University of Pennsylvania ESE206: Electrical Circuits and Systems II Lab. MOSFET (Field Effect Transistor) Lab 1:

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1 Unversty of Pennsylvana ESE06: Electrcal Crcuts and Systems II Lab MOSFET (Feld Effect Transstor) Lab 1: NMOS Measurements and Characterzaton 1. Objectves The objectves of ths frst Feld Effect Transstors (FETs) lab s: 1. To understand the operaton of the MOSFET. To measure the I-V characterstcs and to determne the transstor parameters: a. Threshold voltage V t, b. Transconductance parameter k n ' W/L c. Channel length modulaton parameter λ (or Early voltage V A ) 3. To llustrate amplfcaton usng a MOSFET. Background A MOSFET transstor s a three termnal semconductor devce n whch current, flowng from the dran-source termnals, s controlled by the voltage on the gate termnal ( Fgure 1a). The current-voltage characterstcs of a NMOS transstor are shown n Fgure 1b. In order for current to flow the gate voltage V GS has to be larger than the threshold voltage V t. (a) JVdS/005 1

2 (b) Fgure 1 (a) NMOS transstor showng two dfferent symbols (wth basng voltages) and (b) ran current I vs. dran-source voltage V S wth gate-source voltage V GS as a parameter (V GS =1 to 3V n steps of 0.V). There are two regons n whch the transstor operates dependng on the voltages one apples. The frst regon s called the "trode" regon and the second one s called "saturaton" as ndcated n Fgure 1b. The current-voltage relatonshp n each regon s gven below..1 Trode regon (v S < v GS -V t ) When the dran to source voltage v S s smaller than v GS -V t, the transstor operates n the trode regon. The currents-voltage relatonshp s gven by, ' W 1 = kn ) vs vs L when v S < (v GS - V t ) (1) The parameter k n ' = μ n C ox s the transconductance parameter, μ n s the moblty, C ox s the oxde thckness and W/L the rato of the channel (or gate) wdth over length. Notce that when v S s much smaller than v GS -V t, the current can be approxmated as follows, ' W k n ) vs when v S << (v GS - V t ). () L Thus the transstor acts as a voltage-controlled resstor whose value s gven by (provded that v GS > V t ), JVdS/005

3 r S 1 = (3) ' W kn ) L.. Saturaton regon (v S > v GS -V t ) When the dran to source voltage exceeds the value v GS -V t, the channel wll be pnchedoff and the current can be wrtten as 1 W L ' = k n ) when v S > (v GS - V t ) (4) In the expresson above, the dran current s ndependent of the dran-source voltage what mples that the transstor acts as an deal current source n ths regon. Ths s only an approxmaton. In realty, the current wll vary slghtly wth the dran voltage. Ths varaton can be modeled by addng a parameter λ, called the channel length modulaton parameter, as shown n the followng expresson, 1 ' W = k ( ) n vgs (1 + λvs ) when v S > (v GS - V t ) (5) L The channel length modulaton parameter λ s usually pretty small (typcal values are 0.0 V -1 ). The output resstance of the transstor n saturaton can now be wrtten as, r 1 V o = I λ = I A (6). n whch I s the dran current, and V A (=1/λ) s the Early voltage..3 etermnaton of the transstor characterstcs. The man parameters that characterze a MOSFET are the threshold voltage V t, the parameter (k n ' W/L) and the channel length modulaton parameter λ. The frst two parameters can be easly found by plottng the square root of the dran current versus the gate voltage v GS, when the transstor s n saturaton. Indeed, usng equaton (4) one can wrte that 1 W ' = kn V ) (7) L t The ntersecton wth the horzontal axs gves the threshold voltage V t. and the slope of ths graph corresponds to, JVdS/005 3

4 Slope = 1 k ' n W / L. (8).4 PMOS transstor The PMOS transstor has smlar characterstcs. The only dfference s that the polarty of the voltages changes, as shown n Fgure. Fgure : PMOS transstor showng two symbols and drecton of the current flow. The values of the threshold voltage V t (for enhancement transstor), and of λ and k p ' are now negatve. The current expressons are then gven by, Trode regon: ' W 1 = k p ) vs vs L when v S > (v GS - V t ) (9) Saturaton regon: 1 W L ' = k p ) when v S < (v GS - V t ) (10) If can takes nto account the channel length modulaton, the current s gven by 1 ' W = k p ) (1 + λvs ) when v S < (v GS - V t ) (11) L JVdS/005 4

