In this laboratory course various electronic properties of a red light emitting diode (LED) were measured. Aim of this these experiments was to become acquainted with the use of a sourcemeter unit (SMU) Keithley 2600 Series Sourcemeter), with the remote controll of the SMU with Python scripts and to draw conclusions about the measured device from the collected data. For further information about the atomic processes taking place in an LED, it is refered to the Physics of Semiconductors course from Prof. Hadley.

As part of this laboratory session the following three measurements were performed on a red LED:

Characteristic Curve at Room-Temperature

In this measurement the following Python script was used to record the characteristic curve of the red LED. This script operates the SMU as a voltage source and performs a voltage sweep, while recording the current that flows through the LED. To use this script the Keithley 2600 Python library has to be stored in the same folder as the script. Furthermore it has to be made sure, that the correct channel of the SMU is connected to the LED.

from KeithleyV15 import SMU26xx
import matplotlib.pyplot as plt
import time
import datetime
import csv
import numpy as np

"""
EXAMPLE: characteristic curve of a diode

Schematic:

--------------
    |            |
  -----         -|-
  | A | SMU     \|/
  -----         -|-
    |            |
    --------------

"""

""" ******* Connect to the Sourcemeter ******** """

# initialize the Sourcemeter and connect to it
# you may need to change the IP address depending on which sourcemeter you are using
sm = SMU26xx("TCPIP0::129.27.158.188::inst0::INSTR")

# get one channel of the Sourcemeter (we only need one for this measurement)
smu = sm.get_channel(sm.CHANNEL_A)

""" ******* Configure the SMU Channel A ******** """
#define a variable "current range" to be able to change it quickly for future measurements

current_range = 1e-4
current_range_for_name = str(current_range)

# reset to default settings
smu.reset()
# setup the operation mode of the source meter to act as a voltage source - the SMU generates a voltage and measures the current
smu.set_mode_voltage_source()
# set the voltage and current parameters
smu.set_voltage_range(20)
smu.set_voltage_limit(20)
smu.set_voltage(0)
smu.set_current_range(current_range)
smu.set_current_limit(current_range)
smu.set_current(0)

""" ******* For saving the data ******** """

#some suggestions for improvement of the data saving
	#the file name should identify the file uniquely
	#therefore specifying for example the voltage range, and the
	#type of characteristic (U/I, U/R, etc...) is suggested

# Create unique filenames for saving the data
time_for_name = datetime.datetime.now().strftime("%Y_%m_%d_%H%M%S")
filename_csv = 'Diode_' + 'current_range_' + current_range_for_name + '_' + time_for_name +'.csv'
filename_pdf = 'Diode_' + 'current_range_' + current_range_for_name + '_' + time_for_name +'.pdf'

#initializing a CSV file, to which the measurement data will be written - if this script is used to measure another characteristic than the U/I curve, this has to be changed
# Header for csv
with open(filename_csv, 'a') as csvfile:
        writer = csv.writer(csvfile, delimiter=';',  lineterminator='\n')
        writer.writerow(["Voltage / V", "Current / A"])

""" ******* Make a voltage-sweep and do some measurements ******** """

#some suggestions for the improvement of the measurement process
	#it would be nice to have a primary measurement, in which the
	#type of junction was be determined and a parameter was set
	#that indicated the forward direction of the junction
	#this primary measurement could also be executed and inlcuded
	#in the file name
	#this procedure would spare us the time of finding the forward
	#direction

# define sweep parameters
sweep_start = -1.5
sweep_end = 5
sweep_step = 0.025
steps = int((sweep_end - sweep_start) / sweep_step)

# define variables we store the measurement in
# rename if needed
data_current = []
data_voltage = []

# enable the output
smu.enable_output()

# step through the voltages and get the values from the device
for nr in range(steps):
    # calculate the new voltage we want to set
    voltage_to_set = sweep_start + (sweep_step * nr)
    # set the new voltage to the SMU
    smu.set_voltage(voltage_to_set)
    # get current and voltage from the SMU and append it to the list so we can plot it later
    [current, voltage] = smu.measure_current_and_voltage()
    data_voltage.append(voltage)
    data_current.append(current)
    print(str(voltage)+' V; '+str(current)+' A')
    # Write the data in a csv
    with open(filename_csv, 'a') as csvfile:
        writer = csv.writer(csvfile, delimiter=';',  lineterminator='\n')
        writer.writerow([voltage, current])

# disable the output
smu.disable_output()

# properly disconnect from the device
sm.disconnect()

""" ******* Plot the Data we obtained ******** """

#use plt instead of semilogy to plot in chartesian axis
plt.semilogy(data_voltage, np.abs(data_current),'x-', linewidth=2)

# set labels and a title
#suggestion: include some more information about the measurement (ref CSV file title)
plt.xlabel('Voltage / V', fontsize=14)
plt.ylabel('Current / A', fontsize=14)
plt.title('Characteristic curve of a diode', fontsize=14)
plt.tick_params(labelsize = 14)

plt.savefig(filename_pdf)
plt.show()

diode_characteristic.py

With this programm the diodes U/I curve was measured for different current ranges. The results are shown in the following plot.

This picture shows the U/I characteristics of the red LED for different current ranges of the SMU. The measurement shows the impact of the current range on the recieved data. High current ranges usually are used to measure higher currents, but with the tradeoff that they sacrifice resolution for lower currents. For the current ranges 1e-4 to 1e-6 clearly the lower end of the measurement range was hit, which lead to this data. For the current ranges 1e-7 and 1e-8 a comparison of the low voltage and reverse voltage behaviour with other diodes could be performed to evaluate the influence of the SMU on the measurement. Furthermore the photoelectric effect should be detectable when operating the LED next to a window on a bright day. Some experiments dedicated to this effect could be performed.

