NPN bipolar transistor measurements with a sourcemeter

This page describes how to use a Keithley 2600 Series Sourcemeter to measure the characteristics of an NPN bipolar transistor. We will start with a common base configuration.

Common base
The base is grounded and a current is passed from the emitter to the base so that the emitter-base junction is forward biased (\(I_E\) is negative for an NPN transistor). The current between the collector an base is then plotted as a function of collector-base voltage. Most of the electrons injected into the base diffuse across it and are collected at the reverse biased collector-base junction. The collector current is almost equal to the emitter current. Although the currents are the same at the input and output, the voltage at the output can be much larger than at the input. This circuit exhibits power gain.

The base is the middle lead of the transistor. Connect the Hi of channel A to one of outside leads of the transistor, the Hi of channel B to the other outside lead of the transistor and the common ground to the base at the middle lead.

from KeithleyV15 import SMU26xx
import matplotlib.pyplot as plt
import time
import csv
import datetime
""" EXAMPLE: common base characteristic of an NPN Transistor  """

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

# initialize the SM and connect to it
sm = SMU26xx("TCPIP0::129.27.158.188::inst0::INSTR")

# enable the debug output (= lots of print commands)
# smu.enable_debug_output()

# get the two channels of the smu
smua = sm.get_channel(sm.CHANNEL_A)
smub = sm.get_channel(sm.CHANNEL_B)

# Create unique filenames for saving the data

time_for_name = datetime.datetime.now().strftime("%Y_%m_%d_%H%M%S")
filename_csv = 'npn_common_base' + time_for_name +'.csv'
filename_pdf = 'npn_common_base' + 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", "Current / A"])

""" ******* Configure the SMU Channel A (collector-base) ******** """

# reset to default settings
smua.reset()
# setup the operation mode and what will be shown at the display
smua.set_mode_voltage_source()
smua.display_current()

# set the measurement speed to med (so we can sweep really faster than in normal mode)
smua.set_measurement_speed_med()

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

# define the initial parameters for the channel
smua.set_voltage_range(20)
smua.set_voltage_limit(10)
smua.set_voltage(0)
smua.set_current_range(2E-2)
smua.set_current_limit(2E-2)
smua.set_current(0)

""" ******* Configure the SMU Channel B (emitter-base) ******** """

# reset to default settings
smub.reset()
# setup the operation mode and what will be shown at the display
smub.set_mode_current_source()
smub.display_voltage()

# set the measurement speed to med (so we can sweep really faster than in normal mode)
smub.set_measurement_speed_med()

# define the initial parameters for the channel
smub.set_voltage_range(2)
smub.set_voltage_limit(2)
smub.set_voltage(0)
smub.set_current_range(0.01)
smub.set_current_limit(0.01)
smub.set_current(0)

""" ******* do the actual measurement ******** """

# define base currents we want to run the sweep on
current_steps = [0, -2e-3, -4e-3, -6e-3, -8e-3, -10e-3]

# define sweep parameters for the collector base voltage
voltage_start = 10
voltage_end = -2
settling_time = 0
sweep_steps = 300

# enable the outputs
smua.enable_output()
smub.enable_output()

# do a voltage sweep for every base current we defined
for bias_current in current_steps:
    # some output so that we know what the smu is doing
    print("Emitter current = " + "{0:.2f}".format(bias_current*1000) + " mA")
    # set the emitter current
    smub.set_current(bias_current)
    # sweep and record the collector emitter voltage
    # we use the build in sweep function ... which is much faster than setting every value as a separate command
    [current, voltage] = smua.measure_voltage_sweep(voltage_start, voltage_end, settling_time, sweep_steps)
    data_voltage.append(voltage)
    data_current.append(current)
    plt.plot(voltage, current)
# 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 outputs
smua.disable_output()
smub.disable_output()

# properly disconnect from the SMU
sm.disconnect()

""" ******* Plot the data we obtained ******** """

# generate a nice looking legend
#legend = []
#for i in current_steps:
  #   legend.append("Ib = " + "{0:.2f}".format(i*1000) + " mA")
#plt.legend(legend, loc='upper left')

# set labels and a title
plt.xlabel('Collector-base voltage / V', fontsize=14)
plt.ylabel('Collector current / A', fontsize=14)
plt.title('Output characteristic of an NPN transistor', fontsize=14)
plt.tick_params(labelsize = 14)

plt.savefig(filename_pdf)
# show the plot
plt.show()

npn_common_base.py

Review the script to see what it does and then run it. Swap the outer leads (emitter and collector) and run the script again. One of these orientations is forward-active mode (which is usually what is wanted) and the other is reverse-active mode. The transistor will work in either direction but it suffers from punchthrough at low voltages in reverse-active mode. In forward-active mode, the emitter is connected to channel B. For negative collector-base voltages, both junctions are forward biased and the collector current is dominated by the forward biased collector-base junction. When both junctions are forward biased, this is called saturation.

