devices to operate at extremely high power levels and still dissipate the large amounts of excess heat generated.

High Saturated Electron Drift Velocity (cm/sec)

SiC devices can operate at high frequencies (RF and microwave) because of the high saturated electron drift velocity of SiC

Collectively, these above mentioned properties allow SiC devices to offer tremendous benefits over Si devices in a large number of industrial and military applications.

3.3.2 Advantages of Gallium Arsenide (GaAs) over Silicon (Si)

GaAs is an III–V compound semiconductor composed of the element Gallium (Ga) from column III and the element Arsenic (As) from column V of the periodic table of the elements. The energy band gap of GaAs at room temperature is 1.42 eV. GaAs is a direct band gap semiconductor, which means that the minimum of the conduction band is directly over the maximum of the valance band. Transitions between the valance band and the conduction band require only a change in energy, and no change in momentum, unlike indirect band-gap semiconductors such as Silicon (Si). This property makes GaAs a very useful material for the manufacture of light emitting diodes and semiconductor lasers, since a photon is emitted when an electron changes energy levels from the conduction band to the valance band.

GaAs has several advantages over silicon [5] for operation in the microwave region because of higher mobility and saturated drift velocity and the capability to produce devices on a semi-insulating substrate.

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Fig.3.5: Drift velocity of electrons in GaAs and Si as a function of the electric field [5]

3.3.3 Applications and Benefits of SiC based Semiconductor Devices

The most beneficial advantages that SiC-based electronics offers over semiconductor-based devices are in the areas of high-temperature and high-power device operation [21]:

High-Temperature Device Operation:

The wide bandgap energy and low intrinsic carrier concentration of SiC allow SiC to maintain semiconductor behavior at much higher temperatures than silicon, which in turn permits SiC semiconductor device functionality at much higher temperatures than Si [21].

Depending upon specific device design, the intrinsic carrier concentration of silicon generally confines silicon device operation to junction temperatures <300°C. SiC’s much smaller intrinsic carrier concentration theoretically permits device operation at junction temperatures exceeding 800°C.

The ability to place uncooled high-temperature semiconductor electronics directly into hot environments will enable important benefits to automotive, aerospace, and deep-well drilling industries [21]. In the case of automotive and aerospace engines, improved electronic telemetry and control from high-temperature engine regions are necessary to more precisely control the combustion process to improve fuel efficiency while reducing polluting emissions. High-temperature capability eliminates performance, reliability, and weight penalties associated with liquid cooling, fans, thermal shielding, and longer wire runs needed to realize similar functionality in engines using conventional silicon (Si) or semiconductor electronics [3].

High-Power Device Operation:

The high breakdown field and high thermal conductivity of SiC coupled with high operational junction temperatures theoretically permits extremely high-power densities and efficiencies in SiC devices [21]. SiC’s high breakdown field and wide energy bandgap enable much faster power switching than other power-switching devices.

SiC’s ability to operate at high junction temperatures permits much more efficient cooling to take place, so that heat sinks and other device-cooling hardware (i.e., fan cooling, liquid cooling, air conditioning, heat radiators, etc.) that are needed to keep high-power devices from overheating can be made much smaller or even eliminated [4].

The rectifying metal–semiconductor Schottky barrier contacts to SiC are useful for commercialized SiC metal–semiconductor field-effect transistors (MESFETs). The rectifying contacts permit extraction of Schottky barrier heights and diode ideality factors by current–voltage (I–V) electrical measurement techniques.

The key material advancement that enabled reliable operation was the development of semi-insulating SiC substrates (needed to minimize parasitic device capacitances) with far less charge trapping induced than the previously developed vanadium-doped semi-insulating SiC wafers [7].

The high breakdown field and wide energy bandgap permit operation of SiC metal–semiconductor Schottky diodes at much higher voltages (above 1 kV) than is practical with silicon based Schottky diodes that are limited to operation below ~200 V.

Silicon Carbide (SiC) is a semiconductor material combining several unique features such as high breakdown field, high electron saturation velocity, and high thermal conductivity. These properties make it a promising candidate for RF power transistors [2]. During the last ten year, the interest in SiC RF power transistors has increased considerably. Substantial progress has been made in SiC technology leading to a variety of experimental SiC RF transistors capable of operation in the gigahertz range. Commonly these transistors are n-channel field-effect transistors (MESFETs).

The physical and electronic properties of SiC make it the foremost semiconductor material for short wavelength optoelectronic, high temperature, radiation resistant, and high-power/high-frequency electronic devices [2].

The purpose of this study thesis is to justify whether SiC is the only material that is capable of giving superior performance [7] in terms of high drain current and low noise spectral density and hence electrical characteristics are compared with that of few other semiconductor materials considered here such as Si, GaAs & GaN for determining the suitability for high frequency performance and high temperature operation.

3.4 Comparative Study Analysis on MESFETs Using Different Substrates

In this comparative study analysis of MESFET, we are using different substrates as Silicon (Si), Silicon Carbide (4H-SiC) & Gallium Arsenide (GaAs). Now, the drain current ‘Id’ versus drain-source voltage ‘Vds’ are plotted for each individual substrate material.

The Id vs. Vds plots are obtained by calculating the corresponding drain current for different values of gate-source voltage ‘Vgs’ i,e. from -1.5 to 0 V ( at step size of 0.5 V), over a range of Vds from 0 to 5 V (at step size of 0.5 V).

3.4.1 I-V Characteristics of MESFET using Si, SiC & GaAs Substrates

For Si, SiC and GaAs MESFETs, we have considered a standard value for gate length ‘L’ and gate width ‘W’. The value of doping density ‘Nd’ is also kept constant for these three substrates as mentioned in this section. The purpose for keeping these parameter values constant for the three substrate material is to obtain the required amount of drain current ‘Id’ and perform a comparative study and analysis for the I-V characteristics of MESFET device using various substrate materials.

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