Current range: e-4		Current range: e-5		Current range: e-6		Current range: e-7		Current range: e-8	
-1.4993E+00	-3.6681E-08	-1.4994E+00	-2.9206E-10	-1.4993E+00	-1.0681E-10	-1.4994E+00	-3.1483E-11	-1.4994E+00	-2.6889E-11
-1.4495E+00	-1.1683E-08	-1.4497E+00	9.1791E-11	-1.4496E+00	1.5974E-10	-1.4747E+00	-3.0506E-11	-1.4746E+00	-6.1727E-12
-1.3993E+00	-9.8109E-09	-1.3995E+00	9.2983E-11	-1.3994E+00	1.6332E-10	-1.4497E+00	-3.4869E-11	-1.4496E+00	-1.1907E-11
-1.3495E+00	-9.1434E-09	-1.3496E+00	1.0252E-10	-1.3495E+00	1.6248E-10	-1.4248E+00	-2.0325E-11	-1.4247E+00	-7.8702E-12
-1.2996E+00	-9.2626E-09	-1.2997E+00	1.0967E-10	-1.2996E+00	1.6367E-10	-1.3995E+00	-6.1393E-12	-1.3994E+00	-1.3079E-11
-1.2494E+00	-9.1672E-09	-1.2495E+00	1.0252E-10	-1.2495E+00	1.6534E-10	-1.3746E+00	2.0266E-13	-1.3744E+00	-2.1066E-11
-1.1995E+00	-8.8096E-09	-1.1996E+00	1.1563E-10	-1.1996E+00	1.6081E-10	-1.3496E+00	1.2159E-12	-1.3495E+00	-1.8810E-11
-1.1496E+00	-8.8573E-09	-1.1497E+00	1.0014E-10	-1.1497E+00	1.6284E-10	-1.3247E+00	-6.8426E-12	-1.3245E+00	5.3585E-11
-1.0993E+00	-9.0003E-09	-1.0995E+00	1.0014E-10	-1.0995E+00	1.5914E-10	-1.2997E+00	-2.7418E-13	-1.2996E+00	-7.8565E-11
-1.0494E+00	-8.8215E-09	-1.0497E+00	1.0371E-10	-1.0496E+00	1.5724E-10	-1.2748E+00	3.8266E-12	-1.2747E+00	-4.0622E-11
-9.9959E-01	-8.7380E-09	-9.9966E-01	9.5367E-11	-9.9968E-01	1.6272E-10	-1.2495E+00	7.7009E-12	-1.2494E+00	-2.8157E-11
-9.4937E-01	-9.0480E-09	-9.4948E-01	1.0014E-10	-9.4945E-01	1.5378E-10	-1.2246E+00	-4.9233E-12	-1.2244E+00	-2.7106E-11
-8.9950E-01	-9.2268E-09	-8.9959E-01	1.0729E-10	-8.9958E-01	1.5962E-10	-1.1996E+00	1.5855E-12	-1.1995E+00	-2.2975E-11
-8.4961E-01	-9.0957E-09	-8.4967E-01	9.5367E-11	-8.4969E-01	1.6367E-10	-1.1747E+00	1.3351E-12	-1.1745E+00	6.3539E-13
-7.9939E-01	-9.1195E-09	-7.9950E-01	1.0371E-10	-7.9950E-01	1.7071E-10	-1.1497E+00	1.1599E-11	-1.1495E+00	-1.9049E-11
-7.4949E-01	-9.0003E-09	-7.4961E-01	9.6560E-11	-7.4961E-01	1.7858E-10	-1.1248E+00	1.3959E-11	-1.1245E+00	-1.6837E-11
-6.9930E-01	-9.3460E-09	-6.9941E-01	9.4175E-11	-6.9941E-01	1.7273E-10	-1.0995E+00	1.0443E-11	-1.0993E+00	6.0034E-12
-6.4940E-01	-9.0718E-09	-6.4951E-01	9.7752E-11	-6.4952E-01	1.7488E-10	-1.0745E+00	6.3777E-12	-1.0744E+00	1.4356E-11
-5.9951E-01	-9.0480E-09	-5.9963E-01	9.4175E-11	-5.9963E-01	1.7226E-10	-1.0496E+00	3.0994E-12	-1.0495E+00	-5.5336E-11
-5.4929E-01	-9.1553E-09	-5.4942E-01	9.0599E-11	-5.4943E-01	1.6213E-10	-1.0247E+00	4.1843E-12	-1.0245E+00	-5.0069E-11
-4.9941E-01	-8.9526E-09	-4.9952E-01	9.1791E-11	-4.9955E-01	1.5938E-10	-9.9972E-01	4.3154E-12	-9.9951E-01	-3.7415E-11
-4.4948E-01	-8.9765E-09	-4.4964E-01	9.5367E-11	-4.4969E-01	1.6666E-10	-9.7445E-01	1.9550E-12	-9.7426E-01	-2.1622E-11
-3.9930E-01	-8.9526E-09	-3.9941E-01	1.0371E-10	-3.9944E-01	1.6987E-10	-9.4951E-01	4.7898E-11	-9.4930E-01	-1.4006E-11
-3.4940E-01	-9.0122E-09	-3.4955E-01	9.5367E-11	-3.4953E-01	1.7834E-10	-9.2456E-01	1.9636E-10	-9.2440E-01	-9.7513E-12
-2.9950E-01	-9.0718E-09	-2.9967E-01	9.7752E-11	-2.9964E-01	1.8406E-10	-8.9961E-01	-6.7890E-11	-8.9950E-01	-9.7990E-12
-2.4929E-01	-9.0838E-09	-2.4944E-01	9.5367E-11	-2.4944E-01	1.8334E-10	-8.7469E-01	-9.7716E-11	-8.7455E-01	-6.2299E-12
-1.9939E-01	-8.8334E-09	-1.9955E-01	9.7752E-11	-1.9955E-01	1.8072E-10	-8.4977E-01	-6.3157E-11	-8.4958E-01	-7.2718E-12
-1.4956E-01	-8.9049E-09	-1.4971E-01	1.0252E-10	-1.4970E-01	1.7667E-10	-8.2452E-01	-5.2285E-11	-8.2435E-01	-8.2851E-12
-9.9340E-02	-8.3089E-09	-9.9475E-02	9.4175E-11	-9.9502E-02	1.8060E-10	-7.9955E-01	-3.3474E-11	-7.9940E-01	-2.0337E-12
-4.9441E-02	-8.4281E-09	-4.9603E-02	1.0252E-10	-4.9605E-02	1.7309E-10	-7.7462E-01	-2.2876E-11	-7.7447E-01	1.6305E-11
-3.6955E-04	-7.8440E-09	-5.1022E-04	1.0848E-10	-4.6969E-04	1.8144E-10	-7.4964E-01	-4.8149E-11	-7.4953E-01	2.0504E-13
4.9536E-02	-7.9870E-09	4.9403E-02	-1.6928E-10	4.9448E-02	-9.1553E-11	-7.2472E-01	-6.2871E-11	-7.2458E-01	-1.5566E-11
9.9409E-02	-8.2851E-09	9.9251E-02	-3.3617E-10	9.9323E-02	-2.4891E-10	-6.9949E-01	1.1146E-11	-6.9935E-01	-3.1995E-11
1.4959E-01	-8.0466E-09	1.4952E-01	1.0014E-10	1.4953E-01	1.7250E-10	-6.7452E-01	7.1526E-12	-6.7441E-01	-2.3723E-12
1.9945E-01	-8.6188E-09	1.9933E-01	9.6560E-11	1.9936E-01	1.7762E-10	-6.4955E-01	2.5988E-12	-6.4944E-01	3.4690E-13
2.4934E-01	-8.5950E-09	2.4921E-01	1.0133E-10	2.4926E-01	1.7989E-10	-6.2460E-01	8.3923E-12	-6.2448E-01	-1.4043E-12
2.9956E-01	-8.5831E-09	2.9946E-01	1.0371E-10	2.9946E-01	1.8144E-10	-5.9965E-01	2.0826E-11	-5.9953E-01	-1.3753E-11
3.4948E-01	-8.5712E-09	3.4933E-01	1.0610E-10	3.4941E-01	1.8930E-10	-5.7472E-01	1.0777E-11	-5.7461E-01	-3.7819E-11
3.9938E-01	-8.0943E-09	3.9924E-01	9.8944E-11	3.9928E-01	1.9312E-10	-5.4944E-01	6.6996E-12	-5.4936E-01	-1.0079E-11
4.4956E-01	-8.4639E-09	4.4946E-01	1.0490E-10	4.4951E-01	1.8954E-10	-5.2447E-01	1.4424E-12	-5.2442E-01	-7.3445E-12
4.9949E-01	-8.6427E-09	4.9937E-01	1.0371E-10	4.9938E-01	1.9264E-10	-4.9952E-01	-5.0545E-12	-4.9949E-01	-2.3458E-11
5.4936E-01	-8.4758E-09	5.4926E-01	1.0729E-10	5.4927E-01	1.8668E-10	-4.7461E-01	-1.2171E-11	-4.7454E-01	4.9853E-11