Common emitter
A bipolar transistor is put in a common emitter configuration to determine the current gain. Connect the Hi of channel A to the collector, the Hi of channel B to the base, and the common Lo to the emitter. Download the npn_common_emitter.py script.

from KeithleyV15 import SMU26xx
import matplotlib.pyplot as plt
import time
import csv
import datetime
""" EXAMPLE: common emitter characteristics of an NPN Transistor  """

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

# initialize the SM and connect to it
sm = SMU26xx("TCPIP0::129.27.158.188::inst0::INSTR")

# enable the debug output (= lots of print commands)
# smu.enable_debug_output()

# get the two channels of the smu
smua = sm.get_channel(sm.CHANNEL_A)
smub = sm.get_channel(sm.CHANNEL_B)

# Create unique filenames for saving the data

time_for_name = datetime.datetime.now().strftime("%Y_%m_%d_%H%M%S")
filename_csv = 'npn_common_emitter' + time_for_name +'.csv'
filename_pdf = 'npn_common_emitter' + 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", "Current / A"])

""" ******* Configure the SMU Channel A (collector-emitter) ******** """

# reset to default settings
smua.reset()
# setup the operation mode and what will be shown at the display
smua.set_mode_voltage_source()
smua.display_current()

# set the measurement speed to med (so we can sweep really faster than in normal mode)
smua.set_measurement_speed_med()

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

# define the initial parameters for the channel
smua.set_voltage_range(20)
smua.set_voltage_limit(20)
smua.set_voltage(0)
smua.set_current_range(0.05)
smua.set_current_limit(0.05)
smua.set_current(0)

""" ******* Configure the SMU Channel B (base-emitter) ******** """

# reset to default settings
smub.reset()
# setup the operation mode and what will be shown at the display
smub.set_mode_current_source()
smub.display_voltage()

# set the measurement speed to med (so we can sweep really faster than in normal mode)
smub.set_measurement_speed_med()

# define the initial parameters for the channel
smub.set_voltage_range(2)
smub.set_voltage_limit(2)
smub.set_voltage(0)
smub.set_current_range(1E-3)
smub.set_current_limit(1E-3)
smub.set_current(0)

""" ******* do the actual measurement ******** """

# define base currents we want to run the sweep on
current_steps = [0, 2e-5, 4e-5, 6e-5, 8e-5, 10e-5]

# define sweep parameters for the collector emitter voltage
voltage_start = 0
voltage_end = 20
settling_time = 0
sweep_steps = 300

# enable the outputs
smua.enable_output()
smub.enable_output()

# do a voltage sweep for every base current we defined
for bias_current in current_steps:
    # some output so that we know what the smu is doing
    print("Base current = " + "{0:.2f}".format(bias_current*1E6) + " uA")
    # set the base current
    smub.set_current(bias_current)
    # sweep and record the collector emitter voltage
    # we use the build in sweep function ... which is much faster than setting every value as a separate command
    [current, voltage] = smua.measure_voltage_sweep(voltage_start, voltage_end, settling_time, sweep_steps)
    data_voltage.append(voltage)
    data_current.append(current)
    plt.plot(voltage, current)
# 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 outputs
smua.disable_output()
smub.disable_output()

# properly disconnect from the SMU
sm.disconnect()

""" ******* Plot the data we obtained ******** """
# generate a nice looking legend
#legend = []
#for i in current_steps:
  #   legend.append("Ib = " + "{0:.2f}".format(i*1000) + " mA")
#plt.legend(legend, loc='upper left')

# set labels and a title
plt.xlabel('Collector-emitter voltage / V', fontsize=14)
plt.ylabel('Collector current / A', fontsize=14)
plt.title('Output characteristic of an NPN transistor', fontsize=14)
plt.tick_params(labelsize = 14)

plt.savefig(filename_pdf)
# show the plot
plt.show()

npn_common_emitter.py

Review this script to see what it does then run it. The current gain is the ratio of the collector current to base current,

It is also possible to swap the emitter and collector terminals here and you will once again see that punchthrough is an issue.

The slope of the curves is caused by the Early effect. Use the data file to make plots of \(\beta=I_C/I_B\) as a function of \(V_{CE}\).







Semiconductor Laboratory