FET Prepared by Engr. JP Timola Reference: Electronic Devices by Floyd FET (field-effect transistor) unipolar devices - unlike BJTs that use both electron and hole current, they operate only with one type of charge carrier The term field-effect relates to the depletion region formed in the channel of a FET as a result of a voltage applied on one of its terminals (gate). very high input resistance nonlinear characteristics - generally not as widely used in amplifiers as BJTs except where very high input impedances are required. JFET Preferred device in low-voltage switching applications - generally faster than BJTs when turned on and off. The two main types: junction field-effect transistor (JFET) metal oxide semiconductor field-effect transistor (MOSFET). type of FET that operates with a reverse-biased pn junction to control current in a channel fall into either of two categories, n channel or p channel
BASIC OPERATION VDD - provides a drain-tosource voltage and supplies current from drain to source VGG - sets the reverse-bias voltage between the gate and the source JFET biased for conduction Greater V GG narrows the channel which increases the resistance of the channel and decreases I D. Less V GG widens the channel which decreases the resistance of the channel and increases I D
JFET Symbols JFET AT V GS =0 V GS = Gate-Source Voltage V GS(off) = Gate-Source Cut-off Voltage V DS = Drain-Source Voltage I D = Drain Current I DSS = Zero Gate Voltage Drain Current Drain Characteristic Curve Pinch-Off Voltage For V GS = 0 V, the value of V DS at which I D becomes essentially constant is the pinch-off voltage, V P. Breakdown breakdown occurs at point C when I D begins to increase very rapidly with any further increase in V DS. Breakdown can result in irreversible damage to the device, so JFETs are always operated below breakdown and within the active region
Cutoff Voltage The value of V GS that makes I D approximately zero is the cutoff voltage, V GS(off), The JFET must be operated between V GS = 0 V and V GS(off). For this range of gate-to-source voltages, I D will vary from a maximum of I DSS to a minimum of almost zero. Note Pinch-off occurs for V DS values less than V P when V GS is nonzero V GS(off) and V P are always equal in magnitude but opposite in sign. BIASED P-CHANNEL JFET For the JFET, V GS(off) = -4 V and I DSS = 12 ma. Determine the minimum value of V DD required to put the device in the constant-current current region of operation when V GS = 0 V. If VDD is increased to 15 V, what is the drain current? I D JFET Universal Transfer Characteristic I DSS V GS 1 VGS ( off ) 2
Determine I D for V GS = -3 V for the 2N5459 JFET A 2N5459 JFET indicates that it typically has I DSS = 9 ma and V GS(off) = - 8V (maximum). Using these values, determine the drain current for V GS = 0, -1 V and -4 V MOSFET has no pn junction structure instead, the gate of the MOSFET is insulated from the channel by a silicon dioxide (SiO 2 ) layer MOSFET Two basic types - Enhancement (E) -Depletion (D) Sometimes called IGFETs (insulatedgate FETs) Enhancement MOSFET (E-MOSFET) operates only in the enhancement mode and has no depletion mode Enhancement MOSFET (E-MOSFET) conductivity of the channel is enhanced by increasing the gate-to-source voltage and thus pulling more electrons into the channel area. For any gate voltage below the threshold value, there is no channel.
Enhancement MOSFET (E-MOSFET) E-MOSFET schematic symbols Depletion MOSFET (D-MOSFET) The drain and source are diffused into the substrate material and then connected by a narrow channel adjacent to the insulated gate Depletion MOSFET (D-MOSFET) can be operated in either of two modes the depletion or the enhancement mode sometimes called a depletion/enhancement MOSFET. Depletion MOSFET (D-MOSFET) Depletion Mode Since the gate is insulated from the channel, either a positive or a negative gate voltage can be applied. The n-channel MOSFET operates in the depletion mode when a negative gate-tosource voltage is applied and in the enhancement mode when a positive gateto-source voltage is applied. These devices are generally operated in the depletion mode.
Enhancement Mode D-MOSFET schematic symbols Other types of MOSFET Laterally Diffused MOSFET (LDMOSFET) VMOSFET TMOSFET Dual-Gate MOSFETs E-MOSFET Transfer Characteristic The E-MOSFET uses only channel enhancement. Therefore, an n- channel device requires a positive gate-to-source to voltage, and a p- channel device requires a negative gate-to-source voltage. E-MOSFET Transfer Characteristic there is ideally no drain current until V GS reaches a certain nonzero value called the threshold voltage, V GS(th). The equation for the E-MOSFET transfer characteristic curve is 2 ( ) I K V V D GS GS th
The datasheet for a 2N7002 E-MOSFET gives I D(on) = 500 ma(minimum) at V GS =10 V and V GS(th) =1 V. Determine the drain current for VGS = 5 V. D-MOSFET Transfer Characteristic The square-law expression for the JFET curve also applies to the D-MOSFET curve For a certain D-MOSFET, I DSS =10 ma and V GS(off) = -8 V. (a) Is this an n-channel or a p channel? MOSFET BIASING E-MOSFET Common Bias Configurations (b) Calculate I D at V GS = -3 V (c) Calculate I D at V GS = 3 V. E-MOSFET Bias MOSFET BIASING V V R 2 GS DD R1 R2 Determine V GS and V DS for the E-MOSFET circuit. Assume this particular MOSFET has minimum values of I D(on) = 200 ma at V GS = 4 V and V GS(th) = 2 V. V V GS DS V V I R DS DD D D for drain-feedback bias circuit
MOSFET BIASING D-MOSFET Common Bias Configurations D-MOSFET Bias V V I R DS DD DSS D Determine the drain-to-source voltage in the circuit. The MOSFET datasheet gives V GS(off) = -8 V and I DSS = 12 ma.