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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: S₇P₅A₅₁ (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.

Saturation Region: gate-source voltage, v_GS > V_T, and gate-drain voltage, v_GD <V_T.
Channel is on at the source and off at the drain.
drain current i_D almost independent of drain-source voltage (v_DS) and depends on only the gate voltage.
i_D = K(v_GS - V_T)²(1 + v_DS/V_A)
i_D = K(v_GS - V_T)². If V_A is assumed to be very large.
MOSFET behaves like a voltage-controlled current source.

Triode or Ohmic Region: gate-source voltage, v_GS > V_T, and gate-drain voltage, v_GD >V_T.
Chanel is on at the source and at the drain.
drain current i_D is strongly dependent on both drain-source voltage (v_DS) and gate-source voltage, v_GS
drain current i_D = K[2(v_GS - V_T)v_DS - v²_DS]
MOSFET behaves like a voltage-controlled resistor (gate voltage is the controlling volatage).
Voltage-controlled resistor widely used in integrated circuits.

Math

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