5.9959E-01	-8.3923E-09	5.9951E-01	1.0610E-10	5.9947E-01	1.8299E-10	-4.4964E-01	-1.2589E-11	-4.4952E-01	-5.9128E-12
6.4947E-01	-8.5831E-09	6.4939E-01	1.0490E-10	6.4937E-01	1.7750E-10	-4.2472E-01	-1.1718E-11	-4.2463E-01	4.4709E-11
6.9938E-01	-8.3447E-09	6.9931E-01	1.1206E-10	6.9926E-01	1.7333E-10	-3.9943E-01	-1.0455E-11	-3.9937E-01	4.4742E-11
7.4957E-01	-8.6665E-09	7.4951E-01	1.0014E-10	7.4950E-01	1.7440E-10	-3.7450E-01	-1.0145E-11	-3.7437E-01	-1.1802E-10
7.9945E-01	-8.2135E-09	7.9935E-01	1.0490E-10	7.9937E-01	1.7428E-10	-3.4953E-01	-3.2663E-12	-3.4946E-01	-5.3725E-11
8.4935E-01	-8.3327E-09	8.4928E-01	1.0133E-10	8.4925E-01	1.7881E-10	-3.2458E-01	-3.3855E-12	-3.2450E-01	-1.5506E-11
8.9956E-01	-7.8917E-09	8.9948E-01	9.8944E-11	8.9946E-01	1.8454E-10	-2.9965E-01	-4.4346E-12	-2.9955E-01	-2.7030E-11
9.4945E-01	-7.9632E-09	9.4939E-01	1.0610E-10	9.4933E-01	1.8752E-10	-2.7438E-01	-4.2558E-12	-2.7434E-01	1.8764E-12
9.9936E-01	-8.1182E-09	9.9927E-01	1.2040E-10	9.9924E-01	2.0397E-10	-2.4941E-01	-7.8678E-12	-2.4935E-01	-1.5119E-11
1.0496E+00	-7.9989E-09	1.0495E+00	1.4901E-10	1.0494E+00	2.2793E-10	-2.2445E-01	-8.4162E-12	-2.2441E-01	-1.9554E-11
1.0995E+00	-8.0347E-09	1.0994E+00	2.1696E-10	1.0993E+00	2.9481E-10	-1.9948E-01	-5.8889E-12	-1.9944E-01	-2.1079E-11
1.1494E+00	-7.6294E-09	1.1493E+00	4.0054E-10	1.1493E+00	4.7815E-10	-1.7455E-01	-6.4373E-12	-1.7450E-01	-1.0865E-11
1.1996E+00	-7.5817E-09	1.1995E+00	9.0599E-10	1.1995E+00	9.7668E-10	-1.4965E-01	-5.0664E-12	-1.4960E-01	-7.7069E-12
1.2495E+00	-5.9128E-09	1.2494E+00	2.2388E-09	1.2494E+00	2.3133E-09	-1.2440E-01	-4.6492E-12	-1.2437E-01	-1.3813E-11
1.2994E+00	-1.6093E-09	1.2993E+00	5.8115E-09	1.2993E+00	5.9111E-09	-9.9440E-02	-4.5419E-12	-9.9428E-02	-1.6019E-11
1.3497E+00	8.2970E-09	1.3495E+00	1.5564E-08	1.3495E+00	1.5751E-08	-7.4506E-02	-3.6120E-12	-7.4484E-02	-1.6320E-11
1.3996E+00	3.4618E-08	1.3994E+00	4.1748E-08	1.3994E+00	4.2271E-08	-4.9591E-02	-4.0889E-12	-4.9560E-02	-1.2102E-11
1.4498E+00	1.0729E-07	1.4496E+00	1.1365E-07	1.4496E+00	1.1512E-07	-2.4619E-02	-5.2214E-12	-2.4579E-02	-1.3255E-11
1.4996E+00	3.0527E-07	1.4995E+00	3.0982E-07	1.4994E+00	3.1373E-07	-5.2929E-04	-6.3896E-12	-4.4107E-04	-1.5663E-11
1.5495E+00	8.5874E-07	1.5493E+00	8.5953E-07	1.5493E+00	8.7002E-07	2.4424E-02	-4.2462E-11	2.4493E-02	-7.1224E-11
1.5997E+00	2.4930E-06	1.5995E+00	2.4826E-06	1.5560E+00	1.0000E-06	4.9391E-02	-9.0718E-12	4.9453E-02	-1.2716E-11
1.6496E+00	7.6615E-06	1.6494E+00	7.6133E-06	1.5560E+00	1.0000E-06	7.4325E-02	-7.1049E-12	7.4415E-02	-1.1486E-11
1.6995E+00	2.6373E-05	1.6609E+00	1.0000E-05	1.5561E+00	1.0000E-06	9.9258E-02	-5.8770E-12	9.9280E-02	-1.1143E-11
1.7480E+00	1.0000E-04	1.6609E+00	1.0000E-05	1.5562E+00	1.0000E-06	1.2420E-01	-7.5340E-12	1.2421E-01	-1.0524E-11
1.7481E+00	1.0000E-04	1.6610E+00	1.0000E-05	1.5562E+00	1.0000E-06	1.4945E-01	-6.3539E-12	1.4950E-01	-7.6628E-12
1.7481E+00	1.0000E-04	1.6610E+00	1.0000E-05	1.5562E+00	1.0000E-06	1.7434E-01	-5.9962E-12	1.7442E-01	-1.2786E-11
1.7482E+00	1.0000E-04	1.6610E+00	1.0000E-05	1.5563E+00	1.0000E-06	1.9930E-01	-5.8055E-12	1.9934E-01	-6.5208E-11
1.7481E+00	1.0000E-04	1.6610E+00	1.0000E-05	1.5563E+00	1.0000E-06	2.2425E-01	-4.9710E-12	2.2432E-01	4.9198E-12
1.7481E+00	1.0000E-04	1.6610E+00	1.0000E-05	1.5563E+00	1.0000E-06	2.4919E-01	-5.4240E-12	2.4924E-01	2.0075E-12
1.7482E+00	1.0000E-04	1.6611E+00	1.0000E-05	1.5564E+00	1.0000E-06	2.7413E-01	-5.5432E-12	2.7417E-01	-3.7468E-12
1.7482E+00	1.0000E-04	1.6611E+00	1.0000E-05	1.5564E+00	1.0000E-06	2.9942E-01	-5.2810E-12	2.9948E-01	-5.5206E-12
1.7482E+00	1.0000E-04	1.6611E+00	1.0000E-05	1.5564E+00	1.0000E-06	3.2437E-01	-6.3658E-12	3.2446E-01	3.5524E-12
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												1.4453E+00	1.0000E-07	1.3297E+00	1.0000E-08
												1.4453E+00	1.0000E-07	1.3297E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3297E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3297E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3298E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3297E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3298E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3299E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3298E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3298E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3299E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3298E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3299E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3298E+00	1.0000E-08
												1.4455E+00	1.0000E-07	1.3299E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3300E+00	1.0000E-08
												1.4454E+00	1.0000E-07	1.3299E+00	1.0000E-08
												1.4455E+00	1.0000E-07	1.3300E+00	1.0000E-08
												1.4455E+00	1.0000E-07	1.3300E+00	1.0001E-08
												1.4455E+00	1.0000E-07	1.3301E+00	1.0000E-08
												1.4455E+00	1.0000E-07	1.3300E+00	1.0000E-08
												1.4455E+00	1.0000E-07	1.3301E+00	1.0001E-08
												1.4455E+00	1.0000E-07	1.3301E+00	1.0001E-08
												1.4455E+00	1.0000E-07	1.3300E+00	1.0000E-08
												1.4455E+00	1.0000E-07	1.3300E+00	1.0000E-08

