Analog Electronic Circuit foundation--Field effect tube

Fet, is a voltage control device, does not absorb the signal source current, does not consume the signal source power, the input impedance is high, the temperature characteristic is good, the anti-jamming ability is strong, facilitates the integration.
The JFET uses the width of the depletion zone to change the breadth of the conductive channel to control the drain current, and the MOSFET uses the electric field effect of the semiconductor surface to control the current by changing the conductive channel by the number of inductive charges.
One, junction type FET (JFET)

classification of 1.1-junction field effect Tubes
N-Channels, p- channels

1.2 How the junction FET works (N-channel)
coupled with a reverse voltage uds, a current ID is formed between the source and the drain, and by changing ugs, the width of the two PN junction barrier layer can be changed, thus changing the channel resistance and changing the drain current ID.
    (1) control of the voltage VGS between the gate source and the ID
When the green part is depletion layer, when the leakage source is shorted, and the negative voltage VGS between the grid sources is applied, the two PN junctions in the junction field effect tube are in the inverse state. (b) is shown.
with the negative increase of the VGS, the reverse bias voltage added to the PN junction increases, and the depletion layer widens. Because of the low doping concentration of N-channel, the depletion layer is mainly concentrated on the channel side. The depletion layer widens, making the channel narrower, the channel resistance increased, and (b) shown.
when the VGS negative increases to a certain value, the depletion layer on both sides of the knot expands inward to meet each other, the channel is completely clamped, and the resistance between the drain source will tend to infinity, as shown in (c). Corresponding to this time the leakage source between the voltage VGS called clamp off pressure, with VGS (off).
    (2) leakage source voltage effect of VDS on channel
when Vgs>vgs (off) is a certain value, if a positive voltage is added between the drain source VDS,VDS will generate an electric field in the channel that points from the drain point to the source, which causes the majority of carrier electrons in the N channel to drift along the channel from the source to the drain to form the drain current ID.
because of the resistance of the conductive channel, the ID flows through the channel to produce a pressure drop, so that the potential of the channels in the channel is no longer equal, resulting in the depletion zone from the drain to the source is gradually narrowed, the wedge-shaped distribution, (a) shown.
with the increase of the VDS, the ID increases, the channel unequal width becomes obvious, when the VDS increases to a certain value, the near-drain end of the two depletion zone meet, this situation is known as pre-clamping, (b) shown.
continue to increase the VDS, the clip breakpoint will extend to the source direction, the near-drain end of the clamping zone, (c) shown.
since the gate to clip breakpoint a reverse voltage VGA is constant VGS (off), so the clip breakpoint to the source voltage between the constant Vgs-vgs (off), and the increase of the VDS will be all added to the drain and clip breakpoint between the clamping zone, forming a strong electric field. In this case, the multi-sub free electrons traveling from the drain to the clip breakpoint will be shifted to the drain by the electric field in the clamping area once the clip breakpoint is reached and the drain current is formed.
In General, the pinch zone only accounts for a very small portion of the channel length, so the increase of the VDS causes the movement of the clip breakpoint can be ignored, clip breakpoint to the source of the channel length can be considered approximate, while the clip breakpoint to the source of the voltage is a certain value, so can be approximated to the ID is not with the constant value of VDS.

characteristic curve of 1.3-junction field-effect tube

second, Insulated Gate type field effect Tube (MOSFET)

2.1 Classification of Insulated Gate Type field effect Tubes
N-Channel, p-channel, enhanced, depleted

2.2 How the Enhanced MOS FET works (n-channel)
    

2.2 enhanced MOS Fet output characteristic curve (n-channel)

    VGS (TH)   is the DS short connection, the MoS tube forms the opening voltage of the conductive channel

variable resistance zone
    conditions: vgs>vgs (TH) Span class= "Apple-converted-space" >&NBSP; , Vds<vgs–vgs (TH)

< Span class= "Apple-converted-space" >< Span class= "Apple-converted-space" >
    W, L is the width and length of the channel, and Cox is the gate capacitance of the unit area.
    at this time the MoS tube can be regarded as a resistor with VGS control.

    (2) saturated zone
    conditions: Vgs>vgs (TH) &NBSP; , Vds>vgs–vgs (TH)

The ID is controlled only by VGS and is almost unrelated to VDS and is characterized by a voltage-controlled current source.

(3) cutoff area
Condition: Vgs<vgs (TH) , which is the working area when the channel is not formed
Id=0,mos Tube Disconnect

Analog Electronic Circuit foundation--Field effect tube