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When Boltzmann's Law is applied to the motion of electrons in an n-p-n transistor, it can be shown that the collector current is given by the equation

I_c = I_o exp[(q V_BE)/(K_B T)]

where

I_o = A(T) exp [(- f/(K_B T)]

In the second equiation, A(T) is a slowly varying function of temperature and f is the height of the barrier in the absence of an applied voltage.

In the lab, students test the validity of the first equation by measuring Ic as a function of VBE in an n-p-n transistor at three different temperatures (room temperature, in a dry ice-alcohol mixture and in mineral oil heated to ~91û C). For each temperature, graphs of log(Ic) as a function of VBE are made and the value of the ratio q/kB is computed from the slope of the plotted data. Next, the height of the barrier in electron volts, f/q , is computed from a plot of ln[IoT-3/2] vs. 1/T . Finally, students are asked to estimate the validity of the assumption that A(T) µ T-3/2 based on their data.

Course Level

Intermediate: P49

Student Handouts

Equipment

Number of set-ups available: Per lab station: 2 multimeters 1 DC power supply - HP Harrison 6201B 1 transistor control box 1 transistor on a rod/associated wiring 6 male/female banana adaptors 1 hot plate - VWR Dylatherm 1 low temperature thermometer (-100 to 50 degrees C) 1 thermometer (-20 to 150 degrees C) 1 styrofoam cylindrical container for dry/alcohol mixture 1 styrofoam container to hold extra dry ice 1 600 ml beaker 1 aluminum pan/stand inside mineral oil isopropal alcohol dry ice 2 pair of safety glasses 2 lab coats 1 MSDS sheet for isopropal alcohol 1 box of Kimwhips 1 pair of gloves for handling hot and cold items 1 ring stand 1 ring stand clamp 1 three pronged claps 5 cycle semi-log graph paper

Circuit Diagram

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