Drexel University

ECE-E302, Electronic Devices

PN Junction Diodes

Oct. 2013 BN ZW

Operation of a PN Junction

Objectives:

  • Investigate the DC current-voltage characteristics of the PN junction diode.
  • Experimentally verify theoretical model developed in lecture and extract ideality factor, and reverse saturation current.
  • Produce a piece-wise linear model for the diode.
  • Compare closed form, piece-wise linear model, and PSPICE simulations with experiment.

Theory of Operation:

In practice an ideality factor n is phenomenologically added to the above closed-form description in order to better model the diode I-V, resulting in:

(1)

Where Id is DC current through diode, and Vd is the voltage across the diode.Additionally:

lo= reverse saturation current

q = electron charge (1.6 x 10-19 C)

k = Boltzmann's constant (1.38 x 10-23 J/K)

T = absolute temperature in Kelvin degree

n = ideality factor, 1 ≤ n ≤ 2

For forward bias and voltages V greater than the turn-on voltage (0.2 V for germanium, 0.7 V for silicon, 1.2 V for GaAs), the “ 1 ” term in equation 1 can be ignored.

Equation (1) describes a nonlinear relation between the current Id and the voltage Vd of the diode, and causes difficulty in analysis of electronic circuits. For example, consider the simplest circuit we can construct as shown on the right. Assuming VDD = 1V and R = 500 Ω, we are asked to find ID, and VD. Writing the loop equation, we have:

(2)

And the diode equation is

These are two equations with two unknowns (Id, and Vd), which cannot be solved in closed form, but can be solved numerically, or graphically. We can also produce a piece-wise linear model of the diode for quick calculations. Finally we can use electronic circuit simulators such as PSPICE in order to find Id and Vd.

Figure 2 shows the current-voltage relation of a diode as well as its approximation by a piecewise linear model. The circuit components for the piecewise linear model are also shown.

Experiment:

Note: All of the work below should be done during the lab, not afterwards.

  • Set up the circuit of Figure 3, below.

Figure 3. Experimental set-ups for forward bias (left) and reverse bias (right)

  • Vary the DC power supply, and construct a table with measured Id vs Vd.
  • Use the curve tracer board and measure Id vs Vd.
  • From the measured data, determine the reverse saturation current I0 and ideality factor n.
  • From the measured data, determine piecewise linear circuit elements, namely, the turn-on voltage V0 and the series resistance rD.
  • Simulate the circuit of Figure 1 in PSPICE, use parameters you have extracted above. Note rD is RS in PSPICE.

Now assume the DC supply is 2V. Find Id and Vd from:

a) Closed form model of Equation 1,

b) Graphically from I-V you have generated. That is, plot (1) and (2) together. Use Matlab or Excel.

c) Piecewise linear model of Figure 2.

d) PSPICE simulation

e) Measurement

Report:

Use ECE-E302 report template. Explain how you performed parameter extraction for closed from, SPICE simulation, and piecewise linear model. Compare the modeling results with measurements. Explain in what voltage or current range equation (1) adequately describes diode behavior. Is the model valid for high injection regime (read your textbook to learn what this means).

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