Gate Threshold Voltage

Because Vt is not zero, the definition of transconductance also needs to be modified slightly from 

Note that in our analysis, we are making another simplifying assumption — that the transconductance too is a constant. 

Finally, with all this background information, we can start looking closely at what actually happens during the turn-on and turn-off transitions. 

---

Step 6. The doping concentration of the "typ substrate under the gate oxide is adjusted by another boron implantation. Boron passes through the thin gate oxide. This provides the threshold voltage adjustment for the final device. 

The operation principle of these TFTs is identical to that of the metal-oxide-semiconductor field-effect transistor (MOSFET) [617,618]. When a positive voltage Vg Is applied to the gate, electrons are accumulated in the a-Si H. At small voltages these electrons will be localized in the deep states of the a-Si H. The conduction and valence bands at the SiN.v-a-Si H interface bend down, and the Fermi level shifts upward. Above a certain threshold voltage Vth a constant proportion of the electrons will be mobile, and the conductivity is increased linearly with Vg - Vih. As a result the transistor switches on. and a current flows from source to drain. The source-drain current /so can be expressed as [619]... 

In DC-DC converters, we can somewhat slow the turn-on of Fets if we insert a small resistor (10 to 20Q. typically) in series with the decoupling capacitor of their respective driver stages. For example, a small resistor can be placed in series with the bootstrap capacitor of the third-generation switcher family I used to cover. That helped with almost 10 to 20% of customers, but somehow this trick didn t find its way into the applications information section of their datasheet. If the Fets are external, we can try a small resistor in series with the Gate, but this affects both the turn-on and turnoff (with such low threshold voltages, a diode in parallel to the resistor will not do anything). 

FIGURE 9.7 Measured characteristics of fabricated a-Si H TFTs. (a) Transfer characteristics at Fds = 0.1 V for TFTs with different gate width, (b) an example of /DS- Vos characteristics for current-temperature-stress (CTS) measurements, (c) extracted A Vth versus stress time at RT and 80°C, and (d) Cadence Spectre simulation of pixel electrode circuit for threshold voltage shift of a-Si H TFTs are shown. (From Hong, Y., Nahm, J.-Y., and Kanicki, J., IEEE J. Selected Top. Quantum Electron. Org. Light-Emitting Diodes, 10, 1, 2004. With permission.)... 

Relevant for our discussion is the genesis of the sensitivity behaviour in a class of devices all generated from the well known MOSFET structure (ISFET and GASFET). In particular, the influence of charges into the gate oxide on the threshold voltage and MOSFET behaviour under shrinking conditions will be discussed. 

From the transconductance expressions we see that the first (eq. 15) is linear with Vds and the second (eq. 16) is related to both MOSFET gate voltage Vgs and its threshold voltage Vt- In both cases Cox plays an important role. In fact in order to increase the sensitivity, the gate oxide thickness should be as thin as possible. gm,SAT, according to equation 16, depends on Vt and it is known that Vt depends on Vfb-, the flat band voltage, according to ... 

To be consistent with common equations for NMOS transistors, the polarities of the applied voltages have been inverted. The source-gate voltage is denoted Vsg, the source-drain voltage Vsj, and the threshold voltage Vf. Equation (4.3) corresponds to... 

---