Characteristic Curve at different Temperatures

An interesting feature of semiconductor devices is that their current-voltage-characteristic is temperature dependend. To show this dependence the LED was put into the (Vötsch VT4002 climate chamber(again both the Keithley 2600 Python library and the Vötsch VT4002 Python library have to be stored in the same folder as the Pyhton scripts). The following script performs a temperature sweep and when a predefined temperature is reached, the diode characteristic is recorded.

from voetsch import VT4002
from KeithleyV15 import SMU26xx
import matplotlib.pyplot as plt
import time
import datetime
import csv
import numpy as np

#some improvement ideas fo this script
	# this script makes a temperature sweep and performs two
	# meaurements:
		#measureing the voltage needed to drive a bias
		#current through the diode at different temperatures
		
		#record a U/I curve at defined temperatures
	#to improve data management we recommend putting more
	#information in the file names (type of measurement,
	#device measured, temperature at which the U/I curve got
	#measured etc...).

# troughout the script the operation mode of the SMU is
	# sometimes changed needlessly - setting the operation
	# mode just before a measurement is performed would be
	# better

# this script is an expanded version of an old one, so
	# the names of the variables and the places they are
	# defined are a bit confusing

# the implementation of the U/I measurement at specific
	# temperatures is quite brute force. a smarter way would
	# make the script more readable

""" ******* Configure the VT4002 climate chamber ******** """

# define connection parameters
ip_address = "129.27.158.42"
username = "simpacuser"   # fill in the username on the front of the climate chamber
password = "u1s2e3r4"   # fill in the password on the front of the climate chamber

climate_chamber = VT4002(ip_address, username, password)

climate_chamber.enable()

""" ******* Connect to the Sourcemeter ******** """

# initialize the Sourcemeter and connect to it
# you may need to change the IP address depending on which sourcemeter you are using
sm = SMU26xx("TCPIP0::129.27.158.13::inst0::INSTR")

# get one channel of the Sourcemeter (we only need one for this measurement)
smu = sm.get_channel(sm.CHANNEL_A)

""" ******* Configure the SMU Channel A ******** """

# reset to default settings
smu.reset()

#define a variable "current range" to be able to change it quickly for future measurements

current_range = 1E-5
current_range_for_name = str(current_range)

