STRUCTURE OF MOSFET

The Si Guy

13 February 2023

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Suppose we have a piece of metal and some amount of voltage applied across it as shown in the fig.1

(A)

based on the resistance of it there’s fixed amount of current flowing through it according to ohm’s law.

Fig.1

If we apply the same voltage across a piece of insulator fig.1 (B), there won’t be any current since the

resistivity of insulators is infinity (ideally).

Applying the same voltage across a piece of semiconductor fig.1 (C) would give different amount of current in different situations. The resistivity is semiconductors is controllable (can be increased or decreased as needed). But how the resistivity of semiconductors and hence the current can be controlled? Let me explain by example:

Suppose we have a simple structure consisting of a conductive plate, an insulator and a p type piece of

silicon (semiconductor). Illustrated in Fig.2 (A), such a structure operates as a capacitor because the p-

type silicon is somewhat conductive (conduct electrons poorly since there’s no much free electrons as

the

case in conductors).

if a potential difference (Vc) is applied vertically across the structure as shown in Fig.2 (B), the positive

charge placed on the top metal plate attracts negative charges from the piece of silicon forming an N

channel at the interface between the insulator and the semiconductor (silicon).

current can be flown through the channel if we apply a voltage difference across the silicon Fig.2 (C).

Fig.2

That channel is a good conducting path having much lower resistance as compared to the p-type

silicon.

so the piece of silicon can be thought of as a low resistance (the channel) in parallel with high

resistance (the rest of the p-type piece of silicon). the whole resistance is dominated by the channel

resistance, the current prefers to pass through the channel.

the density of electrons in the channel varies with Vc (Q = CV), where C is the capacitance between the

two plates (the metal and the p-type silicon). Hence the resistivity of the channel and the current

flowing through it can be controlled by the control voltage and the structure capacitance (Q=cV).

To apply the voltage across the p-type silicon two metal contacts are attached to it through two heavily-

doped n-type regions (n+) because direct connection of metal to the silicon would not produce a good

“ohmic” contact Fig.3. These two terminals are called “source” (S) and “drain” (D) indicating sourcing

and draining charge carriers.

Fig.3

To apply the control voltage (Vc) a metal contact is attached directly to the top metal plate, there’s

another metal contact attached to the p-type silicon through heavily-doped p-type region (p+)

That leads us to the MOSFET structure shown in Fig.4

MOSFET is consisting of:

A p-type silicon (called substrate).
A layer of silicon dioxide is grown on top of it acts as insulator (dielectric).
On top of it there’s a metal layer called the gate (G) acts as the top plate of the capacitor
(gate,dielectric,substrate).in the early generations, gate was made up of metal (aluminum).
However, it was discovered that noncrystalline silicon (“polysilicon” or simply “poly”) with
heavy doping (for low resistivity) exhibits better fabrication and physical properties. Thus,
today’s MOSFETs employ polysilicon gates.
The source and drain are made up of n+ regions.