MOSFETs

Question 1

Describe the physical structure of a MOSFET device

Answer 1

• p-type substrate body
• n-type source
• n-type drain
• insulation region formed by silicon oxide
• layer of metal placed above insulation region
• metal is deposited on oxide layer to form gate electrode

Question 2

What is the purpose of the oxide layer of a MOSFET device?

Answer 2

It electrically insulates the gate which causes the current at the gate to be extremely small (order of 10^-15)

Question 3

What is the effect of V_GS?

Answer 3

• A positive voltage that is applied to the gate
• Causes free holes to be repelled from the region of the substrate under the gate (channel region) and downward into the substrate
• This creates a depletion region
• Depletion region is populated by the bound negative charge associated with acceptor atoms
• Positive gate voltage attracts electrons from n-type source and drain regions
• when a sufficient number of electrons accumulate near the region under the gate an n-type region is created known as the channel
• If a voltage is applied between drain and source, current flows through the induced n-type channel

Question 4

What is V_t?

Answer 4

The threshold voltage which is the minimum value of V_GS required to create the n-type channel between source and drain

Question 5

What is the typical range of V_t for a MOSFET?

Answer 5

V_t is controlled during device fabrication and lies within the range of 0.5V to 1V

Question 6

What is the effect of the capacitor formed when V_GS is applied?

Answer 6

• The positive top plate (gate electrode) and negative bottom plate (induced n-type channel)
• This produces an electric field in the vertical direction
• This field controls the amount of charge in the channel, conductivity and the current that will flow through when V_DS is applied

Question 7

What is the effect of applying a small V_DS once a n-type channel has been formed?

Answer 7

• Voltage V_DS causes current i_D to flow through induced n-type channel
• current flow i_D will be from drain to source (conventional current)

Question 8

Describe the capacitor region formed when V_GS is applied

Answer 8

• The gate and channel region form a parallel-plate capacitor
• The oxide layer acts as the capacitor dielectric
• Positive gate voltage causes positive charge on the top plate of the capacitor (gate electrode)
• The negative bottom plate of the capacitor is formed by electrons in the induced n-type channel

Question 9

In which direction does current flow in a MOSFET device?

Answer 9

From the drain to the source (conventional current)

From source to drain (electron current)

Question 10

With no bias voltage applied to the gate how can we represent the MOSFET device?

Answer 10

• Two back to back diodes in series between drain and source
• one diode is formed by pn junction between n-type drain and p-type substrate
• other diode is formed by pn junction between p-type substrate and n-type source
• back to back diodes prevent current conduction from drain to source when a voltage v_DS is applied

Question 11

What is the effect on i_D in relation to V_DS and V_GS

Answer 11

• At small values of V_DS, the slope of i_D-V_DS increases linearly and the slope is proportional to V_GS-V_t
• As the values of V_GS are increased above the threshold voltage the slope of the curve becomes steeper
• i_D saturates because the channel is pinched off at the drain end, so V_DS no longer affects the channel (current is constant)

Question 12

What is the equation for current in saturation mode?

Answer 12

Question 13

What are the relative values of current in gate and source?

Answer 13

i_G = 0

i_S = i_D

Question 14

What is the operation of a n-type MOSFET?

Answer 14

Normall-off: No v_GS, channel is closed