Transcript Chapter 4. MOSFET

Chapter 4. MOS Systems
Total 3 hours.
The Adventure of Carriers
The description must now borrow a picture from the classical books of
adventure. To place a negative electrode against a semiconductor with
negative carriers - this is like bringing a ship up to a quay in the Orient, with the
yellow flag of the plague hoisted. The place becomes deserted by its carriers.
Unloading - current - is blocked. But exchange that negative flag of pestilence
for a positive sign and the carriers will return, the contact becoming conducting.
Electrically this is called rectification. In those seafaring tales it was perhaps
possible to induce the carriers to return, without striking the flag, merely by
throwing some gold coins on the quay, thus positively destroying the insulation.
It is possible to destroy the blockade in the semiconductor in a similar fashion
by throwing in some positive holes around which the negative carriers will
gather. This is transistor action. It is a fine thing that the carriers' strike can be
broken up by rather few holes, which do not cost much energy.
Energy Levels
Equilibrium Band Diagram
Equilibrium implies the
existence of a path for
charge flow between the
metal and the silicon other
than the oxide.
But a barrier for electron
flow along both directions
Voltage Drop across Oxide
• Depends upon specific oxide properties.
Flat-Band Condition
Accumulation
Depletion
Inversion
Onset of Strong Inversion
MOS Capacitor
Capacitance of MOS Systems
Capacitance
• Accumulation: Determined by oxide
• Flat band: Debye length (no surface potential)
• Depletion: Width of the surface depletion layer
• Inversion
– Low frequency: Inversion layer population can follow
– High frequency: Inversion layer cannot respond; charge
modulated by the movement of carriers at the far edge of the
depletion layer
– Deep depletion: Inversion layer cannot form at all
Charge in Oxide
Flat
band
Charge-Coupled Devices
• How is an image stored digitally?
pixels, or
picture elements
• How to acquire an image? What are the basic principles
of charge-coupled devices (CCDs)?
Charge-Coupled Device
CCDs consist of almost
entirely close spaced
array of MOS capacitors.
In a MOS capacitor, the
electrical signal is
represented by the
channel (inversion layer)
charge, which can be
transferred to the channel
region of the adjacent
device.
The nine-stage CCD
illustrates how
information is
transferred from left
to right, by changing
gate voltages.