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Consider a Si sample and dope with Boron to prepare an extrinsic semiconductor

Physics

Consider a Si sample and dope with Boron to prepare an extrinsic semiconductor. Explain what kind of extrinsic semiconductor it will be and why? Plot a band diagram and show where the donor/acceptor level will lie with respect to the conduction and valance bands.

Consider a situation that we are preparing such a device and doping is changing in the range as shown in the following plot. Sketch how the total mobility will vary with increasing carrier doping at 300 K. (plot in the following format. (Split the plot into tentative low and high doping regions as separated by the dash-line). Write proper explanation for so. Assign the N? type (ND or NA respectively) accordingly in the plot.

Ww |

 

2. Consider that a semiconductor is doped in such a way, that in thermal equilibrium, at certain n-type region there is a hole concentration with the following spatial distribution:

p = (1 — 4x)1010 cm-3

Assume that throughout this region, the electron mobility is µn = 700 cm’/Vs and the hole mobility is µp = 500 cm’/Vs.

Now derive an expression for and sketch (a) the hole diffusion current and (b) the electric field distribution in this region. Plot both separately in the following format. Consider temperature 300 K.

Lid

 

5

 

[ Mark the starting

 

= point with associated

 

= y-axis value

 

0 - x (em) 1

3. Find the built-in potential for a p-n Si junction at room temperature if the bulk resistivity of Si is 5.5 ? cm. Electron mobility in Si at RT is 1100 cm*/Vs:

µn/µp = 3.1; n1 = 1.4 x 1010 cm-3, Consider, at room temperature all donors and acceptors are ionized.

Calculate the total depletion-layer width for applied bias voltages V = —5 V,

O, and +0.1 V. Consider ¢s; = 11.9.

4. Calculate the bias voltage for the following p-n junctions: Consider two ideal p-n junctions made of two different semiconducting materials. At the room temperature (300 K) we have two conditions: (Write down the associated equation)

(1) Forward bias total current: 15 mA with turn-on voltage 0.7 V and Reverse bias current 5 uA.

(i) Forward bias total current: 15 mA with turn-on voltage 1.2 V and Reverse bias current 0.1 uA.

If the currents are measured for two different diodes having exactly the same physical dimensions, identify which 1s made of silicon and which is the GaAs diode? (Marks will be given after writing proper justification)

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