SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

9.6. IMPORTANT ISSUES IN REAL MOSFETS 471

S D

N+

A

A'

B B’

nweak inversion

EFN source EFN(x)

EFN drain

φBS

nsource = NCexp –(φBS)/kT

Along BB'

N+

nsource

n(x) = nsource(1-x/LCH)

e

e

Figure 9.24: Band diagrams for anMOSFET at the source, drain, and along the direction of
transport for a device where diffusion current dominates

whereVg <Vth. This gives us the desired subthreshold slope in current of 60 mV/decade.
Deviations from this can occur if

1. Charge sharing occurs i.e. the gate charge is not imaged in the semiconductor but on the
   electrodes as well (short channel effect, or traps in the system)

2.Coxis small or the aspect ration is small

3. Voltage division occurs - for example due to poor contacts


4. Gate leakage occurs.


5. Leakage through the buffer occurs.

Figure 9.25 shows the impact that non-idealities have on subthreshold leakage.