Problem 1. Consider the low frequency C_lf/C_ox versus C_G curve in figure 1. It is for a MOS capacitor with a p-type substrate (N_A = N_A1), a metal gate and V_FB = 0. Draw the C_if/C_Cox­ versus V_G curve for the device if the metal gate is replaced with a p-type poly-Si gate doped to N_A = N_A1. Explain your answer.

Figure 1. lf CV characteristics of a MOS capacitor

Problem 2. The subthreshold I_D-V_G curves of a MOSFET are shown in Figure 2 before and after stressing the device. Determine the interface trap density change DD_it (in cm^-2eV^-1) induced by the stress. T = 300K, e_ox = 3.9, t_ox = 10 nm. (Read off the relevant values from the semi-log plot below)

Figure 2. Transfer characteristics of a MOSFET before and after stress

You may find the following useful: ΔD_it = C_ox/2.3kT (1/slope before -1/slope after)

3. Why do the low frequency and high frequency CV curves differ in inversion?

4. Using the transfer characteristics shown in Figure 3 (V_D = 2V; W/L = 10:1).

Calculate the following:

a. The threshold voltage

b. The field effect mobility

Figure 3. Transfer characteristics of a TFT