5 .5 Precauton when handlng MOSFETs MOSFET transstors are easly damaged, manly as a result of statc dscharge at the gate termnal. For that reason, one should handle MOSFETs wth care and not touch the pns of the package. In many cases one uses a ground strap to ground oneself when handlng MOSFETs. 3. Pre-lab assgnment 3.1 Read the secton on "The MOSFET as an Amplfer and Swtch," (secton 4.4, Sedra-Smth, 5 th edton). 3. Consder the crcut of Fgure 4 n whch the gate and dran termnals of the NMOS are shorted together. Assume that the transstor has a V t =0.7V and a k n '= 0.15mA/V and W/L= 5. a. In what regon s the transstor operatng (Trode, Saturaton)? b. Calculate the dran current for the followng gate-to-source voltages, v GS =v S =0, 1,, 3, 4, and 6V. Plot the current vs. voltage (use excel or Matlab). 3.3 The same transstor as above s used as a resstor, shown n Fgure 5, wth the dransource voltage V S =0.V. a. Calculate the value of the dran-source resstance r S for the followng gate voltages: v GS = 3, 4, 5, 6, 7, 8, 9 and 10V. b. Plot the value of the resstance as a functon of the gate voltage (usng excel or Matlab). 3.4 Consder the crcut of Fgure 6 n whch the NMOS transstor s used as a basc common-source amplfer. The transstor characterstcs are gven n Fgure 1b.and the Appendx A. a. Usng the graph of Fgure 1b (see appendx A) and the load lne, fnd graphcally the output voltage v S =v o as a functon of the gate voltage v GS, for v GS varyng from 1 to 3V n steps of 0.V. Use the same procedure as explaned n the textbook secton (Transfer Characterstc) of Sedra-Smth 5 th ed.). Plot the output voltage v S =v o as a functon of the nput voltage v n = v GS. Use excel or Matlab to draw ths graph (called the transfer functon). b. Fnd the value of the slope of the graph at the pont where the output s 5V. Ths slope corresponds to the amplfcaton of the amplfer. c. For the pre-lab nclude the two graphs (.e. the I S -v GS ) wth load lne and the Transfer characterstc (v S -v GS ). JVdS/005 5

6 4. In-Lab Experments Parts 1 - C4007 MOS transstor array (data sheet from Natonal Semconductor) mcrofarad capactors - 10 kohm resstor Power supples Osclloscope wth FFT module gtal multmeter (Voltage and Current meter) Procedure You wll be usng the C4007 MOSFET array that contans three NMOS and three PMOS transstors as shown n Fgure 3. The key pont to remember when usng ths array s that the substrate of the NMOS (bulk connecton) s connected to pn 7 and should always be connected to the most negatve supply voltage. Pn 14 s the substrate of the PMOS and must be connected to the most postve supply voltage n the crcut! Fgure 3: The C4007 MOSFET array. Pn 7 s connected to the substrate of the NMOS and should be connected to the most negatve voltage of the crcut; pn 14 s the bulk of the PMOS and should be connected to the most postve voltage n the crcut. (Source: Natonal Semconductor C4007 atasheet) 4.1. I S v GS characterstcs and determnaton of V t and k n ' W/L The goal of ths experment s to determne the dran current as a functon of the gate voltage when the transstor s n saturaton. From ths characterstc you can determne the threshold voltage and the transconductance parameter. a. Buld the crcut of Fgure 4. You can use any of the three NMOS transstors of the C4007 array. The node numbers gven on the schematc assume that you use the transstor between the pns 3, 4 and 5. o not forget to connect the substrate pn 7 to the ground. Place a 0.1 mcrofarad capactor between Pn 14 and the ground. Notce that the transstor s always n saturaton snce v GS = v S or v GS - V t < v S. JVdS/005 6

