L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 1 I. Review of JFET (DRAW symbol for n-channel type, with grounded source) 1. "normally on" device A. current from source to drain when V G = 0 no need to turn on B. what device is normally off? C. making V G negative decreases I D D. V G not normally > 0 V i) because that would forward bias G-S diode ii) iii) I G no longer zero, depletion layer electric field gone so gate no longer in control of I D II. MOSFET'S ("metal-oxide-semiconductor FETs") 1. advantages A. V G can be positive or negative and still control I D, so biasing is easier B. device draws no gate current from source, so higher z in for amplifiers--does not load down source voltage (controller) (CONSTRUCT and SHOW example touch switch circuit of Fig. 1)
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 2 2. uses A. logic ICs (CMOS) B. low power circuits, such as wristwatches C. mixers D. power amplifiers (VMOSFETs) 3. the normally-on (D, depletion-mode) type MOSFET A. symbols and construction (Fig. 2, 2A)
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 3 metal gate oxide insulator D n semiconductor channel G p semiconductor substrate S Fig. 2A Depletion mode MOSFET construction (n-channel) i) has same terminals as JFET ii) iii) iv) but gate is insulated from channel no bipolar junction oxide is the insulator small capacitor between gate and channel formed v) n-channel has arrow pointing in vi) vii) p-channel has arrow pointing out note difference with bipolar transistor how it works on the atomic level not important for our purposes, but covered in textbook B. performance (SHOW using circuit of Fig. 2B
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 4 LND150 S G D i) same as JFET when V G negative or zero (a) I D = I DSS when V G = 0 (b) making V G more negative decreases I D (c) until V G reaches V GS(off), when I D = 0 ii) but still works for V G > 0 V (reverse V GS power supply) (a) (b) I D increases over I DSS (I DSS no longer the highest possible drain current) but still no gate current, unlike JFET because gate is insulated from channel (c) I D still = I DSS x (1 - V GS /V GS(off) ) 2 iii) characteristic curves (Fig. 3)
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 5 I D V GS = + 0.2 V I DSS V GS = 0 V V GS =- 0.2 V V GS = V GS(off) V DS Fig. 3 Characteristic curves for depletion mode MOSFET iv) example--find I D in Fig. 4 C. utility solution: I D = 10 ma x (1 - +1/-2) 2 = 10 ma x 1.52 = 22.5 ma (> I DSS ) i) not used very much in practice
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 6 ii) but an important step in evolution to the most important and useful type of MOSFET, the... 4. normally off (E-type, enhancement mode) type MOSFET A. symbol and construction (Fig. 5 and 5A) metal gate oxide insulator D n semiconductor channel G p semiconductor substrate, completely blocks the channel S Fig. 5A Enhancement mode MOSFET construction (n-channel) i) dashed channel line on the symbol shows that device is normally off ii) because p-type silicon substrate region completely interrupts the channel
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 7 B. performance (SHOW, using circuit of Fig. 6) 2N7000 S G D i) off (I D = 0) when V G (= V GS ) = 0, unlike JFET ii) iii) iv) like bipolar transistor, no negative bias needed used in CMOS IC's as V G increases, no current until a certain level reached, V GS(th), the threshold voltage v) V GS(th) usually more than 0.7 V, unlike transistor vi) vii) viii) also unlike transistor, never any gate (base) current like bipolar transistor circuit, varying V DS doesn't change I D much after you get out of saturation region (SHOW by varying V DD supply) characteristic curves? (fill in actual observed values on Fig. 7 and Fig. 8)
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 8 I D V GS = V GS = V GS = V GS = V GS(th) V DS Fig. 7 Characteristic curves for enhancement mode MOSFET
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 9 I D V DS = +10 V V GS V GS(th) Fig. 8 Characteristic curve for enhancement mode MOSFET ix) I DSS? (meaningless) x) I D increases rapidly with V GS idealized formula: I D = k x (V GS - V GS(th) ) 2 with k a constant C. VMOSFET (SHOW example) i) stands for "vertical" MOSFET ii) iii) iv) because of geometry (refer to text) same as E-type MOSFET, but can handle more drain current replacing bipolar transistors in many applications v) example application (touch switch circuit of Fig. 1) (a) draw circuit diagram
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 10 (b) (c) (d) could you do this with single bipolar transistor? why? what determines sensitivity to touch? could you vary circuit to make it normally on? 5. parameters for all MOSFET's (SHOW data sheet for 2N7000) A. I D(max) B. V GS(max) (insulator breakdown) C. V DS(max) (important when device off or on?) D. P D(max) (I D x V DS, how much heat can device dissipate...) III. Comparison of FET Devices--identify the type of FET from the given data 1. V GS = +0.5 V, I D = 1 ma, I G = 40 µa 2. V GS = +0.1 V, I D = 0 ma, I G = 0 µa 3. characteristic curves of Fig. 9
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 11 I D V GS = + 0.1 V V GS = 0 V V GS =- 0.1 V V GS = - 0.5 V V DS Fig. 9 Characteristic curves for unknown MOSFET 4. characteristic curves of Fig. 10
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 12 I D 3 ma - 1 V Fig. 10 Characteristic curve for unknown MOSFET V GS 5. characteristic curves of Fig. 11
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 13 I D 3 ma - 1 V Fig. 11 Characteristic curve for unknown MOSFET V GS 6. characteristic curves of Fig. 12
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 14 I D V GS = + 6 V V GS = + 5 V V GS = + 4 V V GS = + 3 V V DS Fig. 12 Characteristic curves for unknown MOSFET IV. MATERIALS 1. n-channel VMOS power MOSFET (IR511) 2. LND150 n-channel D-type MOSFET 3. 2N7000 n-channel E-type MOSFET 4. DC supply 5. 6.3-VDC lamp 6. 10-MΩ resistor 7. DMM
L.107.4 MOSFETS, IDENTIFICATION, CURVES. PAGE 15 8. ammeter