Pinch-off voltage

The result can be simplified by making use of the above-defined pinch-off voltage [Eq. (14.54)1 and dielectric capacitance of the semiconducting layer. Further simplifications result from the assumptions that Cs 2> C, (Eq. (14.57)), and that the dopant and carrier concentrations arc equal (Eq. (14.58)). 

Under the hypotheses that no = N and rj ds, the threshold voltage is equal to the pinch-off voltage Vp = Vth, and all the terms in the second row of Eq. [3.2] can be neglected to obtain Eq. [3.3]. In this case, the field-effect fibrous transistor can be considered as a planar thin film FET. 

The lower boundary of the active region is controlled by the condition that the channel be pinched off at the drain end. To meet this condition the basic requirement is that the gate-to-channel bias voltage at the drain end of the channel, Vgd> he greater than the pinch-off voltage Vp. For the example under consideration with Vp = —4 V, this means that Vgd = Vgs — Vps must be more negative than —4 V. Therefore, Vps — Vgs > -F 4 V. 

Pinch-off voltage, Vp The voltage that when applied across the gate-to-channel PN junction will cause the conducting channel between drain and source to become pinched off. This is also represented as Vgs(off)-... 

Turn-on Voltage Forward voltage V p highly Pinch-off voltage Vp not very... 

At the saturation point on the drain side, the drain voltage equals the pinch-off voltage, which implies... 

The drain current-voltage characteristics for an arbitrary MESFETwith an external pinch-off voltage of —2 V (when added to the built-in voltage in terms of amplitude equals to Vp) is shown in Figure 8.24. 

Figure 8.24 Typical output current—voltage characteristics of an arbitrary MESFET with an external pinch-off voltage of 2V. In two-piece models applicable to the cases where velocity saturation occurs, it is assumed that the mobility is constant in certain region of the channel under the gate toward the source and velocity is saturated in the rest. The boundary depends on the drain voltage and the gate dimension.

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