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f repair 4 in particular it is the most widely used low voltage less than 200 v switch it can be found in a wide range of applications such as most power supplies dc to dc converters low voltage motor controllers and many other applications history edit see also mosfet vmos ldmos and insulated gate bipolar transistor the mosfet was invented at bell labs between 1955 and 1960 5 6 7 8 9 10 it was a breakthrough in power electronics generations of mosfets enabled power designers to achieve performance and density levels not possible with bipolar transistors 11 in 1969 hitachi introduced the first vertical power mosfet 12 which would later be known as the vmos v groove mosfet 13 the same year the dmos double diffused mosfet with self aligned gate was first reported by y tarui y hayashi and toshihiro sekigawa of the electrotechnical laboratory etl 14 15 in 1974 jun ichi nishizawa at tohoku university invented a power mosfet for audio which was soon manufactured by yamaha corporation for their high fidelity audio amplifiers jvc pioneer corporation sony and toshiba also began manufacturing amplifiers with power mosfets in 1974 16 siliconix commercially introduced a vmos in 1975 13 the vmos and dmos developed into what has become known as vdmos vertical dmos 16 john moll s research team at hp labs fabricated dmos prototypes in 1977 and demonstrated advantages over the vmos including lower on resistance and higher breakdown voltage 13 the same year hitachi introduced the ldmos lateral dmos a planar type of dmos hitachi was the only ldmos manufacturer between 1977 and 1983 during which time ldmos was used in audio power amplifiers from manufacturers such as hh electronics v series and ashly audio and were used for music and public address systems 16 with the introduction of the 2g digital mobile network in 1995 the ldmos became the most widely used rf power amplifier in mobile networks such as 2g 3g 17 and 4g 18 alex lidow co invented the hexfet a hexagonal type of power mosfet at stanford university in 1977 19 along with tom herman 20 the hexfet was commercialized by international rectifier in 1978 13 20 the insulated gate bipolar transistor igbt which combines elements of both the power mosfet and the bipolar junction transistor bjt was developed by jayant baliga at general electric between 1977 and 1979 21 the superjunction mosfet is a type of power mosfet that uses p columns that penetrate the n epitaxial layer the idea of stacking p and n layers was first proposed by shozo shirota and shigeo kaneda at osaka university in 1978 22 david j coe at philips invented the superjunction mosfet with alternating p type and n type layers for which a us patent was awarded in 1988 23 applications edit main article list of mosfet applications power mosfet see also ldmos applications mosfet insulated gate bipolar transistor and rf cmos applications nxp 7030al n channel trenchmos logic level fet irf640 power mosfet die the power mosfet is the most widely used power semiconductor device in the world 3 as of 2010 update the power mosfet accounts for 53 of the power transistor market ahead of the insulated gate bipolar transistor 27 rf power amplifier 11 and bipolar junction transistor 9 24 as of 2018 update over 50 billion power mosfets are shipped annually 25 these include the trench power mosfet which sold over 100 billion units up until february 2017 26 and stmicroelectronics mdmesh superjunction mosfet which has sold 5 billion units as of 2019 update 22 power mosfets are commonly used for a wide range of consumer electronics as well as transportation technology including a wide range of vehicles 27 28 power mosfets are widely used in automotive electronics 29 30 27 rf dmos also known as rf power mosfet is a type of dmos power transistor designed for radio frequency rf applications it is used in various radio and rf applications 31 32 power mosfets including dmos ldmos and vmos are commonly used for a wide range of other applications 33 34 35 basic structure edit fig 1 cross section of a vdmos showing an elementary cell note that a cell is very small some micrometres to some tens of micrometres wide and that a power mosfet is composed of several thousand of them several structures had been explored in the 1970s when the first commercial power mosfets were introduced however most of them have been abandoned at least until recently in favour of the vertical diffused mos vdmos structure also called double diffused mos or simply dmos and the ldmos laterally diffused mos structure the cross section of a vdmos see figure 1 shows the verticality of the device it can be seen that the source electrode is placed over the drain resulting in a current mainly vertical when the transistor is in the on state the diffusion in vdmos refers to the manufacturing process the p wells see figure 1 are obtained by a diffusion process actually a double diffusion process to get the p and n regions hence the name double diffused power