94, MOSFETS The MOSFET is a abbreviation of Metal Oxide Semiconductor Field Effect rransistor, Like JFET it has a source, gate and drain. It is also called as |GPET (Insulated gate field effect transistor) because MOSFET is insulated (rom the channel. Basically the MOSFETs are of a two types namely ‘depletion type MOSFET” and “Enhance-type MOSFET”. MOSFET 1 See, ee Depletion mode MOSFET Enhancement only MOSFET (it includes both depletion and enhancement mode) eee P channel N channel P channel N channel the N channel MOSFET consists of a highly doped 'P' type substrate into which two highly doped 'N' regions are diffused as shown in figure 6. These 'N’ regions which will acts as souree and drain. dioxide (SiO,) is grown over the surface of A thin layer of insulating silicon cut into the oxide layer, allowing to move the structure and free electrons ar between source and drain. The metal area is overlaid on the entire oxide layer, similarly metal contacts tre made to source and drain as shown in figure 32.DyOrain v Jo 9s of ss i Substrate F Figure 32 : Cross Sectional view of N Channel MOSFET and symbot ‘The metal area of the gate, in conjugation with the Insulating dicleetrie wy layer and the semiconductor channel form a parallel plate capacitor, ‘The SiO, layer insulates the gate from the channel due to which a nogligihs gate current flows even if the biasing is applied to gate. It eausos no PN junetin is existing in MOSFET like JFET thus, it is alo known as Insulated (ny Field Effect Transistor (IGFET). ‘The difference between the JFET and MOSFET are i) MOSFET has only one P region which Is called substrate, L¢., there ls w PN junction like JPET, i) ‘The gate is insulated from the conducting channel by metal oxide insulating film, s0 it is also called as Insulated gate FET, {il) In case of MOSFET both positive and negative voltages can be applied the gate as it is insulated from the channel, For negative bias voltuge acts as depletion MOSFET, while with positive gate bias it acts as enhancement MOSFET, ly) Gate and channel form a parallel plate capacitor in the MOSFET #880 the silicon dioxide acts as dicleetric, 25. DEPLETION MODE MOSFET ‘The depletion type MOSFET ean be operated in two different models as # below 4, Depletion Mode : Tho dovioo opornton in thiv mod, when the gate is negative, ‘b. Enchancement mode ; ‘he device , when tne reaiee ‘operates in this: mode,pepletion mode he figure 88(a) shows a MOSFET wi ith a negative gate to source volt when Vos = 0a significant current a ee hen negative voltage is + Wee 8° Is applied on the gate, electrons accumulate on it, if one plate of capacitor (gate) is Negatively charged, induces a positive charge in the other plate (channel). Because of this, free electrons in the vicinity of positive charges are repelled away in the channel ¢ Asa-result of this, the channel is depleted of free electrons passing through the channel! thus the conduction between source to drain is reduced. Thus as the value of negative gate to source voltage is increased, the value of drain current decreases. © Ata sufficient negative value of gate to source voltage called Vas(off) the channel is totally depleted of free electrons and therefore the drain current reduces to zero. Thus with the negative gate voltage, the operation of MOSFET is similar to that of a JFET. © From the above discussion, the negative gate voltage depletes the channel of free electron thus, the working of a MOSFET, with a negative gate voltage, is called "Depletion mode operation’. (a) Depletion mode of MOSFET (&) Enhancement mode of MOSFET Figure 33 : Biasing of MOSFET . Enhancement mode ‘The figure 3(b) shows a MOSFET with a positive gate to source voltage. ‘The positive gate voltage increases the number of free electrons pale the channel. ‘The greater the gate voltage, the greater is the number dlectrons passing through the channel. This nereases 1c, enchances the conduetiod Of the channel, this positive gate voltage operation of MOSFET Is called *tchancement mode of MOSFET. .26, ENHANCEMENT - ONLY MOSFET ‘The enhancement type MOSFET has no depletion mode and it operate in enhancement mode, It differs in construction from the depletion type \iosy., in the sense that it has no physical channel. ‘ ‘The figure 34 shows the basic structure of the N Channel enchanceme MOSFET: It may be noted that the P type substrate extends the sitic layer completely as shown in figure 34. It must be noted that this MOSFET is always operated with the positive g to source voltage (Vas). When gate to source voltage is be tries to force free electrons from source to drain. 10, the Voy supp But the presence of P region does not permit the electrons to pass through i ‘Vhus there is no drain current for Vos = 0. Due to this fact the "Enhancemen, type MOSFET” is also called "Normally OFF MOSFET". SiO, layer Figure 34 : Biasing for Enhancement only MOSFET d to the gate, it induces a negative eel nt to the silicon dioxide layer. ‘The Inti" | would attracting the free electrons from‘The minimum gate to source voltage (V qailed “threshold voltage” ‘s), Which produces invertion layer is and is designated by the symbol Vas(th)- When the voltage Vos is less than Vos(th), no current flows from drain to source. However, when the voltage V, connects the is greater than Vos(th), the inversion layer drain and source and we get significant values of current. 