7 Fgure 4: NMOS transstor n saturaton used to measure -v GS characterstc. b. Vary the gate voltage (v GS = v S ) from 0 to 6V n steps of 1V and record the correspondng dran current. c. For the report: Plot the I -v GS graph. Also, calculate the square root of and plot the - v GS relatonshp. Use the method outlned n secton.3 to fnd the threshold voltage V t (the ntersecton wth the horzontal axs) and the transconductance parameter (k n ' W/L). 4. I - v S characterstcs and determnaton of the output resstance ro and λ The objectve of the followng experment s to measure the output characterstcs of the NMOS transstor: -v S wth v GS as a parameter. From ths graph you wll be able to determne the output resstor r o and the channel length modulaton parameter λ. Fgure 5: Crcut to measure the output characterstcs of a transstor. a. Buld the crcut of Fgure 5 (or modfy the crcut of Fgure 3). Use the same transstor as you used for the prevous experment. b. Keep the gate voltage constant at 3V and measure the dran current whle varyng the dran voltage from 0, 0.5, 1.0, 1.5,, 4, 6, 8 to 10V. c. o the same of a gate voltages v GS of 5V. d. For your report: Plot the two curves on the same graph. etermne the output resstance r o (.e. the nverse of the slope of the graphs) at a dran voltage of 5V for each graph. Notce that the output resstance decreases wth current, as one expects from expresson (6). Fnd the correspondng value of λ and the Early JVdS/005 7

8 voltage V A. It s lkely that the values you fnd for λ are slghtly dfferent for each graph. In that case, take the average value as the value for the transstor. 4.3 I - v GS for small values of v S (determnaton of the resstance of a MOSFET) In ths experment you wll keep the value of the dran voltage small so that the transstor operates n the trode regon. Snce v S s kept small the transstor acts as a resstor wth a value that s determned by the gate voltage (see expresson (3) above). The goal of ths experment s to expermentally determne the resstor values r S for varous gate voltages. a. In the crcut of Fgure 5, set the voltage v S =0.V so that the transstor wll act as a electroncally controlled resstor. b. Vary the gate voltage v GS from 3 to 10V n steps of 1V, and record the correspondng dran current S. Fnd also the value of the resstor r S = (v S / S ). c. For the report: determne the resstance r S from the measurements and plot the value of r S as a functon of the gate voltage v GS. Also, calculate the value of the resstance accordng to the expresson (3) and usng the measured values of V t and k n ' W/L. Plot the measured and calculated resstor values on the same graph. Notce the 1/x relatonshp. 4.4 Large-Sgnal Operaton: Transfer characterstc The objectve of ths measurement s to determne the transfer characterstc of the MOSFET amplfer, shown n Fgure 6. You wll determne the output versus the nput voltage. The graph you'll obtan wll be smlar to the one n Fg. 4.6(b) of the textbook (Sedra-Smth, 5 th ed., secton 4.4.1). From ths characterstc you wll be able to determne the amplfcaton of ths crcut. Fgure 6: Common Source NMOS amplfer. a. Buld the crcut of Fgure 6. b. Vary the gate voltage between 0 and 6V, and record the output voltage v S. You wll notce that the output wll change rapdly at one pont. When that happens JVdS/005 8