# setup the operation mode
	# current source makes the SMU generate a current and
	# measure the corresponding voltage (this gets changed
	# below, to measure a characteristic curve)
# setup the operation mode
smu.set_mode_current_source()
# set the voltage and current parameters
smu.set_voltage_range(2)
smu.set_voltage_limit(2)
smu.set_voltage(0)
smu.set_current_range(current_range)
smu.set_current_limit(current_range)
# the bias current is the current, that is applied to the
# diode in forward direction, while the temperatur dependent
# voltage gets measured
bias_current = 1E-6
smu.set_current(bias_current)
smu.display_voltage()

""" ******* For saving the data ******** """

# Create unique filenames for saving the data
time_for_name = datetime.datetime.now().strftime("%Y_%m_%d_%H%M%S")
filename_csv = 'VvsT' + time_for_name +'.csv'
filename_pdf = 'VvsT' + time_for_name +'.pdf'

# Header for csv
with open(filename_csv, 'a') as csvfile:
        writer = csv.writer(csvfile, delimiter=';',  lineterminator='\n')
        writer.writerow(["Voltage / V", "Temperature / C", "Current / A"])

# Create unique filenames for saving the data
#some suggestions for improvement of the data saving
	#the file name should identify the file uniquely
	#therefore specifying for example the bias current
	#and or the voltage range is suggested.

filename_csv_v = 'Diode_' + time_for_name +'.csv'
filename_pdf_v = 'Diode_' + time_for_name +'.pdf'

# Header for csv
with open(filename_csv_v, 'a') as csvfile:
        writer_v = csv.writer(csvfile, delimiter=';',  lineterminator='\n')
        writer_v.writerow(["Voltage / V", "Current / A"])

""" ******* Define voltage-sweep paramters ******** """

# define sweep parameters
sweep_start = 10
sweep_end = 40

# define variables we store the measurement in
data_temperature = []
data_voltage = []
data_current = []

# enable the output
smu.enable_output()

""" ******* Sweep the temperature ******** """

# define sweep parameters Voltage
sweep_start_v = -1.5
sweep_end_v = 2
sweep_step_v = 0.025
steps_v = int((sweep_end_v - sweep_start_v) / sweep_step_v)
print(steps_v)
smu.set_current(0)
smu.set_mode_voltage_source()
# define variables we store the measurement in
data_current_v = []
data_voltage_v = []

for T in range(sweep_start,sweep_end):
    temp = climate_chamber.get_actual_temperature()
    print('Temperature (°C): ' + str(temp))
    
    climate_chamber.go_to_temperature(T, 0.2, 60)

if 09.8<temp<10.2:
# Create unique filenames for saving the data
        time_for_name = datetime.datetime.now().strftime("%Y_%m_%d_%H%M%S")
        current_range_for_name = str(current_range)
        filename_csv_v = 'Diode_' + time_for_name +'.csv'
        filename_pdf_v = 'Diode_' + time_for_name +'.pdf'

# step through the voltages and get the values from the device
        for nr in range(steps_v):

# calculate the new voltage we want to set
            voltage_to_set_v = sweep_start_v + (sweep_step_v * nr)
        # set the new voltage to the SMU

smu.set_voltage(voltage_to_set_v)
        # get current and voltage from the SMU and append it to the list so we can plot it later
            [current_v, voltage_v] = smu.measure_current_and_voltage()
            data_voltage_v.append(voltage_v)
            data_current_v.append(current_v)
            print(str(voltage_v)+' V; '+str(current_v)+' A;'+ str(voltage_to_set_v) +'V')
        # Write the data in a csv
            with open(filename_csv_v, 'a') as csvfile:
                writer_v = csv.writer(csvfile, delimiter=';',  lineterminator='\n')
                writer_v.writerow([voltage_v, current_v])
        smu.set_current(bias_current)

if 19.8<temp<20.2:
# Create unique filenames for saving the data
        time_for_name = datetime.datetime.now().strftime("%Y_%m_%d_%H%M%S")
        current_range_for_name = str(current_range)
        filename_csv_v = 'Diode_' + time_for_name +'.csv'
        filename_pdf_v = 'Diode_' + time_for_name +'.pdf'

# Header for csv
        with open(filename_csv_v, 'a') as csvfile:
            writer_v = csv.writer(csvfile, delimiter=';',  lineterminator='\n')
            writer_v.writerow(["Voltage / V", "Current / A"])
    # step through the voltages and get the values from the device
        for nr in range(steps_v):

# calculate the new voltage we want to set
            voltage_to_set_v = sweep_start_v + (sweep_step_v * nr)
        # set the new voltage to the SMU

if 29.8<temp<30.2:
# Create unique filenames for saving the data
        time_for_name = datetime.datetime.now().strftime("%Y_%m_%d_%H%M%S")
        current_range_for_name = str(current_range)
        filename_csv_v = 'Diode_' + time_for_name +'.csv'
        filename_pdf_v = 'Diode_' + time_for_name +'.pdf'

# calculate the new voltage we want to set
            voltage_to_set_v = sweep_start_v + (sweep_step_v * nr)
        # set the new voltage to the SMU

if 39.8<temp<40.2:
# Create unique filenames for saving the data
        time_for_name = datetime.datetime.now().strftime("%Y_%m_%d_%H%M%S")
        current_range_for_name = str(current_range)
        filename_csv_v = 'Diode_' + time_for_name +'.csv'
        filename_pdf_v = 'Diode_' + time_for_name +'.pdf'

# calculate the new voltage we want to set
            voltage_to_set_v = sweep_start_v + (sweep_step_v * nr)
        # set the new voltage to the SMU

# disable the output
smu.disable_output()  
smu.set_mode_current_source()
    #Read out the current temperature and print the result to the console
temp = climate_chamber.get_actual_temperature()
print(' ')
print('Temperature (°C): ' + str(temp))

#get current and voltage from the SMU and append it to the list so we can plot it later
[current, voltage] = smu.measure_current_and_voltage()
data_voltage.append(voltage)
data_current.append(current)
data_temperature.append(temp)
print(str(voltage)+' V; '+str(temp)+' C; '+str(current)+' A')
    # Write the data in a csv
with open(filename_csv, 'a') as csvfile:
    writer = csv.writer(csvfile, delimiter=';',  lineterminator='\n')
    writer.writerow([voltage, temp, current])

