PHT.301 Physics of Semiconductor Devices
29.04.2022


Problem 1

Describe a green laser diode. What kind of semiconductor is needed to make it green? Make a sketch of the device indicating the contacts.

Solution


Problem 2

(a) An n-channel MOSFET is in weak inversion. This means that the electron concentration near the semiconductor-oxide interface is smaller than the acceptor doping concentration. Draw the charge density along a line normal to the gate, oxide, and body of the MOSFET. Indicate the position of the acceptor concentration on the charge density axis.

(b) Draw the corresponding electric field that goes with the charge density drawn in (a).

(c) Consider the drain current. What is the dominant current transport mechanism when the gate voltage is above the threshold voltage and what is the dominant current transport mechanism when the gate voltage is below the threshold voltage?

Solution


Problem 3

Draw the minority carrier concentration in a pnpn thyristor biased in the forward blocking mode. Make another drawing of the minority carrier concentration in forward conducting mode. Indicate the depletion widths in the drawings.

Solution


Problem 4

(a) A pnp bipolar transistor is used in a common base configuration where $|V_{CE}|= 20$ V. What are approximately the voltages at the emitter and the collector? Assume that the voltage at the base is zero. Indicate the polarity (+ or -) of the emitter and collector voltages.

(b) If the base doping of this transistor is increased, what will happen to the gain factor $\alpha$ and the Early effect? Explain why these changes take place.

(c) The collector is lightly doped. How is a Schottky contact avoided between the collector and the metal contact? What determines how wide the lightly doped part of the collector should be?

Solution




Quantity

Symbol

Value

Units

electron charge

e

1.60217733 × 10-19

C

speed of light 

c

2.99792458 × 108

 m/s

Planck's constant

h

6.6260755 × 10-34

J s 

reduced Planck's constant

$\hbar$

1.05457266 × 10-34

J s

Boltzmann's constant

 kB

1.380658 × 10-23

J/K

electron mass

me

9.1093897 × 10-31

kg 

Stefan-Boltzmann constant

σ

5.67051 × 10-8

W m-2 K-4

Bohr radius

a0

0.529177249 × 10-10

m

atomic mass constant

mu

1.6605402 × 10-27

kg

permeability of vacuum

μ0

4π × 10-7

N A-2

permittivity of vacuum

ε0

8.854187817 × 10-12

F m-1

Avogado's constant

NA

6.0221367 × 1023

mol-1