Analyze the threshold voltage of N-channel MOS field effect transistors（）

Threshold voltage of N-channel MOS field effect transistor

The difference between an actual MOS tube and an ideal MOS tube is that the positive charge in the oxide layer needs to be considered(Unit area) and the contact potential difference between metal and semiconductor，the impact.

1. There is always a positive charge formed by impurities or defects at the interface between silicon and oxide layer, it is equivalent to increasing the gate voltage, which actually increases the threshold voltage by a negative value-。

the size of is related to the crystal plane orientation of silicon material, and its typical value is shown in Table 1.2-1:

2. The contact potential difference between the gate material and the substrate material in the channel regionimpact on the threshold voltage VT.

If the work function of the semiconductor is greater than the work function of the metal, the electrons on the side of the metal will flow to the side of the semiconductor when the electrons are allowed to be exchanged, and its effect is similar to the positive charge in the oxide layer, which actually increases the threshold voltage by a negative value , Its size is equal to the contact potential difference between metal and silicon. its value determines the Fermi potential of the gate and substrate materialsthe difference between:

For aluminum grids,(Gate) = 0.6 volts; and for silicon gates,the value of (gate) varies with gate doping and concentration. Now the data provided in document [10] is listed as 1.2-2.

Obviously, the actual threshold voltage VT of the n-channel MOS tube can be added to the threshold voltage VˊT of the ideal MOS tube

Obtained, the relationship is:

where, called flat band voltage. The above formula is applicable to both reinforced pipes and depletion pipes. For reinforced pipes, as long as the conditions are met:

which is, appropriately increase the substrate concentration, reduce the positive charge Qss in the oxide layer, you can do, Constitute an n-channel enhancement type MOS tube. If the positive charge Qss in the oxide layer is large or the lining corrosion concentrationtoo small, it is possible to make，an n-channel depletion type field effect transistor is formed.

For the threshold voltage VT of the p-channel MOS tube, it can be written as:

If satisfiedunder the conditions, the P-groove reinforced tube is obtained. if, Then get P-channel depletion tube. It can be seen from the above formula that the P-channel depletion tube is not easy to realize.

Depend on（1.2-12）、（1.2-14）formula and Table 1.2-2 can get the threshold voltage expression of aluminum gate and silicon gate, as shown in Table 1.2-3.

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