# disable sourcemeter output
smu.disable_output()

# properly disconnect from the device
sm.disconnect()

""" ******* Plot the Data we obtained ******** """

plt.plot(data_temperature, data_voltage, 'x-', linewidth=2)

# set labels and a title
plt.xlabel('Temperature / C', fontsize=14)
plt.ylabel('Voltage / V', fontsize=14)
plt.title('Temperature-Voltage characteristic of a diode', fontsize=14)
plt.tick_params(labelsize = 14)

plt.savefig(filename_pdf)
plt.show()

#Bring the climate chamber to room temperture so you can safely open it
climate_chamber.go_to_temperature(20, 2, 60)

climate_chamber.disable()

smu.reset()

sweep_diode_temperature.py

The characterisitc curves for the temperatures 10°C, 20°C and 30°C are shown in the figure below:

10°C				20°C				30°C	
Voltage / V	Current / A	Voltage / V	Current / A	Voltage / V	Current / A
-1.5001E+00	-2.7418E-10	-1.5001E+00	-2.9087E-10	-1.5001E+00	-6.9141E-10
-1.4751E+00	-6.9141E-11	-1.4751E+00	-8.3447E-11	-1.4751E+00	-5.1260E-11
-1.4501E+00	1.5497E-11	-1.4501E+00	2.1458E-11	-1.4501E+00	6.4373E-11
-1.4251E+00	3.9339E-11	-1.4251E+00	2.0266E-11	-1.4251E+00	7.6294E-11
-1.4001E+00	6.1989E-11	-1.4001E+00	2.1458E-11	-1.4001E+00	9.0599E-11
-1.3751E+00	3.9339E-11	-1.3751E+00	2.7418E-11	-1.3751E+00	9.5367E-11
-1.3501E+00	4.8876E-11	-1.3501E+00	2.8610E-11	-1.3501E+00	9.0599E-11
-1.3251E+00	4.5300E-11	-1.3251E+00	3.9339E-11	-1.3251E+00	1.0490E-10
-1.3001E+00	5.1260E-11	-1.3001E+00	6.0797E-11	-1.3001E+00	8.3447E-11
-1.2751E+00	5.3644E-11	-1.2751E+00	4.0531E-11	-1.2751E+00	9.7752E-11
-1.2501E+00	6.9141E-11	-1.2501E+00	3.5763E-11	-1.2501E+00	1.0133E-10
-1.2251E+00	3.8147E-11	-1.2251E+00	4.2915E-11	-1.2251E+00	8.3447E-11
-1.2001E+00	3.8147E-11	-1.2001E+00	3.8147E-11	-1.2001E+00	9.5367E-11
-1.1751E+00	7.1526E-11	-1.1751E+00	3.8147E-11	-1.1751E+00	9.1791E-11
-1.1501E+00	5.9605E-11	-1.1501E+00	3.6955E-11	-1.1501E+00	9.8944E-11
-1.1251E+00	5.2452E-11	-1.1251E+00	3.2187E-11	-1.1251E+00	1.1206E-10
-1.1001E+00	4.6492E-11	-1.1001E+00	3.5763E-11	-1.1001E+00	8.3447E-11
-1.0751E+00	4.2915E-11	-1.0751E+00	7.1526E-11	-1.0751E+00	9.2983E-11
-1.0501E+00	5.0068E-11	-1.0501E+00	4.7684E-11	-1.0501E+00	1.0252E-10
-1.0251E+00	6.1989E-11	-1.0251E+00	4.7684E-11	-1.0251E+00	9.6560E-11
-1.0001E+00	5.6028E-11	-1.0001E+00	3.6955E-11	-1.0001E+00	1.0014E-10
-9.7508E-01	6.6757E-11	-9.7509E-01	5.6028E-11	-9.7508E-01	1.0490E-10
-9.5009E-01	9.5367E-11	-9.5010E-01	7.0334E-11	-9.5009E-01	9.1791E-11
-9.2506E-01	5.0068E-11	-9.2507E-01	5.0068E-11	-9.2506E-01	1.0371E-10
-9.0007E-01	5.7221E-11	-9.0008E-01	5.6028E-11	-9.0007E-01	9.1791E-11
-8.7509E-01	8.5831E-11	-8.7510E-01	6.6757E-11	-8.7509E-01	1.0490E-10
-8.5006E-01	8.3447E-11	-8.5007E-01	5.1260E-11	-8.5007E-01	9.8944E-11
-8.2508E-01	7.2718E-11	-8.2509E-01	4.7684E-11	-8.2508E-01	9.4175E-11
-8.0006E-01	6.7949E-11	-8.0006E-01	7.2718E-11	-8.0005E-01	1.0967E-10
-7.7507E-01	6.0797E-11	-7.7507E-01	4.0531E-11	-7.7506E-01	8.8215E-11
-7.5008E-01	6.3181E-11	-7.5008E-01	5.0068E-11	-7.5007E-01	8.9407E-11
-7.2506E-01	6.1989E-11	-7.2506E-01	3.8147E-11	-7.2506E-01	1.0967E-10
-7.0007E-01	6.5565E-11	-7.0007E-01	3.9339E-11	-7.0006E-01	9.1791E-11
-6.7504E-01	8.5831E-11	-6.7505E-01	6.0797E-11	-6.7504E-01	9.6560E-11
-6.5004E-01	6.5565E-11	-6.5005E-01	5.6028E-11	-6.5005E-01	1.1206E-10
-6.2507E-01	8.7023E-11	-6.2507E-01	8.1062E-11	-6.2506E-01	9.1791E-11
-6.0005E-01	1.0014E-10	-6.0005E-01	6.5565E-11	-6.0004E-01	9.1791E-11
-5.7506E-01	7.1526E-11	-5.7506E-01	5.1260E-11	-5.7505E-01	1.1683E-10
-5.5003E-01	6.3181E-11	-5.5004E-01	5.9605E-11	-5.5003E-01	9.1791E-11
-5.2504E-01	6.9141E-11	-5.2505E-01	4.1723E-11	-5.2504E-01	9.8944E-11
-5.0006E-01	8.8215E-11	-5.0006E-01	7.3910E-11	-5.0005E-01	1.1206E-10
-4.7503E-01	8.1062E-11	-4.7503E-01	7.1526E-11	-4.7502E-01	8.7023E-11
