FET

What is a JFET?

A type of FET that operates with a reverse-biased PN junction to control current in a channel

What is JFET short for?

Junction field-effect transistor

Draw a basic structure representation of n-channel of a JFET

Draw a basic structure representation of p-channel of a JFET

Are the p-type regions both connected to the gate-lead?

Yes, they are connected internally. This allows to create a depletion region symmetrically from both sides in the n-channel, which allows for better control on the current

Draw the symbol for a n-channel JFET

Draw the symbol for a p-channel JFET

How is the drain current controlled in a JFET

As you increase the voltage to the gate, the channel narrows (depletion region increases), which increases the resistance of the channel and decrease the drain current (Vise versa for decreasing Vgg)

Draw the drain characteristic curve for JFET (V_GS = 0V)

What is the pinch-off voltage (V_p)

(For V_GS = 0V) The value of V_DS where I_D becomes constant

What is the cutoff voltage V_GS(Off)

It is where I_D = 0

Explain what happens to the depletion region to allow an ohmic region to exist

The channel resistance is constant-ish because the depletion region is not large enough

How is the drain current constant in the active region?

As V_DS increases, the reverse-biased voltage V_GD produces a depletion region large enough to offset the increase V_DS and therefore, I_D is constant

Draw a family of drain characteristics for JFET where V_GS from 0V to -5V

The knee is equal to V_GS - V_p and you can find the effective I_DSS from the Shockley's equation

What is the Shockley's equation

What is the difference between a p-channel JFET biasing compared to n-channel?

The polarity of the voltages

What is the relationship between V_P and V_GS(Off) ?

They are same in magnitude but opposite in sign

Draw the JFET universal transfer characteristics

Draw the JFET universal transfer characteristics and drain characteristics curves on the same graph

What is forward transconductance g_m

It is an AC quantity that is defined as a change in drain current ΔI_D divided by corresponding in gate to source voltage ΔV_GS with the drain-to-source voltage constant

What is the formula for forward transconductance g_m

Unit: S (Siemans)

Show how you find g_m from the transfer characteristic curve

Plot g_m in terms of V_GS

How do you calculate g_m

How do you calculate g_m0 / g_sf(max)

Note V_GS =0V

How do you calculate input resistance for a JFET?

Explain how JFET have high input resistance

JEFT operate with gate-source junction reverse-biased → high input resistance (Which means no current is drawn → no load / voltage drop when connected to source)

Draw a self-biasing circuit for n-channel and p-channel

Why do we have R_G if it has no effect in the circuit

Force the gate to be 0V and isolate AC signal from ground in amp application

What are the two ways of setting the Q-point of a self-biasing JFET

1. Use the transfer characteristic curve for a particular JFET (Use this when some values are unknown)

2. Use Shockley's equation (More practical)

How to use the transfer characteristic curve to find the Q-point

• Get the gate voltage

• Get two points

  • First point → Where I_D = 0A

  • Second second point → Where V_GS = 0V

• Get the line equation (if needed)

• Find the point of intersection (This is your Q-point)

If I want to do a midpoint bias, what should by I_D and V_GS should be

I_D = 0.5I_DSS and V_GS = V_GS(Off) / 3.4

Draw a circuit of a voltage-divider bias (JFET)

What are the two types of MOSFET we do?

• Enhanced MOSFET

• Depletion MOSFET

What is the difference between JFET and MOSFET

The gate of the MOSFET is insulated from the channel by a silicon dioxide layer

What is the difference between FET and BJTs

• FETs are unipolar devices while BJTs are bipolar devices

• BJTs are current controlled devices while FETs are voltage controlled

• JFETs have a very high input impedance while BJTs do not

Draw a basic construction of a E-MOSFET

What mode does E-MOSFET operate in

Only in enhancement mode

Draw schematic symbol for E-MOSFET

Draw a basic structure of D-MOSFET

What mode does D-MOSFET work in?

Enhancement and depletion

What makes D-MOSFET able to have positive and negative voltage apply to the gate

The gate is insulated from the channel

Draw the E-MOSFET transfer characteristic curve

What is the formula for the E-MOSFET characteristic curve?

How do you find K for the E-MOSFET

Draw the D-MOSFET characteristic curve

What is the formula for the D-MOSFET

Same as the JFET

Why is E-MOSFET useful and common in IC applicaiton

• Simpler construction

• Used in digital ICs

What are the two biasing configuration for E-MOSFET

What is the gate voltage for a drain-feedback bias?

V_GS = V_DS

Why is there R_G ?

It allows AC signal to develop at the gate (isolating it from ground)

Draw n-channel MOSFET as a switch

Draw p-channel MOSFET as a switch

Draw an ideal switching operation load line on the drain characteristic curve

What is one thing you need to make sure when you applying an AC signal at the gate?

Ensure the signal level at the source must not cause the V_GS to drop below the V_GS(th)

Draw a basic operation of a n-channel MOSFET analogue switch

Draw a sampling circuit and input & output waveform

What is the Nyquist frequency

Draw a CMOS

What type of gate is a CMOS?

NOT Gate

What is the main advantage of CMOS?

Consume very little DC power

Draw the two state of a CMOS

Draw a CMOS NAND gate

Draw a truth table for this

Draw a CMOS NOR

Draw a truth table for this

What will the output wave be for this?

What is the purpose of a buffer?

It means that we can provide a high input impedance "buffer" between the input and output to provide some electrical isolation between parts of the circuit, for example between a low power logic circuit and a high-power motor circuit

Draw the gate symbol, truth table and expression for AND gate

Draw the gate symbol, truth table and expression for NOT gate

Draw the gate symbol, truth table for Buffer

Draw the gate symbol, truth table and expression for OR gate

Draw the gate symbol, truth table and expression for NOR gate

Draw the gate symbol, truth table and expression for XOR gate

What happens to the current for MOSFET switching on each rising and falling edge?

A huge spike in current due to the internal capacitive behaviour of the MOSFET

Draw a diagram of D-MOSFET in depletion mode

Draw a diagram of D-MOSFET in enhancement mode