🚀 go-pugleaf

raju@bass.bu.edu (rajminder Singh) writes:
: Hi there! I am  taking a course in Solid State Physics but I still 
: haven't been able to answer the basic question of why the voltage drop
: in a Silicon p-n junction is around 0.7 Volts ? Does this vary significantly
: with the doping level ? What about the effect of temperature, current and
: the energy band gap (E) of the material used in the junction ?
: 
	The voltage drop across a pn junction diode depends on the
current, temperature, etc. according to the well-known V-I
characteristic equation, I = I_s [ exp(eV/nkT) - 1 ] (refer to your favorite
text book for a derivation). So the voltage drop is 
	V = (nkT/e) ln(I/I_s + 1).
The parameter n (actually Greek letter eta) is approximately 2 for Si
diodes, and kT/e is about 25 mV at room temperature, so the voltage drop
is approximately (50 mV)*ln(I/I_s). So you can see that for a typical
diode current, V is around 0.7 V for a wide range of values for I_s
(the reverse saturation current). For example, let's say I = 10 mA, then
the voltage drop for various values of I_s are (approximately):
	I_s (A)		V (volt)
	-------		--------
	 1E-9		  0.81
	 1E-8		  0.69
	 1E-7		  0.58 
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