-4.5004E-01	7.6294E-11	-4.5004E-01	6.5565E-11	-4.5003E-01	1.0729E-10
-4.2502E-01	7.0334E-11	-4.2502E-01	6.3181E-11	-4.2501E-01	1.0014E-10
-4.0003E-01	7.1526E-11	-4.0003E-01	7.8678E-11	-4.0003E-01	9.4175E-11
-3.7503E-01	6.7949E-11	-3.7504E-01	8.2254E-11	-3.7503E-01	1.0371E-10
-3.5001E-01	9.2983E-11	-3.5001E-01	7.0334E-11	-3.5000E-01	8.8215E-11
-3.2502E-01	5.8413E-11	-3.2502E-01	7.0334E-11	-3.2502E-01	1.1087E-10
-2.9999E-01	6.3181E-11	-3.0000E-01	6.9141E-11	-2.9998E-01	8.8215E-11
-2.7500E-01	8.9407E-11	-2.7501E-01	6.4373E-11	-2.7500E-01	1.0490E-10
-2.5001E-01	7.7486E-11	-2.5002E-01	6.0797E-11	-2.5001E-01	1.1087E-10
-2.2499E-01	7.6294E-11	-2.2499E-01	5.8413E-11	-2.2498E-01	8.2254E-11
-2.0000E-01	9.6560E-11	-2.0000E-01	8.1062E-11	-2.0000E-01	8.8215E-11
-1.7498E-01	5.9605E-11	-1.7499E-01	9.1791E-11	-1.7498E-01	9.7752E-11
-1.4998E-01	7.6294E-11	-1.4999E-01	8.1062E-11	-1.4998E-01	1.1087E-10
-1.2500E-01	5.0068E-11	-1.2500E-01	7.2718E-11	-1.2500E-01	9.5367E-11
-9.9976E-02	9.6560E-11	-9.9985E-02	7.2718E-11	-9.9975E-02	9.5367E-11
-7.4988E-02	9.4175E-11	-7.4997E-02	1.0014E-10	-7.4986E-02	1.0133E-10
-4.9964E-02	8.7023E-11	-4.9974E-02	8.3447E-11	-4.9962E-02	9.1791E-11
-2.4977E-02	6.4373E-11	-2.4980E-02	7.8678E-11	-2.4974E-02	1.0967E-10
-2.7657E-05	9.2983E-11	-3.5524E-05	8.8215E-11	-2.8372E-05	1.2875E-10
2.4962E-02	7.6294E-11	2.4959E-02	9.0599E-11	2.4966E-02	1.1325E-10
4.9951E-02	8.8215E-11	4.9946E-02	7.1526E-11	4.9950E-02	1.1683E-10
7.4977E-02	9.2983E-11	7.4966E-02	9.6560E-11	7.4975E-02	1.3113E-10
9.9968E-02	1.0252E-10	9.9959E-02	8.7023E-11	9.9966E-02	1.0610E-10
1.2496E-01	8.4639E-11	1.2495E-01	8.3447E-11	1.2496E-01	1.1206E-10
1.4997E-01	8.1062E-11	1.4996E-01	6.9141E-11	1.4998E-01	1.2755E-10
1.7497E-01	8.4639E-11	1.7496E-01	7.3910E-11	1.7497E-01	1.1206E-10
1.9995E-01	8.2254E-11	1.9994E-01	9.7752E-11	1.9995E-01	1.2755E-10
2.2498E-01	1.0133E-10	2.2497E-01	9.1791E-11	2.2498E-01	1.1325E-10
2.4997E-01	8.3447E-11	2.4996E-01	9.7752E-11	2.4997E-01	1.2040E-10
2.7499E-01	8.5831E-11	2.7498E-01	1.0848E-10	2.7499E-01	1.3232E-10
2.9998E-01	9.7752E-11	2.9997E-01	9.1791E-11	2.9998E-01	1.2517E-10
3.2498E-01	1.0371E-10	3.2497E-01	8.2254E-11	3.2498E-01	1.3948E-10
3.5001E-01	9.6560E-11	3.5000E-01	1.0610E-10	3.5000E-01	1.2636E-10
3.7499E-01	8.3447E-11	3.7498E-01	9.2983E-11	3.7499E-01	1.1444E-10
3.9999E-01	1.0014E-10	3.9998E-01	9.0599E-11	3.9999E-01	1.2636E-10
4.2501E-01	9.1791E-11	4.2500E-01	8.4639E-11	4.2501E-01	1.1563E-10
4.5000E-01	7.8678E-11	4.4999E-01	8.2254E-11	4.5000E-01	1.2159E-10
4.7499E-01	1.0133E-10	4.7499E-01	1.1087E-10	4.7499E-01	1.3709E-10
5.0001E-01	8.4639E-11	5.0001E-01	1.2398E-10	5.0002E-01	1.0729E-10
5.2500E-01	1.1683E-10	5.2500E-01	9.0599E-11	5.2500E-01	1.2875E-10
5.5003E-01	1.0967E-10	5.5003E-01	1.1087E-10	5.5004E-01	1.1325E-10
5.7502E-01	7.8678E-11	5.7501E-01	9.2983E-11	5.7502E-01	1.2040E-10
6.0000E-01	1.0848E-10	6.0000E-01	8.9407E-11	6.0001E-01	1.3232E-10
6.2503E-01	9.8944E-11	6.2503E-01	9.1791E-11	6.2503E-01	1.1444E-10
6.5002E-01	1.0014E-10	6.5001E-01	9.0599E-11	6.5002E-01	1.2517E-10
6.7501E-01	8.3447E-11	6.7501E-01	1.0848E-10	6.7501E-01	1.1206E-10
7.0003E-01	8.3447E-11	7.0002E-01	8.9407E-11	7.0004E-01	1.1683E-10
7.2503E-01	9.4175E-11	7.2502E-01	1.0610E-10	7.2503E-01	1.2875E-10
7.5001E-01	1.2040E-10	7.5001E-01	1.0371E-10	7.5000E-01	1.0967E-10
7.7504E-01	1.0610E-10	7.7503E-01	9.1791E-11	7.7504E-01	1.2875E-10
8.0002E-01	1.1325E-10	8.0002E-01	1.0014E-10	8.0003E-01	1.2279E-10
8.2504E-01	1.2398E-10	8.2504E-01	9.4175E-11	8.2505E-01	1.2517E-10
8.5004E-01	1.0133E-10	8.5003E-01	8.7023E-11	8.5004E-01	1.3471E-10
8.7502E-01	1.0848E-10	8.7502E-01	1.2875E-10	8.7503E-01	1.2636E-10
9.0005E-01	1.0729E-10	9.0004E-01	1.1444E-10	9.0005E-01	1.2875E-10
9.2503E-01	1.0252E-10	9.2503E-01	9.6560E-11	9.2504E-01	1.3351E-10