27. COMPARISON OF JFETS AND MOSFETS JPET and MOSFETS are remarkably well as in their electri differences JF! eT JEET operates only in depletion mode, because the input junetion is always reverse biased. If the gate source (input junction) is forward biased excess carrier in junction enhances the conduction and the gate current is substantial and it is undesirable, 2. Input impedance of JFET is less because gate takes a very small leakage current. 3. Output characteristics in flatter than MOSFET, it indicates the drain resistance in high. 4. Difficult to fabricate than MOSFET, 5. PN junction formed between gate and channel . JPRT does not requires any ‘Additional protection circuits. similar in their operating principles as | characteristies. However, the following are the main MOSFET MOSFET operates in both enhancement and depletion method. . MOSFET has high input impedance than JFET because the gate current is negligible. . Drain resistance is less. . Easier to fabricate ». There is no PN junction, due to the SiO, layer between gate and channel it acts as capacitor. |. MOSFET get damaged easily, if they ‘are not operated properly thus additional protective cireuits are needed. i aLETION MODE MOSFE'T type MOSFI a ISTICS OF DE! 28, CHARAC’ (i) Drain characteristics of Depleti ‘The figure 35 shows the drain characteristics for the N channel depltio, type MOSFET. e Doshi arco ‘mode mo — } exnancement mode c J oston mise a % (a) Drain Characteristics of depeletion (b) Transfer characteristics of type MOSFETs depletion type MOSFET Figure 35 ® These curves are plotted for both negative and positive values of gate to Source voltage (Vas) The euros shown above the Vag = havea pie ‘Value where a8 those below it have a negative value of Vas. © When Vas is zero and ne; the MOSFET operates in the depletion mode... On the other hand, if Vos is zero and positive, the MOSFET operates in the enhancement mode.(The 01 rence the JFET and the depletion MOSFET Ws That JFEY does not operate for positive values o! gate to source voltage Was) © When Vos = 0, there is no conduction takes place between source to dr it Vos < 0, und Vos > OL, increased above zero volt, then drain curret! imearly, As a result of Vos < Os applied to the gate induces positive charged holes in the channel, and also it controls the chant! ‘width. ‘Thus the conduetion (between source to drain is maintained * constant). 1¢., Ip is constant, ow Von > 0 the ute Induces more electrons in channel side, it 8 eS pies “4 the channel of ee width and maintains “flow through it as Jn figure 85(a),Tp vs Vas (Vop -» omabenl) for an N channe} depletion 8 curve, that the region AB of the (i) Transfer Characteristic of Depletion-type MOS PET e 35(b) shows whe the transfer characteristies type MOSFET. It may be noted from th tic is similar to that of JFET character This curve ext The value of Ings s for the positive values 01 gate-to- epresents the current from drai The drain current at any point along the the relation, urce voltage Veg also, to-source with Vag = 0. transfer characteristic is given by jaan toile aa pis a 1 may be noted that even if Vs = 0, the device has a drain current equal to Ipss- Due to this faet it is called normally - ON MOSFET. © Indepletion mode, when Vs = 0 maximum current will flow between souree to drain thus Ip = Ipss (refer point B) similarly, when Vgs increased continuously, after @ certain extend the positive charges induced by gate complctly depletes the channel thus no drain current i.e., Vos = Vp (refer point A) In enhancement mode of increasing Vos > 0 more free electrons are induced in the channel, thus it enhances the electron result in which Ip increases as shown in figure 35. 29, CHARACTERISTICS OF ENHANCEMENT - ONLY MOSFET () Drain characteris’ Vaxw), Me me ae Wm Bae pars parrot sie As 0b 21 Vai jax ‘The drain eharacteristies for N- channel enhancement: type MOSFET is shown in figure 36(a), It may be noted from this figure 86(b), that the gate-to- Source voltage (Vu) is less than threshold voltage, Vog(th), there is no drain current. Mowever, in actual practice, an extremly small value of deain current does flow through the MOSFET. This current flow is due to the presence of ‘hormally generated electrons in the P type substrate. When the value of Vos Is kept above Vag(th), a significant drain current flows. ‘The value of drain current increases with Increase in gate to source {Lis because of the fact that the width of inversion layer widens for increased Value of Voy and therfore allows more number of free electrons to pass through |. The drain current reachos {ts saturation value above cortain value '0 source voltage (Vin): m sie aeVes (volts) (b) Transfer characteristics (a) Drain characteristics of MOSFET Enhancement type MOS! Figure 36 (ii) Transfer Characteristics for Enhancement-type MOSFET The figure 36(b) shows the transfer Characteristics for N-channe! enie°° type MOSFET, It may be noted from this figure 36(b) there is no drain when the gate-to-source voltage, Vag = 0, However, if Vas is inerease® the threshold voltage, Vag(th), the drain current at any point along '™ is given by the relation, Jy © K[ Yax~ Vay(th)]? — Where k is @ constant, whose value dee?” the type of MOSFET.