Instrumentation & Power Electronic Systems 1 1

Metal Oxide Semiconductor FET: MOSFET MOSFET also known as insulated-gate field-effect transistors (IGFET) is preferred in power electronics due to its ability of fast switching especially in timing circuits. MOSFET has a "Metal Oxide" gate (silicon dioxide- usually a glass, with insulating properties), which is electrically insulated from the semiconductor‘s N-channel or P-channel. This isolation of the controlling gate makes the input resistance of the MOSFET extremely high in the Mega-ohms region (infinite), thus switching loss at input side can controlled and stabilized. As the gate terminal is isolated from the main current carrying channel "NO current flows into the gate”, so MOSFET acts as a voltage controlled resistor (like JFET). MOSFET is specially used in digital complementary metal oxide semiconductor (CMOS) logics.

MOSFET Symbols n-channel MOSFET p-channel MOSFET drain gate body A circle is sometimes used on the gate terminal to show active low input drain gate body source or n-channel MOSFET p-channel MOSFET

MOSFET FAMILY-TREE

Metal Oxide Semiconductor FET: MOSFET

MOSFET Operation Output current (Drain current-ID) in a MOSFET is controlled by the gate-source voltage VGS. VGS controls the thickness of the channel

MOSFET Modes of Operations Two basic types of MOSFETs: Depletion MOSFETs (D-MOSFETs) : can be operated in either the depletion mode or the enhancement mode (Negative VGS). Enhancement MOSFETs (E-MOSFETs) : can be operated only in the enhancement mode (Positive VGS) . D-MOSFET: Zero bias E-MOSFET: Zero bias

Depletion-type: MOSFET Channel Mode n-channel depletion-type MOSFET p-channel depletion-type MOSFET

N-channel MOSFET with & without channel formation with channel formed: ON state (D-MOSFET) without channel formed: OFF state (E-MOSFET)

D- MOSFET Modes Status D-MOSFETs can operate in the depletion as well as in enhancement mode. Zero bias VGS=0: Gate is shorted to the source, so drain current equals the rating of the component. (IS: shorted-gate drain current). Depletion mode VGS<0: Negative VGS forces free electrons away from the gate, forming a depletion layer that cuts into the channel. So ID<IDSS. Enhancement mode VGS>0: Positive VGS attracts free electrons in the substrate toward the channel while driving valence-band holes away from the channel. As a result, material to the right of the channel effectively becomes n-type material, results in a wider channel. So ID>IDSS.

The Depletion MOSFET In depletion MOSFET, conduction channel is physically implanted (rather than induced), so channel conducts even if VGS=0. If value of VGS is positive, channel is further enhanced (more free electrons are attracted to the channel, conductivity increases). If value of VGS is negative, free electrons are repelled, conductivity of the channel is decreased, phenomenon known as channel depletion. If value of VGS becomes sufficiently negative, all of the free electrons in the channel will be repelled; thus channel is completely depleted! A channel that is completely depleted cannot conduct. In other words, the depletion MOSFET is in cutoff! Negative value of VGS at which the channel is completely depleted is the threshold voltage Vt for a depletion mode . In other words, to have a conducting channel, VGS must be greater than the threshold voltage Vt: (VGS >Vt)

Depletion MOSFET Characteristics Curves

Depletion MOSFET Summary: Lateral Structure

N-Channel Enhancement Mode MOSFET E-MOSFETs are only restricted to enhancement-mode operation. When an E-MOSFET is zero biased, p-type substrate makes contact with the SiO2 insulator, so there is no channel for conduction b/w drain and source terminals and ID=0A. When VGS is placed positive, time where VGS exceeds the threshold voltage of the component (VGS(th)), a channel is formed. This allows a current to pass through the component. When VGS>VGS(th),ID=positive E-MOSFET transconductance curve E-MOSFET operation

Output characteristics of a n-channel enhancement-type power-MOSFET Output Drain characteristics Transfer characteristic 15