9.5003E-01	8.2254E-11	9.5003E-01	1.0729E-10	9.5003E-01	1.2755E-10
9.7505E-01	9.5367E-11	9.7505E-01	1.1683E-10	9.7505E-01	1.2398E-10
1.0000E+00	1.2040E-10	1.0000E+00	1.2755E-10	1.0000E+00	1.4663E-10
1.0250E+00	9.2983E-11	1.0250E+00	1.1206E-10	1.0250E+00	1.5497E-10
1.0501E+00	1.0371E-10	1.0500E+00	1.1921E-10	1.0501E+00	1.7047E-10
1.0750E+00	1.3232E-10	1.0750E+00	1.2398E-10	1.0750E+00	2.2411E-10
1.1001E+00	1.2517E-10	1.1001E+00	1.8239E-10	1.1001E+00	2.5630E-10
1.1251E+00	1.3232E-10	1.1251E+00	2.2411E-10	1.1251E+00	3.4094E-10
1.1500E+00	1.9193E-10	1.1500E+00	2.6941E-10	1.1500E+00	4.7565E-10
1.1751E+00	2.4557E-10	1.1751E+00	3.9458E-10	1.1751E+00	7.1764E-10
1.2000E+00	3.3736E-10	1.2000E+00	5.6028E-10	1.2000E+00	1.0610E-09
1.2250E+00	4.5180E-10	1.2250E+00	8.8573E-10	1.2250E+00	1.7214E-09
1.2500E+00	7.1049E-10	1.2500E+00	1.3876E-09	1.2500E+00	2.7478E-09
1.2750E+00	1.1063E-09	1.2750E+00	2.2113E-09	1.2750E+00	4.4000E-09
1.3000E+00	1.7893E-09	1.3000E+00	3.6073E-09	1.3000E+00	7.1216E-09
1.3250E+00	2.9099E-09	1.3250E+00	5.9283E-09	1.3250E+00	1.1541E-08
1.3500E+00	4.8983E-09	1.3500E+00	9.7537E-09	1.3500E+00	1.8810E-08
1.3750E+00	8.1790E-09	1.3750E+00	1.6104E-08	1.3750E+00	3.0705E-08
1.4000E+00	1.3716E-08	1.4000E+00	2.6661E-08	1.4000E+00	5.0163E-08
1.4250E+00	2.3024E-08	1.4250E+00	4.4162E-08	1.4250E+00	8.2059E-08
1.4500E+00	3.8787E-08	1.4500E+00	7.3298E-08	1.4500E+00	1.3443E-07
1.4750E+00	6.5354E-08	1.4750E+00	1.2188E-07	1.4750E+00	2.2093E-07
1.5000E+00	1.1040E-07	1.5000E+00	2.0323E-07	1.5000E+00	3.6501E-07
1.5250E+00	1.8725E-07	1.5250E+00	3.4082E-07	1.5250E+00	6.0682E-07
1.5500E+00	3.1888E-07	1.5500E+00	5.7491E-07	1.5500E+00	1.0173E-06
1.5750E+00	5.4708E-07	1.5750E+00	9.7930E-07	1.5750E+00	1.7272E-06
1.6000E+00	9.4532E-07	1.6000E+00	1.6855E-06	1.6000E+00	2.9751E-06
1.6250E+00	1.6519E-06	1.6250E+00	2.9492E-06	1.6250E+00	5.2306E-06
1.6500E+00	2.9359E-06	1.6500E+00	5.2783E-06	1.6500E+00	9.4557E-06
1.6750E+00	5.3384E-06	1.6750E+00	9.7241E-06	1.6523E+00	9.9999E-06
1.6999E+00	9.9999E-06	1.6762E+00	9.9999E-06	1.6523E+00	9.9999E-06
1.7000E+00	1.0000E-05	1.6762E+00	9.9999E-06	1.6524E+00	9.9999E-06
1.7000E+00	1.0000E-05	1.6762E+00	9.9999E-06	1.6524E+00	9.9999E-06
1.7000E+00	9.9999E-06	1.6762E+00	9.9999E-06	1.6524E+00	9.9999E-06
1.7000E+00	9.9999E-06	1.6762E+00	9.9999E-06	1.6524E+00	9.9999E-06
1.7000E+00	1.0000E-05	1.6762E+00	9.9999E-06	1.6524E+00	9.9999E-06
1.7000E+00	1.0000E-05	1.6762E+00	9.9999E-06	1.6524E+00	9.9999E-06
1.7000E+00	9.9999E-06	1.6762E+00	9.9999E-06	1.6524E+00	9.9999E-06
1.7000E+00	9.9999E-06	1.6762E+00	9.9999E-06	1.6524E+00	9.9999E-06
1.7000E+00	9.9999E-06	1.6763E+00	1.0000E-05	1.6524E+00	9.9999E-06
1.7001E+00	9.9999E-06	1.6763E+00	9.9999E-06	1.6524E+00	9.9999E-06
1.7001E+00	1.0000E-05	1.6763E+00	9.9999E-06	1.6524E+00	9.9999E-06

An intersting analysis of this measurement would be the operating temperature of the diode. Therefore the step size of the voltage sweep should be decreased to get more data points for higher voltages. With this additional data a comparision to the next measurement could be performed and the operating temperature could be estimated.

Temperature Dependence of the Diode Voltage

In the temperature sweep programm also a second measurement is implemented. This second measurement operates the SMU in current mode and measures the voltage that is needed to drive a predefined bias current of 1e-6 A through the LED for different temperatures. The results of this temperature sweep are shown in the following figure:

As expected with a higher divice temperature less voltage is needed to drive the same current through the LED.

Red Light Emitting Diode

Characteristic Curve at Room-Temperature

Characteristic Curve at different Temperatures

Temperature Dependence of the Diode Voltage