ENE 311 Lecture 11: J-FET and MOSFET
FETs vs. BJTs Similarities: Amplifiers Switching devices Impedance matching circuits Differences: FETs are voltage controlled devices. BJTs are current controlled devices. FETs have a higher input impedance. BJTs have higher gains. FETs are less sensitive to temperature variations and are more easily integrated on ICs. 2
Current Controlled vs oltage Controlled Devices January 2004 ELEC 121 3
Transfer Characteristics The input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT In a BJT, β (hfe) defined the relationship between I B (input current) and I C (output current). In a JFET, the relationship (Shockley s Equation) between GS (input voltage) and I D (output current) is used to define the transfer characteristics, and a little more complicated (and not linear): I D = I DSS 1 - As a result, FET s are often referred to a square law devices GS P 2 January 2004 ELEC 121 4
FET Types JFET: Junction FET MOSFET: Metal Oxide Semiconductor FET D-MOSFET: Depletion MOSFET E-MOSFET: Enhancement MOSFET 5
There are two types of JFETs n-channel p-channel The n-channel is more widely used. JFET Construction There are three terminals: Drain (D) and Source (S) are connected to the n-channel Gate (G) is connected to the p-type material 6
JFET Operation: The Basic Idea JFET operation can be compared to a water spigot. The source of water pressure is the accumulation of electrons at the negative pole of the drain-source voltage. The drain of water is the electron deficiency (or holes) at the positive pole of the applied voltage. The control of flow of water is the gate voltage that controls the width of the n-channel and, therefore, the flow of charges from source to drain. 7
N-Channel JFET Symbol 8
JFET Operating Characteristics There are three basic operating conditions for a JFET: GS = 0, DS increasing to some positive value GS < 0, DS at some positive value oltage-controlled resistor 9
JFET Operating Characteristics: GS = 0 Three things happen when GS = 0 and DS is increased from 0 to a more positive voltage The depletion region between p-gate and n-channel increases as electrons from n-channel combine with holes from p-gate. Increasing the depletion region, decreases the size of the n-channel which increases the resistance of the n-channel. Even though the n-channel resistance is increasing, the current (I D ) from source to drain through the n- channel is increasing. This is because DS is increasing. 10
JFET Operating Characteristics: Pinch Off If GS = 0 and DS is further increased to a more positive voltage, then the depletion zone gets so large that it pinches off the n-channel. This suggests that the current in the n- channel (I D ) would drop to 0A, but it does just the opposite as DS increases, so does I D. 11
JFET Operating Characteristics: : Saturation At the pinch-off point: Any further increase in GS does not produce any increase in I D. GS at pinch-off is denoted as p. I D is at saturation or maximum. It is referred to as I DSS. The ohmic value of the channel is maximum. 12
JFET Operating Characteristics As GS becomes more negative, the depletion region increases. 13
JFET Operating Characteristics As GS becomes more negative: The JFET experiences pinch-off at a lower voltage ( P ). I D decreases (I D < I DSS ) even though DS is increased. Eventually I D reaches 0 A. GS at this point is called p or GS(off).. Also note that at high levels of DS the JFET reaches a breakdown situation. I D increases uncontrollably if DS > DSmax. 14
The region to the left of the pinch-off point is called the ohmic region. The JFET can be used as a variable resistor, where GS controls the drain-source resistance (r d ). As GS becomes more negative, the resistance (r d ) increases. JFET Operating Characteristics: oltage-controlled Resistor r d = 1 r o GS P 2 15
Transfer (Transconductance) Curve From this graph it is easy to determine the value of I D for a given value of GS It is also possible to determine IDSS and P by looking at the knee where GS is 0
Plotting the JFET Transfer Curve Using I DSS and p ( GS(off) ) values found in a specification sheet, the transfer curve can be plotted according to these three steps: Solving for GS = 0 Step 1 I D = I DSS I D = I DSS 1 GS P 2 Step 2 Solving for GS = p ( GS(off) ) I D = 0A I D = I DSS 1 GS P 2 Solving for GS Step 3 = 0 to p I D = I DSS 1 GS P 2 17
JFET Transfer Characteristics The transfer characteristic of input-to-output is not as straightforward in a JFET as it is in a BJT. In a BJT, β indicates the relationship between I B (input) and I C (output). I = f ( I ) =β I C B B In a JFET, the relationship of GS (input) and I D (output) is a little more complicated: I D = I DSS 1 where GS is the control variable, P and I DSS are constants. GS P 2 NOTE: When GS = 0, I D = I DSS When GS = P, I D = 0 ma 18
p-channel JFETS The p-channel JFET behaves the same as the n-channel JFET, except the voltage polarities and current directions are reversed. 19
p-channel JFET Characteristics As GS increases more positively The depletion zone increases I D decreases (I D < I DSS ) Eventually I D = 0 A Also note that at high levels of DS the JFET reaches a breakdown situation: I D increases uncontrollably if DS > DSmax. 20