Question

Consider below I-V characteristics of a GaAs JFET. VG = 0 V -0.1 -0.3 -0.5 -0.7 VD (V)

a) Is this a n-channel or p-channel? why?

b) Does this appear to be a long channel or a short channel device? Why?

c) Based on the above IV curve you want to estimate Vt, explain your approach to obtain it.

d) The high frequency limit of operation of a JFET is dependent on the dimension and physical properties of the device. Describe three different approaches to improve the high frequency response in a JFET

Answer 1

a) n-channel

A N-Channel JFET is a JFET whose channel is composed of primarily electrons as the charge carrier. This means that when the transistor is turned on, it is primarily the movement of electrons which constitutes the current flow.

Gate Depletion region Insulator Source NCHAN. Drain

VDs Source Drain P-type Gate VGs N-type Channel P-type Gate Gate Depletion Layer

b) Beyond VDS = VP, there is a short pinch-off channel of length, ℓpo.As VDS increases, most of additional voltage simply
drops across ℓpo as this region is depleted of carriers and hence highly resistive. The voltage drop across channel length, Lch remain as VP. Beyond pinch-off then ID = VP/RAP (VDS>VP).

Pinched off charıncl I. ch The pinched-off channel and conduction for VD VP (-5 V)

What happens when negative voltage, says VGS = -2V, is applied to gate with respect to source (with VDS=0).The p+n junction are now reverse biased from the start, the channel is narrower,and channel resistance is now larger than in the VGS =0 case.

18 mA DS DS (G I 3.6 mA P Pinched off DS (a) The JFET with a negative Vce voltagc has a narrower n channel at the start (b) Compared to the V- 0 case, the same VDs gives less Ip as the channel is narrower. (c) The channel is pinched off at V 3 V soner than the Ve 0 case where it was Vn 5V GS = U casc wncrC It was VDS

c)

Gate Channel 16 BreakDown Pinch oFF or Active Region V = +0.5 V 12 DS 20 Drain Source Voltage VosIn Volts 12 16 24 Circuit Diagram for Determining Drain Characteristic with External-Bias for An N-Channel JFET Drain Characteristic with External-Bias External Bias Characteristic of JFET

It is observed that as the negative gate bias voltage is increased

(1) The maximum saturation drain current becomes smaller because the conducting channel now becomes narrower.

(2) Pinch-off voltage is reached at a lower value of drain current ID than when VGS = 0. When an external bias of, say – 1 V is applied between the gate and the source, the gate-channel junctions are reverse-biased even when drain current, ID is zero. Hence the depletion regions are already penetrating the channel to a certain extent when drain-| source voltage, VDS is zero. Due to this reason, a smaller voltage drop along the channel (i.e. smaller than that for VGS = 0) will increase the depletion regions to the point where 1 they pinch-off the current. Consequently, the pinch-off voltage VP is reached at a lower 1 drain current, ID when VGS = 0.

(3) The ohmic region portion decreases.

(4) Value of drain-source voltage VDS for the avalanche breakdown of the gate junction is reduced.

Value of drain-source voltage, VDS for breakdown with the increase in negative bias voltage is reduced simply due to the fact that gate-source voltage, VGS keeps adding to the I reverse bias at the junction produced by current flow. Thus the maximum value of VDS I that can be applied to a FET is the lowest voltage which causes avalanche breakdown. It is also observed that with VGS = 0, ID saturates at IDSS and the characteristic shows VP = 4 V. When an external bias of – 1 V is applied, the gate-channel junctions still require -4 V to achieve pinch-off. It means that a 3 V drop is now required along the channel instead of the previous 4.0 V. Obviously, this drop of 3 V can be achieved with a lower value of drain current, Similarly when VGS = – 2 V and – 3 V, pinch-off is achieved with 2 V and 1 V respectively, along the channel. These drops of 2 V and 1 V are, of course, achieved with further reduced values of drain current, ID. It is further observed that when the gate-source bias is numerically equal to pinch-off voltage, VP (-4 V in this case), no channel drop is required and, therefore, drain current, ID is zero. The gate-source bias voltage required to reduce drain current, ID to zero is designated the gate-source cut-off voltage, VGS /0FF) and, as explained,

Hence for working of JFET in the pinch-off or active region it is necessary that the following conditions be fulfilled.

VP < VDS < VDS (max)

VGS (OFF)< VGS < 0

0 < ID < IDSS

Hence for working of JFET in the pinch-off or active region, it is necessary that the following conditions be fulfilled.

V_P < V_DS < V_{DS (max)}

V_{GS (OFF)}< V_GS < 0

0 < I_D < I_DSS

The threshold voltage equals the sum of the flatband voltage, twice the bulk potential and the voltage across the oxide due to the depletion layer charge,

where the flat band voltage, V_FB, is given by:

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