Problem Set #10
ECEN 3320 Spring 2021
Semiconductor Devices
Due on April 7, 2021
1. Consider a silicon npn transistor with NE = 10
16/cm3, NB = 10
15/cm3, a channel area of
10−8 cm2 and emitter and base lengths wE = wB = 1µm. You can assume recombination
cu
ents are small compared with the emitter electron and hole cu
ent. Find:
(a) the built-in potential, φi, across the base emitter junction,
(b) the n and p side depletion widths, xn and xp, of the base emitter junction,
(c) the IE,n,0 and IE,p,0,
(d) the α and
(e) the β.
2. We want to fa
icate a high speed npn bipolar junction transistor (BJT) with a β = 100.
The base is to be doped at NB = 10
16 cm−3 and the collector at NC = 10
14. The ratio of the
emitter to base length, wE/wB, is required to be 11/32. High speed requires that the base is
as thin as possible. Find:
(a) the doping level that must be used in the emitter in order to satisfy the requirement
that β = 100,
(b) the minumum value for base length that we can use and still have the device operate as
a transistor (that is, not be permeated by depletion regions), and
(c) and the transit time for an electron to traverse the base, using the maximum electric
field at the base emitter junction, EEB,max to find the drift velocity.
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