1. 1. A photoresistor is formed from a square 1 cm x 1 cm slab of GaAs. Light of wavelength 830 nm falls onto it at a power density of 1, generating electron-hole pairs of 10 m s lifetime. i) if each incident photon produces one EHP, what is the resistance of the GaAs slab. (Assume the carrier concentration with no light is negligible). ii) Explain whether a similar measurement on a slab of AlSb would yield a similar result. Assignment # 4 Given February 4, 2000 Due February 11, 2000 W/m 2

2. 2. A Hall effect measurement is made on a semiconductor sample with length 0.5 cm, width 0.2 cm, and cross-sectional area of 10 -2 cm 2. With a constant current supply, 0.4 mA flow through the length of the sample. Determine: a)The resistivity of the sample if the potential V x across the length of the sample is 1.0 V. b)The carrier type. A magnetic field of 0.75 T is applied normal to the sample, and a Hall voltage V H = 0.1 V is measured (across the width with polarity as indicated). - see next page -

3. Question 2 cont’d: c)The carrier concentration. d)The carrier mobility. e)The values of V x and V H if the carrier concentration is doubled. 3. For Si that is very lightly doped with N A =3.7  10 10 cm -3, calculate the equilibrium electron and hole concentrations ( n i = 1.5  10 10 cm -3, T = 300K ). Find the displacement of the Fermi energy level E F from the band gap position. Note: You can use 4.Problem 3.10, p 93, textbook, 4 th edition