mosfets have a different structure from the lateral mosfet as with most power devices their structure is vertical and not planar in a planar structure the current and breakdown voltage ratings are both functions of the channel dimensions respectively width and length of the channel resulting in inefficient use of the silicon real estate with a vertical structure the voltage rating of the transistor is a function of the doping and thickness of the n epitaxial layer see cross section while the current rating is a function of the channel width this makes it possible for the transistor to sustain both high blocking voltage and high current within a compact piece of silicon ldmos are power mosfets with a lateral structure they are mainly used in high end audio power amplifiers 16 and rf power amplifiers in wireless cellular networks such as 2g 3g 17 and 4g 18 their advantage is a better behaviour in the saturated region corresponding to the linear region of a bipolar junction transistor than the vertical mosfets vertical mosfets are designed for switching applications so they are only used in on or off states on state resistance edit fig 2 contribution of the different parts of the mosfet to the on state resistance when the power mosfet is in the on state see mosfet for a discussion on operation modes it exhibits a resistive behaviour between the drain and source terminals it can be seen in figure 2 that this resistance called r dson for drain to source resistance in on state is the sum of many elementary contributions r s is the source resistance it represents all resistances between the source terminal of the package to the channel of the mosfet resistance of the bond wires of the source metallisation and of the n wells r ch this is the channel resistance it is inversely proportional to the channel width and for a given die size to the channel density the channel resistance is one of the main contributors to the r dson of low voltage mosfets and intensive work has been carried out to reduce their cell size in order to increase the channel density r a is the access resistance it represents the resistance of the epitaxial zone directly under the gate electrode where the direction of the current changes from horizontal in the channel to vertical to the drain contact r jfet is the detrimental effect of the cell size reduction mentioned above the p implantations see figure 1 form the gates of a parasitic jfet transistor that tend to reduce the width of the current flow r n is the resistance of the epitaxial layer as the role of this layer is to sustain the blocking voltage r n is directly related to the voltage rating of the device a high voltage mosfet requires a thick low doped layer i e highly resistive whereas a low voltage transistor only requires a thin layer with a higher doping level i e less resistive as a result r n is the main factor responsible for the resistance of high voltage mosfets r d is the equivalent of r s for the drain it represents the resistance of the transistor substrate the cross section in figure 1 is not at scale the bottom n layer is actually the thickest and of the package connections breakdown voltage on state resistance trade off edit fig 3 the r dson of mosfets increases with their voltage rating when in the off state the power mosfet is equivalent to a pin diode constituted by the p diffusion the n epitaxial layer and the n substrate when this highly non symmetrical structure is reverse biased the space charge region extends principally on the lightly doped side i e over the n layer this means that this layer has to withstand most of the mosfet s off state drain to source voltage however when the mosfet is in the on state this n layer has no function furthermore as it is a lightly doped region its intrinsic resistivity is non negligible and adds to the mosfet s on state drain to source resistance r dson this is the r n resistance in figure 2 two main parameters govern both the breakdown voltage and the r dson of the transistor the doping level and the thickness of the n epitaxial layer the thicker the layer and the lower its doping level the higher the breakdown voltage on the contrary the thinner the layer and the higher the doping level the lower the r dson and therefore the lower the conduction losses of the mosfet therefore it can be seen that there is a trade off in the design of a mosfet between its voltage rating and its on state resistance citation needed this is demonstrated by the plot in figure 3 body diode edit it can be seen in figure 1 that the source metallization connects both the n and p implantations although the operating principle of the mosfet only requires the source to be connected to the n zone however if it were this would result in a floating p zone between the n doped source and drain which is equivalent to a npn transistor with a non connected base under certain conditions under high drain current when the on state drain to source voltage is in the order of some volts this parasitic npn