Semiconductor Devices and Circuits Fall Midterm Exam. Instructor: Dr. Dietmar Knipp, Professor of Electrical Engineering. Name: Mat. -Nr.

Transcription

1 Semiconductor Devices and Circuits Fall 2003 Midterm Exam Instructor: Dr. Dietmar Knipp, Professor of Electrical Engineering Name: Mat. -Nr.: Guidelines: Duration of the Midterm: 1 hour The exam is a closed book exam. No additional utilities (Laptop, calculator, books, additional paper besides the exam sheet) are allowed. Enter your name and the matriculation number on each page of the exam. Every attempt to communicate with other students or colleagues during the exam will be considered as cheap, which will lead to the exclusion from the exam. You can get 62 points for the midterm exam. The number of points is proportional to the time needed to answer the question. In average it should take 1 minute to answer a question, which accounts for 1 point. The smallest available unit for the answer of a question is 0.5 point. Try to provide short answers. The answer can be shorter than the question. Depending on the question the answer might be only an equation. There will be no chance to repeat the midterm exam. The students who are not able to attend the exam (assuming a medical certificate yellow paper is provided) have to prepare a report on an advanced topic provided by the course instructor. Good Luck!

2 1 Introduction to Semiconductor devices (4 points) 1.1 List 3 of the basic device building blocks of semiconductor devices out of the list of 4 basic building blocks. (3 points) 1.2 What is a single crystal ingot? (1 point) 2 Basic Semiconductor physics (8 points) 2.1. What is a hole in a semiconductor? (1 point) 2.2 The DeBroglie equation is given by λ=h/mv. Explain the equation. (1 point) 2.3 Explain the 1st and the 2nd postulate of Bohr. (2 points) 2.4 Band gap structure Schematically sketch the relationship between the bandgap and the temperature for silicon. (2 points)

3 2.4.2 What means complete ionization in the case of a p-type doped silicon sample at room temperature? (1 point) 2.5 What is the difference between an indirect and a direct semiconductor? (1 point) 3 Semiconductors in thermal equilibrium (15 points) 3.1 Explain the difference between and extrinsic and an intrinsic semiconductor. (2 points) 3.2 Provide the proportionality between the intrinsic carrier concentration and the bandgap of a semiconductor. (1 point) 3.3 How is the bandgap of an intrinsic material related with the conductivity? (2 points)

4 3.4 Explain the concept of minority and majority carriers in a boron doped silicon sample. (2 points) 3.5 Density of states Describe the relationship between the density of states and the energy. (1 point) What is the reason for the difference in the density of states in the conduction and the valence band? (1 point) 3.6 Fermi-Dirac Statistic Describe the energetic position of the Fermi level in conductors, insulators and semiconductors. You can sketch the band diagrams. (3 points) Provide an equation for the Fermi-Dirac statistic for electrons. (1 point) Sketch the Fermi-Dirac statistic for electrons for the temperatures T 1 and T 2. T 1 >T 2 =0K. (1 point)

5 3.6.4 Provide the value of the Fermi-Dirac statistic for the Fermi energy. (1 point) 4. Semiconductors in non-thermal equilibrium (6 points) 4.1 Quasi Fermi Levels Sketch a band diagram of a p-type semiconductor in the case of carrier injection (np>n i 2 ). The band diagram should include the Fermi energy in thermal equilibrium, the quasi Fermi levels, the conduction and the valence band and the intrinsic energy level. (3 points) Sketch the corresponding carrier concentration diagram including the free carrier concentration (in thermal and non-thermal equilibrium) and the donor/acceptor concentration. (3 points)

6 5 Semiconductor Equations (14 points) 5.1 The electronic transport of semiconductors can be described by 5 semiconductor equations. Provide a list of the semiconductor equations. (2.5 points) 5.2 What is the dimension of the diffusion coefficient? (1 point) 5.3 Provide an expression for the specific electron conductivity? (1 point) 5.4 Provide the relationship between the mobility and the diffusion coefficient. (1 point) 5.5 Recombination Explain band-to-band and Acceptor-Donor recombination. Explain the differences. (1.5 point)

7 5.5.2 It is assumed that both processes are radiative processes and the recombination leads to the generation of photons. Compare the emitted photons in respect to their wavelength (2 point) 5.6 How is the bulk potential defined? Provide a schematic band diagram. (2 points) 5.7 How does lattice and impurity scattering affect the drift mobility? Discuss the influence of the temperature on the scatter mechanism. Sketch a graph of the mobility as a function of the temperature and indicate the influence of the scatter mechanism. (3 points) 6 pn junction (15 points) 6.1 Explain charge neutrality of a pn-junction in thermal equilibrium? Provide an equation and a sketch. (2 points)

8 6.2 Sketch the electric field distribution within an abrupt pn-junction in thermal equilibrium, under forward bias and under reverse bias. Explain the differences (3 points). 6.3 Provide the energy band diagram and the carrier concentration diagram of a pn-junction under reverse bias. It can be assumed that all dopants are ionized. N D > N A. (6 points) 6.4 Provide the empirical diode equation. Explain the meaning of the ideality factor. (2 points)

9 6.5 Sketch schematically the influence of the series resistance on the I/V characteristic of a pn-junction under forward bias. (2 points)