PROCESS AND DEVICE SIMULATION OF A POWER

MOSFET USING SILVACO TCAD

TEAM MEMBERS

Name : Roll no.• Anmol Ramraika -120102079• Rajesh Kumar -120108028• Somitra Baldua -120108034• Jitendra Khatik -120108016• Prakhar Tosniwal -120108026

MOSFET- INTRODUCTION• The metal–oxide–semiconductor field-effect transistor (MOSFET) is a

transistor used for amplifying or switching electronic signals.• In enhancement mode MOSFETs, a voltage drop across the oxide induces a

conducting channel between the source and drain contacts via the field effect.

• The term "enhancement mode" refers to the increase of conductivity with increase in oxide field that adds carriers to the channel, also referred to as the inversion layer.

• The channel can contain electrons (called an nMOSFET or nMOS), or holes (called a pMOSFET or pMOS), opposite in type to the substrate, so nMOS is made with a p-type substrate, and pMOS with an n-type substrate (see article on semiconductor devices).

MOSFET : STRUCTURE• The 'metal' in the name MOSFET is now often a misnomer because the

previously metal gate material is now often a layer of polysilicon(polycrystalline silicon).

• Aluminium had been the gate material until the mid-1970s, when polysilicon became dominant, due to its capability to form self-aligned gates.

• Metallic gates are regaining popularity, since it is difficult to increase the speed of operation of transistors without metal gates.

• Likewise, the 'oxide' in the name can be a misnomer, as different dielectric materials are used with the aim of obtaining strong channels with smaller applied voltages.

• Another synonym is MISFET for metal–insulator–semiconductor FET or IGFET(insulated gate field effect transistor).

MOSFET –STRUCTURE(N-MOSFET)

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METAL OXIDE SEMICONDUCTOR STRUCTURE ON p-TYPE SILICON

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t ;

BAND DIAGRAM(CORESPONDING TO N-MOSFET)

Channel formation in nMOS MOSFET shown as band diagra m :

Top

panels: An applied gate voltage bends bands, depleting holes from

surface (left). The charge inducing the bending is balanced by a layer

of negative acceptor-ion charge (right). Bottom panel: A larger

applied voltage further depletes holes but conduction band lowers

enough in energy to populate a conducting channel.

POWER MOSFET(APPLICATIONS)

• Power MOSFETs with lateral structure are mainly used in high-end audio amplifiers and high-power PA systems.

• Power MOSFETs are well known for superior switching speed, and they require very little gate drive power because of the insulated gate. In these respects, power MOSFETs approach the characteristics of an "ideal switch".

• The main drawback is on-resistance RDS(on) and its strong positive temperature coefficient.

STRUCTURE OF POWER MOSFET(N-Power MOSFET)

• A positive voltage applied from the source to gate terminals causes electrons to be drawn toward the gate terminal in the body region.

• If the gate-source voltage is at or above what is called the threshold voltage, enough electrons accumulate under the gate to cause an inversion n-type layer; forming a conductive channel across the body region (the MOSFET is enhanced). Electrons can flow in either direction through the channel.

• Positive (or forward) drain current flows into the drain as electrons move from the source toward the drain. Forward drain current is blocked once the channel is turned off, and drain-source voltage is supported by the reverse biased body-drain p-n junction.

• In N-channel MOSFETs, only electrons flow during forward conduction — there are no minority carriers.

• Switching speed is only limited by the rate that charge is supplied to or removed from capacitances in the MOSFET. Therefore switching can be very fast, resulting in low switching losses. This is what makes power MOSFETs so efficient at high switching frequency.

STRUCTURE OF POWER MOSFET(N –channel MOSFET)

PROCESS SIMULATION

• Modelling of device is carried out using Athena in the Deckbuild software

• As per the required characteristics of power MOSFET the device is modelled by undergoing various process like deposition of oxidant layers, etching, ion implantation etc.

OVERVIEW OF PROCESS SIMULATION

• Developing a good simulation grid• Defining the initial substrate• Performing epitaxial growth• Performing layer deposition and geometrical

etching• Performing ion implantation• Removing implant mask• Performing diffusion • Specification of electrodes

1. SIMULATION GRID

• Using Mesh Define menu to define grid• Defining X and Y boundary values• Defining spacing for the non-uniform grid• Grid is finest in the surface active region of the

MOSFET

2. INITIAL SUBSTRATE

• Using Mesh Initialize menu to define initial substrate

• Selecting semiconductor material as Silicon and impurity material as Phosphorous

• Selecting orientation of silicon as <100> and concentration of impurity as 1*e-18 atoms/cm3

3. EPITAXIAL GROWTH

• Using process menu for performing deposition epitaxial layer on gate surface of MOSFET

• At the temperature of 1200 degree celsius for 10 minutes

4. DEPOSITION AND ETCHING

• Using Process menu to deposit oxide layers and perform geometrical etching of the layer where we want to implant

• Oxide, polysilicon and photoresist layers are deposited along with layer thickness

• Shape of etch is defined and etching is performed

5. ION IMPLANTATION

• Using the Process menu ion implantation is performed

• Implantation of boron with dose of 1*e-14 atm/cm2 and implant energy of 80 KeV is performed

6. REMOVING IMPLANT MASK

• Removal of barrier mask by performing etching

• Removing photoresist material by selecting all option to remove the implant mask

7. SPECIFICATION OF ELECTRODES

• Using Structure menu for specifying the location of electrodes so that device can be used by device simulator for electrical characterization

• Aluminum contact is established by deposition and etching beforehand

DEVICE SIMULATION

• The modelled MOSFET is simulated using Atlas• Device equations are solved to obtain

electrical characteristics of power MOSFET• I-V curve is plotted by Tonyplot