Department of Engineering Science Dec. 11, 2018 Sonoma State University

Test 3EE/CES 432

Physical ElectronicsElectronic and Optoelectronic Devices

To receive credit, you must show your work thoroughly and clearly! Diagrams must be sketched very carefully

1. 1N4001 Consider the LED driver circuit shown below

(a) Assume IF = 12 mA and Vinput = 1.8 V and calculate RF. (4 pts)

(b) Assume VCC = 4.4 V and VLED = 2 V. Calculate VCE. (3 pts)

(c) Assume an AC input voltage of frequency f. The optical power output of the LED should follow the waveform of the input voltage. P(f)/P0 is the relative optical output power of the LED which is expected to be equal to 1 at lower frequencies and drop as frequency increases. Sketch a plot of P(f)/P0 vs f and indicate the optical bandwidth of the LED and its cut-off frequency on the diagram. (3 pts)

1

Department of Engineering Science Dec. 11, 2018 Sonoma State University

2. Erbium doped fiber amplifiers (EDFA) are used in optical fiber communication systems.

(a) What is the main function of EDFA? Why do we need to insert EDFAs in a long-haul system? (4 pts)

(b) What is the pumping mechanism of EDFA? (3 pts)

(i) Electrical: preamplifier(ii) Optical: laser diode(iii) Optical: flash lamp

(c) For fiber optic communications what is the wavelength of the light that (2 pts),

(i) enters EDFA?:

(ii) exits EDFA?:

(d) Sketch an energy diagram describing the amplifying process. Indicate the pump energy, the input and output photons and their wavelengths. (3 pts)

3. What type of feedback is utilized in a laser diode?

(a) Positive feedback or negative feedback (2 pts)

(b) Explain why! Describe how this feedback mechanism is implemented in a laser diode (2 pts)

2

Department of Engineering Science Dec. 11, 2018 Sonoma State University

4. Briefly explain:

(a) Why it is necessary to have population inversion for operation of a laser diode? (3 pts)

(b) Population inversion in laser diodes in achieved by (2 pts)

(i) electrical pumping(ii) optical pumping

5. (a) Sketch the energy band diagram (E-x) of a quantum well laser diode. (2 pts)

(b) How would you choose the energy band gap of the semiconductor layers next to the active layer? Choose an answer and explain why! (2 pts)

(i) Eg > Eg-active

(ii) Eg < Eg-active

6. Consider the I-V axes below. It is divided into four quadrants. Specify the quadrant in which the following semiconductor optical devices operate. Mark each device in their appropriate quadrant: LED, laser diode, solar cell, p-n junction photodiode, PIN photodiode, avalanche photodiode (APD). (10 pts)

3

I

V

12

3 4

Department of Engineering Science Dec. 11, 2018 Sonoma State University

7. The I-V curve shown below is for a 1 cm2 solar cell under 800 W/m2 illumination.(a) Determine the open-circuit voltage and short-circuit current of the device. (2 pts)

(b) Identify an optimum operating point on the curve for this device and specify its Im and Vm. (3 pts)

(c) Calculate the fill factor of the device at the above operating point. (2 pts)

(d) Draw the load line and determine the load resistance for the operating point. (3 pts)

(e) Calculate the efficiency of the cell at this operating point. (3 pts)

4

Department of Engineering Science Dec. 11, 2018 Sonoma State University

8. Briefly describe the following parameters for photodiode detectors. Support each answer with a formula or a sketch.(a) Responsivity (2 pts)

(b) Spectral Response (2 pts)

(c) Signal rise time (2 pts)

9. Estimate the minimum detectable power for the following PIN diode:Responsivity = 0.4 A/W; Dark current = 1.6 nA (4 pts)

10. Describe the following within the context of an optical communication system:(a) Advantages of PIN photodiode. (2 pts)

5

Department of Engineering Science Dec. 11, 2018 Sonoma State University

(b) Advantages of APD photodiode. (2 pts)

(c) Assume the power loss in an optical fiber is 0.3 dB/km. We have a choice to detect the signal at the end of the fiber cable using a PIN or APD photodiode. If we use a PIN diode the maximum length of the cable is 80 km. Calculate the maximum length of the cable if we use an APD if it has an advantage of 9 dB over the PIN diode. (3 pts)

11. Why a p n p junction FET may not work as well as a p+ n p+ transistor? (3 pts)

12. Height of the n-channel in a silicon p+ n p+ FET is 2.4 μm and its width is 8 μm. The length of the channel is 16 μm. The n-channel is doped with 2 x 1016 cm-3 donors. Assume all donors are ionized at room temperature. The dielectric constant of silicon is 11.8. Calculate:

(a) Pinch-off voltage of the device. Sketch the drain current ID vs. drain voltage VD and identify the pinch-off point on the plot. (3 pts)

(b) Conductivity and resistivity of the channel. Assume electron mobility in Si to be 1,100 cm2/V.s(3 pts)

6

Department of Engineering Science Dec. 11, 2018 Sonoma State University

(c) Resistance and conductance of the channel. (2 pts)

(d) Drain current at drain voltage of 2 volts and gate voltage of -1 volt. (2 pts)

(e) Current density at the drain for part (d). (2 pts)

(f) The electric field within the channel due to the drain voltage of part (d). Does the strength of this electric field cause breakdown of the n-channel? Why? (2 pts)

7

Department of Engineering Science Dec. 11, 2018 Sonoma State University

13. High electron mobility transistor (HEMT) is a particular form of MESFET transistors. Describe modulation doping and explain why such a pattern is necessary for achieving a high n-channel conductivity. (2 pts)

14. What type of a semiconducting material (p-type or n-type) is used between the source and drain of a p-MOS field effect transistor? Support your answer with a sketch. (3 pts)

15. Sketch two separate series of ID vs VD curves for n-MOS and p-MOS transistors under three different values of gate voltages. (3 pts)

8