Describe the physical structure of a MOSFET device
- 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
What is the purpose of the oxide layer of a MOSFET device?
It electrically insulates the gate which causes the current at the gate to be extremely small (order of 10-15)
What is the effect of VGS?
- 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
What is Vt?
The threshold voltage which is the minimum value of VGS required to create the n-type channel between source and drain
What is the typical range of Vt for a MOSFET?
Vt is controlled during device fabrication and lies within the range of 0.5V to 1V
What is the effect of the capacitor formed when VGS is applied?
- 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 VDS is applied
What is the effect of applying a small VDS once a n-type channel has been formed?
- Voltage VDS causes current iD to flow through induced n-type channel
- current flow iD will be from drain to source (conventional current)
Describe the capacitor region formed when VGS is applied
- 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
In which direction does current flow in a MOSFET device?
From the drain to the source (conventional current)
From source to drain (electron current)
With no bias voltage applied to the gate how can we represent the MOSFET device?
- 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 vDS is applied
What is the effect on iD in relation to VDS and VGS
- At small values of VDS, the slope of iD-VDS increases linearly and the slope is proportional to VGS-Vt
- As the values of VGS are increased above the threshold voltage the slope of the curve becomes steeper
- iD saturates because the channel is pinched off at the drain end, so VDS no longer affects the channel (current is constant)
What is the equation for current in saturation mode?
What are the relative values of current in gate and source?
iG = 0
iS = iD
What is the operation of a n-type MOSFET?
Normall-off: No vGS, channel is closed
