JFET and MOSFET (electronic device and circuits part 02)

In this article, you will find the Study Notes on Circuits Analysis and Applications of Diodes, BJT, FET and MOSFET which will cover the topic as Introduction, V-I Characteristics, FET small signal model, JFET Biasing, Common Drain amplifier and MOSFET. 

1. JFET (Junction FET)

The FET is a three terminal (i.e. drain, gate and source), unipolar voltage controlled device. There are two types of such devices MOSFET (Metal Oxide Semiconductor FET) and JFET (Junction FET). Again the JFET is classified into n-channel JFET where current conduction occurred due to electrons and p-channel JFET where current conduction occurred due to holes. In JFET gate to source junction is always in reversed bias condition and drain is always high potential than a source.

• Advantages of JFET:
  • Very high input impedance order of 10⁸ − 10¹⁰ ohm.
  • The operation of JFET depends on the bulk material current carriers that do not cross junctions.
  • Very high power gain.
  • Smaller in size and having high efficiency.

2. V-I characteristics of JFET:

A JFET is a voltage controlled device, its drain current depends upon the gate to source voltage and drains to source voltage i.e. I_D (drain current) is a function of V_GS (gate to source voltage) & V_DS (drain to source voltage).

• I_DSS is the maximum drain current for a JFET and is defined by the conditions V_GS = 0 and V_Ds > V_p where V_p is the pinch off voltage.
• For gate-to-source voltages, V_GS less than (more negative than) the pinch-off level, the drain current is 0A (I_D= 0A).
• For all levels of V_GS between 0 V and the pinch-off level, the current I_D will range between I_DSS and 0 A, respectively.

Transfer characteristics:

The transfer characteristics of JFET is between I_D versus V_GS when V_Ds is kept constant.

3. FET small signal model

The ac analysis of a FET configuration requires that a small-signal ac model for the FET. A major component of the ac model will reflect the fact that an ac voltage applied to the input gate-to-source terminals will control the level of current from drain to source. The three major component of ac model are as follows:

• Drain resistance:

     

• Transconductance:

      

• Amplification factor:

       

4. JFET biasing

For the JFET to operate as a linear amplifier, the Q-point should be in the middle of the saturation region, the instantaneous operating point must at all times be confined to the saturation region and the input signal must be kept sufficiently small. For a selection of an appropriate, the operating point for a JFET amplifier stage proper biasing is needed. Similar to BJT, JFET is having three types of DC biasing.

• Fixed-bias configuration
• Self-bias configuration
• Voltage-divider configuration

FET as voltage variable resistor:

• FET is operated in the constant current portion of its output characteristics for the linear applications
• Small signal FET drain resistance R_d varies with applied gate voltage V_GS and FET act like a variable passive resistor

5. Common drain amplifier

FET AC Equivalent Circuit:

  

JFET source follower (common drain) configuration:

• In a CD amplifier configuration, the input is at the gate, but the output is the source

• Input impedance: Z_i = R_G
• Output impedance: 

         

• Voltage gain:

        

5. MOSFET

• MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a unipolar, voltage controlled current device and is a device in which current at two electrodes drain and the source is controlled by the action of the electric field at another electrode gate having in between a semiconductor very a thin metal oxide layer. MOSFET is classified into two types i.e. enhancement type and depletion type.
• There is no direct electrical connection between the gate terminal and the channel of a MOSFET. It is the insulating layer of SiO₂ in the MOSFET construction that account for the very desirable high input impedance of the device.

• MOSFET can be operated with either a positive or a negative gate. When the gate is positive with respect to the source it operates in the enhancement—or E-mode and when the gate is negative with respect to the source, it operates in depletion-mode.

• The equation for the transfer characteristic of E-MOSFETs is given as:

        

        

• The Hybrid-pi CE Transistor Model:
  • The hybrid-pi or Giacoletto model of common emitter transistor model is given below. The resistance components in this circuit can be obtained from the low-frequency h-parameters.
  • For high-frequency analysis transistor is replaced by high-frequency hybrid-pi model and voltage gain, current gain and input impedance are determined.

                                                                                         

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