This report details the results of the electronics laboratory experiment entitled "8: Diodes" (Bath, 1999).
The theory behind the physics involved will be discussed in any good semiconductor or electronics book, such as "Microelectronics" (Millman, 1987).
For each diode, the variation of current with applied forward voltage was measured using the following circuit.
The data was plotted (see next page, graphs 13) and the following results were found.
See Appendix A for details on how the data was collected from the graphs. The value of η was provided on the brief.
The
The values of I_{0} for the
We would expect the values of T (temperature of
the diode) to be very close to room temperature (aprox.
295K). The more ideal the diode, the more likely the result
is to be close to the room temperature (since to calculate
the temperature ideality is assumed). The
The overwhelming source of error was the fluctuations in the power supply caused by the number of appliances being used in the lab at the time. Other errors, such as the limitations of the equipment, were swamped by the fluctuations, and so have not been taken into account when drawing the errors bars.
To measure the reverse bias characteristics of the diodes, the following circuit was used:
Again the data was plotted (see next page, graphs 4 and 5) and the following results were found.
As predicted in the previous section, the
For the
I  =  I_{0}  +  αV 
...where α is some constant of proportionality (the gradient of the line), in this case 0.1S.
For the
I  =  I_{0}  exp  (  βV  ) 
...where β is a constant, in this case 0.548V^{1}.
In both of these cases, the theory (see Appendix A) would suggest that the relationship should be independent of V, if the diodes were ideal. (The ideal relationship is simply I = I_{0}.) That there is some dependence indicates that the diodes are not ideal.
The following circuit was used to examine the performance
of the
An oscilloscope was placed across the 1kΩ resistor, and the frequency was varied from 1kHz to 1MHz for the two diodes. Sketches of the resulting waves are included in Appendix C, at the back.
Qualitatively, the
However, the efficiency of both reduces as the frequency
increases: by 1MHz, the
The following circuit was used to determine the
temperaturecurrent relationship for the
The data collected is plotted on the last graph (see the next page, graph 6).
According to the brief, the temperature dependence of reverse current can be described by
I_{0}  ∝  exp  (   

) 
This can also be written as:
I_{0}  =  A  exp  (   

) 
...where A is the constant of proportionality.
Unfortunately, the data collected did not appear to match this at all, indicating either that the equation provided was incorrect, or that an error was made during the collection of the data.
A least squares fit analysis of the data shows that it fits a positive exponential relation:
I  =  I_{0}  exp  (  γT  ) 
...where I_{0} = 27.2nA and γ = 50.7 mK^{1}. Since there is no theory behind these numbers  they are purely empirical  no value of E_{G} can be derived or estimated.
Note that both sets of data have been used to find the fit. This is because during both the heating and cooling, the temperature of the transistor was lagged with respect to the casing, which is what the probe was in contact with. During the heating, the transistor was slightly cooler than the casing, and during the cooling, the transistor was slightly warmer than the casing. The net effect should be that they cancel out.
The brief (Bath, 1999) stated the following equation as a good approximation of diode behaviour:
I  =  I_{0}  (  exp  ( 

)    1  ) 
...where I is the current through the diode (positive for a forward bias and negative for reverse bias); I_{0} is the reverse saturation current; q is the electronic charge; V is the applied voltage (positive for a forward bias and negative for reverse bias); η is the ideality factor; k is the Boltzmann constant and T is the junction temperature in Kelvin.
For positive and large V (as in forward bias), this equation simplifies to:
I  =  I_{0}  exp  ( 

) 
...as the exponential term dominates the 1 term.
Using Excel, one can get a leastsquaresfit for the equation
y  =  c  exp  (  bx  ) 
...by plotting an "Exponential Trend Line" (Microsoft, 1997). By equating I=y, I_{0}=c, V=x and
b  = 

...values for I_{0}=c and T can be derived.
The following data is given in the relevant databooks for the devices.
These handdrawn sketches are included separately.