JUNCTION FIELD EFFECT TRANSISTOR (JFET)

Transcription

1 CLA 23 JFET

2 JUNCTON FEL EFFECT TRANTOR (JFET) 2 Tyes: 1. n-channel JFET The current carriers in an n-channel JFET are the electrons. 2. -channel JFET The current carriers in a -channel JFET are the holes. ymbols: RAN () RAN () ATE () ATE () OURCE () nchanneljfet n-channel OURCE () -channel JFET NORLAL MOH NOH 2008/09 2

3 imlified cross-section of a JFET n-channel JFET -channel JFET n- -channel n -c channel n Both the regions in the n-channel JFET and the n regions in the -channel JFET are electrically connected. NORLAL MOH NOH 2008/09 3

4 A more ractical cross-section of an n-channel JFET Both the + regions in the n-channel JFET are electrically connected. nsulator io 2 + n + Metal electrode n-channel deletion region NORLAL MOH NOH 2008/09 4

5 Basic oeration of a JFET As an amlifier. The condition o that enables esthe ejfet to oerate eas an amlifier is the - that has to be reverse biased. is to rovide the difference in otential between - to enable the majority current carrier to move from to. is to reverse bias the -. n-channel JFET -channel channel JFET n- channel - channel NORLAL MOH NOH 2008/09 5

6 JFET Characteristics 1. rain characteristic 2. Transfer characteristic rain Transfer characteristic characteristic rain Characteristic 1. = 0 and = 0 No difference in otential between and. No majority carriers flowing from to. Hence, =0. n- channel NORLAL MOH NOH 2008/09 6

7 2. = 0 and is small. = - = 0 = = - = 0 is reverse biased. eletion regions exist. is ositive. Therefore, - is reverse biased. Again, deletion region exists. Comaring - and -, - is more reverse biased. Hence, the deletion region grows wider towards. For a small, the size of the - deletion region does not affect the width of the channel significantly. Under this condition, the deletion region does not influence the current. When increases, will also increase. The versus characteristic is linear. Under this condition, = R and the n-channel is a basically a resistance. Hence, this region of the drain characteristic is known as the ohmic region. n- channel =0 Ohmic region NORLAL MOH NOH 2008/09 7

8 There is a voltage dro along the channel from A to B. A = = 0 and B = =. The voltage becomes more ositive towards B. Hence, the -n junction from to channel becomes more reverse erse biased from A to B. = A channel B n-channel x A B x NORLAL MOH NOH 2008/09 8

9 3. = 0 and is increased. When increases, the to channel -n junction becomes more reverse-biased near. eletion region becomes wider and channel becomes narrower near. The channel is basically a resistor and the effective channel resistance increases when deletion region widens. R AB increases with the increment of. R AB = Δ / Δ. does not increase linearly with anymore. n the sub-linear region, sloe decreases and resistance increases. B A -chann nel n sub-linear region (channel resistance varies) NORLAL MOH NOH 2008/09 9

10 4. =0and is further increased. The reverse biasing of the to -n junction is enough to make the deletion regions meet near. The channel is said to be inched-off. Any further increment to the will no longer increase the. At inched-off, = (sat). (sat) = the voltage across - when inched-off occurs. For > (sat), is fixed. Transistor is in the saturation region and is indeendent of. When =0, (sat) = and = for (sat). B A (sat) = =0 aturation region NORLAL MOH NOH 2008/09 10

11 = Js = neu s where J is the current density, s is the cross section area of the channel, U is the carrier drift velocity, e is the electronic charge and n is the carrier concentration. For an n-channel JFET, = N eu s where N is the electron concentration donor doant concentration. When aroaching inched-off, s becomes very small and U has to become very large to maintain the current flow. The current density J = N eu becomes very high and under this condition, the drift velocity U is at its maximum. B A (sat) = =0 aturation region NORLAL MOH NOH 2008/09 11

12 5. =0and > (sat). E is the electric field. The n-channel is searated from by a sace charge region that has a length of ΔL. From, the electrons move along the channel, got injected into the sace charge region, and subsequently being swet by E to move to. f ΔL << L channel, the electric field in the n-channel does not change from the one when = (sat). is fixed and indeendent of. > (sat) (sat) B E channel A n- ΔL L channel NORLAL MOH NOH 2008/09 12

13 is determined by the channel resistance from A to, and not by the inched-off art of the channel. When > (sat), the excess voltage i.e. - (sat),isacrossδl asthisartisdeletedofcarriersand consequently has high resistivity. has a voltage (sat) = as this is the otential that causes the deletion regions to meet. = (sat) / R A.fΔL<<L channel,then = (sat) /R A is equivalent to (sat) /R AB when = (sat). Hence, although > (sat), remains unchanged. > (sat) (sat) B E nnel n-cha A ΔL L channel NORLAL MOH NOH 2008/09 13

14 f the magnitude of ( )increases, becomes more negative and the - becomes more reverse biased. Hence, a smaller is required to achieve inched-off and the at saturation ti will be smaller than. To oerate a JFET as an amlifier, the JFET has to be biased in the saturation ti region. This meansthat t - (sat). The tid triode region is for - (sat). and are the JFET arameters and secified in the data sheet. B A Triode/ohmic region = saturation/ active region =0 (sat) morenegative (sat) = NORLAL MOH NOH 2008/09 14

15 From the drain characteristic, as becomes more negative, the saturated current,, becomes smaller. When the is negative enough (i.e. when = (off) ), 0. This condition occurs as when = (off), the deletion region becomes large enough that it closes the channel. (off) =- B n A saturation/ ohmic inch-off/ region active region (-2) (-4) (sat) = =0 =-2 =-4 (off) (off) =0 more negative NORLAL MOH NOH 2008/09 15

16 The family of drain characteristic curves shows that when becomes more negative, (or (sat) )and become smaller. is deendent on the width of the channel. The width of the channel is deendent on the deletion region. The deletion region is deendent on the. Hence, is controlling the value of.thisisthereason why the JFET is known as a voltage controlled device. Pinched-off occurs when = (sat). (-1) (-2) (-3) (-4) = (-4) (-1) (-3) (-2) (off) NORLAL MOH NOH 2008/09 16

17 uring inch-off: = = R + R (sat) A '( ) A'( ) When becomes more negative, (sat) reduces and R A '( ) increases. R A '( increases as the ) deletion region increases. Hence, decreases. NORLAL MOH NOH 2008/09 17

18 TRANFER CHARACTERTC = when =0 and = 0 when = (off) =-. and (off) are the JFET arameters which are available in the JFET data sheet. Another imortant JFET arameter is the forward transconductance, g m. g m = Δ / Δ at a fixed and the has to be in the saturation/fixedcurrent/inch-off region. - (off) 0 Δ - (off) 0 Δ NORLAL MOH NOH 2008/09 18

19 g m at = 0 is known as g m0. g m = g m0 1 ( off ) g m0 = 2 ( off ) Y fs n the data sheet g m0 is reresented by. mortant arameters of the JFET n- channel Besides (off), and g m0, another arameter of the JFET is R N. R N = NORLAL MOH NOH 2008/09 19

20 is the - current when the - is short circuited. is given in the data sheet. ince is from the flow of minority carriers, increases with the increment of temerature, T, at a fixed. isthe reverse current at a known. As increases with T, R N will be reduced as R. N = The value of is very small making the value of R N to be very large. Hence, the inut imedance, Z i, is very large. n- channel NORLAL MOH NOH 2008/09 20