Enhancement Mode Metal Oxide Semiconductor Field Effect Transistors

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Enhancement Mode Metal Oxide Semiconductor Field Effect Transistors

*Field-Effect Transistors* (FETs) are another main group of transistors. There are two primary types of FETs: metal-oxide semiconductor field-effect transistors (MOSFETs) and Junction Field Effect Transistors (JFETs). MOSFETs are grouped into *enhancement-mode MOSFETs* and *depletion-mode MOSFETs*. Each of these transistors can be either an *n-channel* device or a *p-channel* device depending on the nature of the *doping*. Figure 123.1 illustrates both the *n-channel enhancement MOSFET* and the *p-channel enhancement MOSFETs*.

(a) Explain the conduction state of the NMOS (figure 123.1c).

(b) Define the following terms: *Threshold Voltage*, *Conductance Parameter*, *Early Voltage*.

(c) Name and characterize the three operating regions of the NMOS transistor.

**The strings**:
S_{7}P_{5}A_{51} (Physical Change).
**The math**:

Pj Problem of Interest is of type *change* (physical change). Transistors are primarily used for signal *amplification* and *switching*. Both are *change* problems.

(a)The NMOS has three terminals: *gate* (similar to the BJT base), *drain* (similar to the collector), and the *source* (similar to the emitter).

It has a *p-material substrate* (or bulk) electrically connected to the *source*. So the *substrate* does not appear as a separate terminal in circuit diagram.

The *gate* consists of a metal film separated from the *p-substrate* by a thin oxide layer (hence called metal oxide semiconductor).

both *drain* and *source* are made of n^{+} material.

Consider figure 123.1c. Without the voltage supply connected to the *gate*, no current will flow in the NMOS since all junctions are *reverse-biased*. This is the *normal off* state of the NMOS.

When a positive voltage is applied to the *gate*, electric field is created (hence called field-effect). The field repels positive charges away from the surface of the *p-substrate*. Consequently a narrow *channel* (green in diagram) consisting of negative charges that are available for conduction, is formed near the surface of the *p-substrate*.

The higher the *gate voltage* (enhancement), the higher the concentration of the negative charges, so the higher the conductivity.

(b) **Threshold Voltage** (V_{T}): the positive voltage that must be exceeded by the *gate voltage* in order to form a *conducting channel* in the NMOS. In other words, NMOS is *on* if *gate voltage* > V_{T}. It is *off* otherwise.
**Conductance Parameter** (K) : ability of the *channel* to conduct. Expressed as a formula as follows:

K = (WμC_{ox})/2L.

W = width of channel; L = length of channel; μ = mobility of charge carrier (electrons in nmos, holes in pmos); C_{ox} = capacitance of oxide layer.
**Early Voltage** (V_{A}): describes the dependence of MOSFET drain current to drain-source voltage (v_{DS}). Usually assumed to approach infinity to indicate the drain current is independent of v_{DS}.

(c) **Cuttoff Region**: gate-source voltage, v_{GS} < V_{T} and gate-drain voltage, v_{GD} <V_{T}

No conduction channel. So, drain current *i _{D}* = 0.

drain current

MOSFET behaves like a

drain current

drain current

MOSFET behaves like a

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The *point* **.** is a mathematical abstraction. It has negligible size and a great sense of position. Consequently, it is front and center in abstract existential reasoning.

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