POWER MOSFET Power MOSFET is a specific type of MOSFET designed to handle large amounts of power. Power MOSFETs are majority carrier devices, so they perform better in high frequency applications where switching power losses are important. Power MOSFETs can withstand of high current and voltage without undergoing destructive failure due to second breakdown (BJT case). Power MOSFET are based on isolated gate, that makes it easy to drive with minimum of power requirement. Power MOSFET’s main advantages are high commutation speed, superior switching speed and good efficiency at low voltages. Power MOSFET’s major drawback is on-resistance RDS (on) and its strong positive temperature coefficient. At high breakdown voltages (>200V) the on-state voltage drop of the power MOSFET becomes higher Power MOSFET is the most widely used as a low-voltage (i.e. less than 200V) switch mode power-supply (SMPS) converter applications. It can be found in most power supplies, DC to DC converters, and low voltage motor controllers.

Power MOSFET Fabrication Process n-Channel Power MOSFET Symbol Source Gate Source n n p+ n - epitaxial layer n+ substrate Drain

Power MOSFET: Lateral v/s Vertical Structure Power MOSFETs have a vertical structure compared to ordinary MOSFET having Lateral structure . D-MOSFET can be built using either a Lateral or vertical structure. In a lateral structure (ordinary MOSFET), current & breakdown voltage ratings are both functions of the channel dimensions (respectively width & length of the channel), resulting in inefficient use of the "silicon estate". Lateral structure is more suitable for integration and provides lower capacitance and higher speed. With a vertical structure (Power MOSFETs), voltage rating are function of the doping and thickness of the N-epitaxial layer, while current rating is a function of the channel width. This makes possible device able to sustain both high blocking voltage and high current within a compact piece of silicon. vertical structure supports higher breakdown voltage, lower on-resistance and higher current capability.

Power MOSFET: Lateral v/s Vertical Structure Lateral Structure Vertical Structure

Power MOSFET: Internal Cross sectional view Power MOSFET: Vertically Diffused (VDMOS) Structure

Power MOSFET Parasitic Components LTO: low-temperature oxide

Power MOSFET Parasitic Components Cgs is the capacitance due to the overlap of the source and the channel regions by the polysilicon gate and is independent of applied voltage. CGD consists of two parts. first is the capacitance associated with the overlap of the polysilicon gate and the silicon underneath in the JFET region. Second part is the capacitance associated with the depletion region immediately under the gate. Cgd is a nonlinear function of voltage. Cds capacitance associated with the body-drift diode, varies inversely with the square root of the drain-source bias. Capacitances of Cgs, Cgs and Cds are determined by the size of the chip (bigger chip, bigger capacitance), and the spacing of the parts of the chip, and by the manufacturing process.

Power MOSFET : Intrinsic body diode & Parasitic BJT Power MOSFET has a parasitic BJT as an integral part of its structure. The body region serves as the base, source as the emitter and the drain as the collector. It is important to keep this BJT OFF of all times by keeping the potential of the base as close to the emitter potential as possible. This is accomplished by shorting the body and the source part of the MOSFET. Otherwise, the potential at the base would turn on the BJT and lead the device into the “latch up” condition, which would destroy the device. Intrinsic body diode with patristic BJT

MOSFET: Merit/ Demerits Advantages Voltage controlled device Low gate losses Parameters are less sensitive to junction temperature No need for negative voltage during turnoff Limitations One disadvantage of MOSFET devices is their extreme sensitivity to electrostatic discharge (ESD) due to their insulated gate-source regions. The SiO2 insulating layer is extremely thin and can be easily punctured by an electrostatic discharge. High-on-state drop as high as 10V Lower off-state voltage capability Unipolar voltage device.

MOSFET: Summary A majority-carrier device: fast switching speed Typical switching frequencies: tens and hundreds of kHz On-resistance increases rapidly with rated blocking voltage Easy to drive The device of choice for blocking voltages less than 500V 1000V devices are available, but are useful only at low power levels (100W)