MOSFET, Construction And Working

MOSFET is developed from the MOS integrated circuit technology. MOSFETs also have comparatively higher ON-state resistance of the device cross-section which increases with the blocking voltage rating of the device. MOSFET cant not be used for a low voltage less than about 500 volts applications where the ON state resistance reaches acceptable values. Internally fast switching speed of these devices can be efficiently used to increase the switching frequency beyond several hundred kHz.

MOSFET operating principle is based on a voltage controlled majority carrier device. In a MOSFET movement of majority carriers are controlled by the voltage applied to the control electrode called the gate, which is insulated by a thin metal oxide layer from the bulk semiconductor body. The electric field produced by the gate voltage modifies the conductivity of the semiconductor in the region between the main current-carrying terminals called the Drain (D) and the Source (S).

Constructional Features of MOSFET

Constructional Features of MOSFET

The first time to think about developing high voltage MOSFETs were by redesigning lateral MOSFET to increase their voltage blocking capacity. Lateral double diffused MOS (DMOS) technology was used. However, it was shortly understood clearly that much larger breakdown voltage and current ratings could be achieved by resorting to a vertically oriented structure. After that vertical DMOS (VDMOS) structure has been accepted by virtually all manufacturers of Power MOSFET.

The first attempts to develop high voltage MOSFETs were by redesigning lateral MOSFET to increase their voltage blocking capacity. The resulting technology was called lateral double diffused MOS (DMOS). However, it was soon realized that much larger breakdown voltage and current ratings could be achieved by resorting to a vertically oriented structure. Since then, vertical DMOS (VDMOS) structure has been adopted by virtually all manufacturers of Power MOSFET

The two n+ end layers labeled as Source and Drain are heavily doped at the same level both. The p type middle layer is called the body or substrate and has a moderate doping level 2 to 3 orders of magnitude lower than n+ regions on both sides. The n- drain drift region has the lowest doping density.

The thickness of the region decided the breakdown voltage of the device. The gate terminal is put in over the n- and p type regions and is covered in non-conducting material to prevent it from the semiconductor body by a thin layer of silicon dioxide also called the gate oxide in the cell structure of MOSFET. The source and the drain region of all cells on a wafer are connected to both metallic contacts to form the Source and the Drain terminals of the complete device.

The thickness of the region determines the breakdown voltage of the device. The gate terminal is placed over the n- and p type regions of the cell structure and is insulated from the semiconductor body by a thin layer of silicon dioxide (also called the gate oxide). The source and the drain region of all cells on a wafer are connected to the same metallic contacts to form the Source and the Drain terminals of the complete device.

MOSFET cell is that the alternating n+ n- p n+ structure embeds a parasitic BJT into each MOSFET cell. The nonzero resistance between the base and the emitter of the parasitic npn BJT arises due to the body spreading resistance of the p type substrate.

In the design of the MOSFET cells, special care is taken so that this resistance is minimized, and switching operation of the parasitic BJT is suppressed. With an effective short circuit between the body and the source, the BJT always remain in cut off and its collector-base junction is represented as an anti-parallel diode called the body diode in the circuit symbol of a Power MOSFET.

Operating Principle of MOSFET

At first, there is no path for any current to flow between the source and the drain terminals since at least one of the p n junctions, source – body, and body-Drain will be reverse biased for either polarity of the applied voltage between the source and the drain. There is no possibility of current injection from the gate terminal either since the gate oxide is a very good insulator.

At initial, there is no path for any current to flow due to somewhere of the reverse block and so no electron flow between the source and the drain terminals since at least one of the p n junctions, source – body, and body-Drain will be reverse biased for either polarity of the applied voltage between the source and the drain. There is a poor possibility of current injection from the gate terminal either since the gate oxide is a very good insulator.

However, the uses of a positive voltage at the gate terminal to the source that covert the silicon surface beneath the gate oxide into an n type layer or channel, thus connecting the Source to the Drain as explained below.

The gate region is composed of gate metallization in the MOSFET, the gate (silicon) oxide layer, and the p-body silicon forms a high-quality capacitor. When a small voltage is applied to this capacitor structure with the gate terminal positive with respect to the source a depletion region forms the shared boundary across which two or more separate components between the SiO2 the silicon as shown in the above Figure.

The positive charge induced on the gate metallization repels the majority hole carriers from the interface region between the gate oxide and the p type body. This exposes the negatively charged acceptors and a depletion region is created.

The positive charge induced on the gate metallization drive or force back i.e repel the majority hole carriers from the interface region between the gate oxide and the p type body. This makes visible the negatively charged acceptors and a depletion region is created.

If we increase in the value of  VGS then it causes the depletion layer to grow in thickness. At the same time, the electric field at the oxide-silicon interface gets larger and begins to attract free electrons as shown in the above Fig. The main source at this stage of the electron is the electron-hole generation by thermal ionization. The holes are repelled into the semiconductor majority forward of the depletion region. The extra holes are neutralized by electrons coming from the source.

As VGS increases further the density of free electrons at the interface becomes equal to the free hole density in the bulk of the body region beyond the depletion layer. The layer of free electrons at the interface is called the inversion layer and is shown in the above Fig. The inversion layer has all the properties of an n type semiconductor and is a conductive path or channel between the drain and the source which permits the flow of current between the drain and the source. Since current conduction in this device takes place through an n- type “channel” created by the electric field due to gate-source voltage it is called Enhancement type n-channel MOSFET.

As  VGS increases more than more free electrons at the interface become equal to the free hole density in the bulk of the body region beyond the depletion layer. The layer of free electrons at the interface is called the inversion layer and is shown in the above Fig. The inversion layer has all the properties of an n type semiconductor and is a conductive path or channel between the drain and the source which provides a path to flow more electrons due to which the flow of current between the drain and the source. Since current conduction in this device takes place through an n- type “channel” that is created by the electric field due to gate-source voltage it is called Enhancement type n-channel MOSFET.

Output i-v Characteristics of MOSFET

Output i-v Characteristics of MOSFET

The output characteristics of a MOSFET is then a plot of drain current (iD) is a function of the Drain –Source Voltage (VDS) with gate-source voltage ( VGS) as a parameter as shown in the above figure of such characteristics.

When a gate-source voltage ( VGS) below the threshold voltage (VGS(th)) the MOSFET operates in the cut-off mode. No drain current flows in this mode and the applied drain-source voltage (VDS) is supported by the body-collector p-n junction. Due to no drain current, the maximum applied voltage should be below the avalanche break down voltage of this junction (VDSS) to avoid the destruction of the device.

When VGS is increased beyond VGS(th) drain current starts flowing. For small values of VDS (VDS < (VGS –VGS(th) ) iD is directly proportional to VDS. Consequently, this mode of operation is called the ohmic mode of operation of MOSFET.

The slope of the VDS –iD characteristics is called the ON-state resistance of the MOSFET rDS(ON) .

Summary

  • MOSFET is a voltage controlled majority carrier device.
  • A Power MOSFET has a vertical structure of alternating p and n layers
  • The main current-carrying terminals of an n channel enhancement mode MOSFET are called the Drain and the Source and are made up of n+ type semiconductor
  • p-type semiconductor body separates n+ type source and drain regions.
  • Current conduction in a MOSFET occurs by flow of electron from the source to the drain through this channel.
  • The on-state resistance of a MOSFET has a positive temperature coefficient.
  • MOSFET does not undergo a second break down.
  • The safe operating area of a MOSFET does not change under Forward and Reverse bias conditions.
  • The gate oxide layer can be damaged by static charge. Therefore MOSFETs should be handled only after discharging oneself through proper grounding

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