2. Introduction to FET’s

3. Current Controlled vs Voltage Controlled Devices

4. Types of Field Effect Transistors (The Classification) » JFET MOSFET (IGFET) n-Channel JFET p-Channel JFET n-Channel EMOSFET p-Channel EMOSFET Enhancement MOSFET Depletion MOSFET n-Channel DMOSFET p-Channel DMOSFET FET

5. Figure: n-Channel JFET. The Junction Field Effect Transistor (JFET)

6. ELEC 121 JFET Construction There are two types of JFET’s: n-channel and p-channel. The n-channel is more widely used. There are three terminals: Drain (D) and Source (S) are connected to n-channel Gate (G) is connected to the p-type material

7. Gate Drain Source SYMBOLS n-channel JFET Gate Drain Source n-channel JFET Offset-gate symbol Gate Drain Source p-channel JFET

8. N-Channel JFET Symbol

9. Figure: n-Channel JFET and Biasing Circuit. Biasing the JFET

10. Figure: The nonconductive depletion region becomes broader with increased reverse bias. (Note: The two gate regions of each FET are connected to each other.) Operation of JFET at Various Gate Bias Potentials

11. Transfer Characteristics The input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT In a BJT,  (h FE ) defined the relationship between I B (input current) and I C (output current). In a JFET, the relationship (Shockley’s Equation) between V GS (input voltage) and I D (output current) is used to define the transfer characteristics, and a little more complicated (and not linear): As a result, FET’s are often referred to a square law devices

12. JFET Operating Characteristics There are three basic operating conditions for a JFET: JFET’s operate in the depletion mode only A.V GS = 0, V DS is a minimum value depending on I DSS and the drain and source resistance B.V GS < 0, V DS at some positive value and C.Device is operating as a Voltage-Controlled Resistor For an n channel JFET, V GS may never be positive* For an p channel JFET, V GS may never be negative*

13. Saturation At the pinch-off point: any further increase in V GS does not produce any increase in I D. V GS at pinch-off is denoted as Vp. ID is at saturation or maximum. It is referred to as I DSS. The ohmic value of the channel is at maximum.

14. Specification Sheet (JFETs)