transistor would be triggered making the mosfet uncontrollable the connection of the p implantation to the source metallization shorts the base of the parasitic transistor to its emitter the source of the mosfet and thus prevents spurious latching this solution however creates a diode between the drain cathode and the source anode of the mosfet making it able to block current in only one direction body diodes may be utilized as freewheeling diodes for inductive loads in configurations such as h bridge or half bridge while these diodes usually have rather high forward voltage drop they can handle large currents and are sufficient in many applications reducing part count and thus device cost and board space to increase efficiency synchronous rectification is often used to minimize the amount of time that the body diode conducts current switching operation edit fig 4 location of the intrinsic capacitances of a power mosfet because of its unipolar nature the power mosfet can switch at very high speed indeed there is no need to remove minority carriers as with bipolar devices the only intrinsic limitation in commutation speed is due to the internal capacitances of the mosfet see figure 4 these capacitances must be charged or discharged when the transistor switches this can be a relatively slow process because the current that flows through the gate capacitances is limited by the external driver circuit this circuit will actually dictate the commutation speed of the transistor assuming the power circuit has sufficiently low inductance further information mosfet gate driver capacitances edit in the mosfet datasheets the capacitances are often named c iss input capacitance drain and source terminal shorted c oss output capacitance gate and source shorted and c rss reverse transfer capacitance source connected to ground the relationship between these capacitances and those described below is c i s s c g s c g d c o s s c g d c d s c r s s c g d displaystyle begin matrix c_ iss c_ gs c_ gd c_ oss c_ gd c_ ds c_ rss c_ gd end matrix where c gs c gd and c ds are respectively the gate to source gate to drain and drain to source capacitances see below manufacturers prefer to quote c iss c oss and c rss because they can be directly measured on the transistor however as c gs c gd and c ds are closer to the physical meaning they will be used in the remaining of this article gate to source capacitance edit the c gs capacitance is constituted by the parallel connection of c oxn c oxp and c oxm see figure 4 as the n and p regions are highly doped the two former capacitances can be considered as constant c oxm is the capacitance between the polysilicon gate and the metal source electrode so it is also constant therefore it is common practice to consider c gs as a constant capacitance i e its value does not depend on the transistor state gate to drain capacitance edit the c gd capacitance can be seen as the connection in series of two elementary capacitances the first one is the oxide capacitance c oxd constituted by the gate electrode the silicon dioxide and the top of the n epitaxial layer it has a constant value the second capacitance c gdj is caused by the extension of the space charge zone when the mosfet is in off state therefore it is dependent upon the drain to gate voltage from this the value of c gd is c g d c o x d c g d j v g d c o x d c g d j v g d displaystyle c_ gd frac c_ oxd times c_ gdj left v_ gd right c_ oxd c_ gdj left v_ gd right the width of the space charge region is given by 36 w g d j 2 ϵ s i v g d q n displaystyle w_ gdj sqrt frac 2 epsilon _ si v_ gd qn where ϵ s i displaystyle epsilon _ si is the permittivity of the silicon q is the electron charge and n is the doping level the value of c gdj can be approximated using the expression of the plane capacitor c g d j a g d ϵ s i w g d j displaystyle c_ gdj a_ gd frac epsilon _ si w_ gdj where a gd is the surface area of the gate drain overlap therefore it comes c g d j v g d a g d q ϵ s i n 2 v g d displaystyle c_ gdj left v_ gd right a_ gd sqrt frac q epsilon _ si n 2v_ gd it can be seen that c gdj and thus c gd is a capacitance whose value is dependent upon the gate to drain voltage as this voltage increases the capacitance decreases when the mosfet is in on state c gdj is shunted so the gate to drain capacitance remains equal to c oxd a constant value drain to source capacitance edit as the source metallization overlaps the p wells see figure 1 the drain and source terminals are separated by a p n junction therefore c ds is the junction capacitance this is a non linear capacitance and its value can be calculated using the same equation as for c gdj other dynamic elements edit equivalent circuit of a power mosfet including the dynamic elements capacitors inductors the parasitic resistors the body diode packaging inductances edit to operate the mosfet must be connected to the external circuit most of the time using wire bonding although...
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