9 vary the nput voltage n small steps of 0.1V so that you can record the transfer characterstc accurately. It wll be useful to make a rough sketch of the characterstc n your lab notebook (or usng excel) so see how the transfer characterstc looks lke. c. For your report: plot the output voltage as a functon of the nput voltage. Fnd the slope of the transfer characterstc around the pont where the characterstc s steepest. The slope s the amplfcaton of the amplfer (.e. the rato of the output to the nput voltage). You'll notce that when the changes n the nput voltage around the Q pont are kept small, the characterstc s qute lnear and gves a constant amplfcaton. For larger values of the nput voltage, one wll see some dstorton n the output sgnal snce the characterstc s not lnear for large nput sgnals. etermne the maxmum value (n peak-to-peak voltage) of the nput sgnal that gves a relatvely constant amplfcaton The MOSFET as a voltage amplfer In ths experment you wll buld a smple common-source voltage amplfer, smlar to the one shown n Fgure 6. From the prevous experment you have notced that the amplfcaton (.e. the change of output voltage over nput voltage) s relatvely large around a narrow range of the nput voltage (see e.g. Fg. 4.6c n Sedra-Smth, 5 th ed.). So, t s mportant to bas the transstor n the regon where the amplfcaton s large. It s for that reason that we need to apply a C nput voltage V1 shown n Fgure 7. The actual nput sgnal v n wll be supermposed on ths C sgnal. Ths can be done by a couplng capactor C1 shown n Fgure 7. The voltage seen at the gate termnal of the transstor wll than be the sum of V1 and v n. Fgure 7: Common source amplfer wth nput sgnal v n s supermposed on the bas voltage V1, usng a couplng capactor C1. a. Buld the crcut of Fgure 7. Adjust the voltage V1 so that the output voltage v o s around 5V (you can the correspondng v GS from the prevous measurement of the transfer characterstc). Use the osclloscope to verfy the value of v o. For the nput sgnal v n use a snusodal sgnal of 5 khz and ampltude of 00 mv (.e. 400 mv peak-to-peak). splay ths sgnal on the osclloscope to verfy t value. b. splay the output v o of the amplfer on the osclloscope together wth the nput sgnal. Measure the ampltude of the output sgnal. What s the correspondng JVdS/005 9

10 value of amplfcaton (.e. the rato of the ampltude of the output snusod to the nput snusod)? What do you notce about the phase relatonshp between the nput and output sgnal? Is the sgnal dstorted? Take a snapshot of the nput and output sgnals for your report. c. You may change the ampltude of the nput and see f the dstorton mproves or deterorates. You can see the effect of the non-lnearty of the amplfer better when you swtch to a trangular nput sgnal. Notce the dstorton (or lack of dstorton). Take a snapshot for your report. d. Swtch the nput back to a snusod wth 5 khz frequency. Set the ampltude so that the output sgnal s not too much dstorted. Take the FFT of the output sgnal and determne the ampltude (n db) and poston (frequency) of the peaks. If the sgnal s not dstorted you should have a sngle peak at a frequency of 5kHz. The presence of peaks at multples of the fundamental frequency of 5kHz ndcates that the sgnal s dstorted. It s normal to see multple peaks for ths smple amplfer. Later we'll dscuss ways to reduce the dstorton. Take a snapshot for your report. e. etermne the cut-off frequences (.e. the 3-dB where the ampltude of the output sgnal has decreased by 3-dB). o ths by changng the frequency of the nput sgnal from a few tens of Hz up to hundred of khz. Record the values of the two 3-dB ponts. What s bandwdth of the amplfer? f. For your report: Compare the value of the amplfcaton determned n ths experment wth the value obtaned from the transfer characterstc (prevous experment of secton 4.4). What s the phase relatonshp between nput and output? Can you explan the phase relatonshp? From the FFT determne the total harmonc dstorton TH of the output sgnal (for defnton of total harmonc dstorton see prevous lab on AM emodulator: AMemodLabOpAmpPart.pdf. The couplng capactor and resstor R1 n Fgure 7 acts as a hgh pass flter. Calculate the correspondng 3-dB pont of ths flter. Express the value n Hz. Compare ths value to the one measured n the lab (low frequency 3-dB pont). References 1. "Mcroelectronc Crcuts, Sedra, Smth, 5 th edton, Oxford Unversty Press, New York, "The Art of Electroncs", Horowtz and Hll, Cambrdge Unversty Press. 3. "C4007M/C4007C ual Complementary Par Plus Inverter" atasheet, Natonal Semconductor, Created by Jan Van der Spegel jan_at_seas.upenn.edu Aprl 5, 005 JVdS/005 10

11 ** Profle: "SCHEMATIC1-CTran" [ C:\My ocuments\classwork\@ese16\pspcesmulatons\nmoschar-schemat... ate/tme run: 03/7/05 14:00:35 Temperature: 7.0 (A) nmoschar-schematic1-ctran.dat (actve).0ma Saturaton Vgs=3V 1.5mA Trode mA mA A 0V V 4V 6V 8V 10V I(M1) V_V ate: March 7, 005 Page 1 Tme: 14:07:04