Ideality factor for silicon diode

The purpose of this experiment is to acquaint the student with the operation of semiconductor diodes. You will use a curve tracer to obtain the current-voltage (I-V) characteristics of a silicon diode. From these characteristics, you will determine several diode parameters including the dynamic resistance, rf = rd; the diode forward resistance, RF = RD; the cut-in voltage, Vγ; the forward diode ideality factor, n; and the breakdown voltage, VBR. All of these terms are defined below. You will find that most of these parameters depend on the current at which that parameter is measured. You will also compare the dc operation of a diode in a circuit with both the calculated and simulated operation.

PRE-LAB:

Review the INTRODUCTION section below. Simulate the diode characteristic using PSpice for comparison with experimentally measured results. Determine rd, Vγ, and n for the 1N4004 diode in your diode characteristic plot. Simulate the circuit shown in Figure 5 for the resistor (R) values shown using a DC sweep test (sweep Vin and R simultaneously; see Part 2 of previous lab for help).

EQUIPMENT:

For this laboratory session you will need the following:

a. Silicon diodes: 1 – 1N4004 diode or equivalent 1N4001 1 – 1N4744 diode These diodes are all silicon diodes with different breakdown voltages and different power handling capabilities. The 1N4744 is a Zener Diode and has the lowest breakdown voltage. It should be used when attempting to obtain the reverse breakdown voltage. The 1N4004 diode will be used extensively in circuits in other experiments.

b. Tektronix Type 571 curve tracer c. Breadboard d. Resistor decade box e. A computer with PSpice

INTRODUCTION:

Diode Structure

Figure 1

Figure 1 shows the physical and schematic circuit symbol of the diode. The band on the diode and the bar on the left of the circuit symbol represent the cathode (n-type material) and must be noted. The p-type material (the anode) in the diode is located to the right. The circuit symbol of the diode is an arrow showing forward bias, when the p-side is positive with respect to the n-side, and the direction of the arrow represents the direction of large current flow.

Ideal Diode Equation

The relationship between the diode current and voltage is given by the diode equation

:

 

  

 −= 1T

D nV

V

SD eII (1)

The terms in Equation (1) are defined as follows:

ID = the diode current (amperes).

VD = the voltage across the diode (volts).

IS = the reverse saturation current or the reverse leakage current (amperes).

IS is a function of the diode material, the doping densities on the p-side and n-side of the diode, the geometry of the diode, the applied voltage, and temperature. IS is usually of the order of 1 μA to 1 mA for a germanium diode at room temperature and of the order of 1 pA = 10

-12 A for a silicon diode at room

temperature. IS increases as the temperature rises.

VT = k T / q = the thermal equivalent voltage = 0.0258 V at room temperature

where

q = 1.6 x 10 -19

Coulombs = the electric charge,

k = 1.38 x 10 -23

J/K = Boltzmann's constant,

T = absolute temperature (Kelvin) [room temperature = 300 K], and

n = the ideality factor or the emission coefficient.

The Ideality Factor (n):

The ideality factor, n, depends on the type of semiconductor material used in the diode, the manufacturing process, the forward voltage, and the temperature. Its value generally varies between 1 and 2. For voltages less than about 0.5 V, n ~ 2; for higher voltages, n ~ 1. (Based on experimental measurements, at higher voltages, typically 1.15 ≤ n ≤ 1.2.)

The ideality factor, n, can readily be found by plotting the diode forward current on a logarithmic axis versus the diode voltage on a linear axis.

Equation (1) indicates that an increase in current ID by a factor of 10 represents an increase in exp(VD / n VT) by a factor of 10, as long as exp(VD / n VT) >> 1. If ΔVD is the change in voltage required to produce a factor-of-10 change in the current, then

( )

( ) mV3.590593.0V0258.030.230.2

30.210ln

⋅====∆

== ∆

nnnVnV Vn V

TD

T

D

And so,