6. Bipolar Diode. Owing to this one-direction conductance, current-voltage characteristic of p-n diode has a rectifying shape shown in Fig. 2.

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Timothy Hines
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different temperatures; - to measure forward and reverse currents flowing in bipolar diode at three different temperatures and to determine the semiconductor material these diodes are made of and the

1 33 6. Bipolar Diode 6.1. Objectives - to experimentally observe temperature dependence of the current flowing in p-n junction silicon and germanium diodes; - to measure current-voltage characteristics of bipolar diodes at different temperatures; - to measure forward and reverse currents flowing in bipolar diode at three different temperatures and to determine the semiconductor material these diodes are made of and the dopants used for fabrication of n- and p-regions Principles When p- and n-type semiconductors are brought in contact with each other, an energy barrier is formed at the junction so that it allows current to flow freely in one direction (forward current under forward bias), but almost blocks the current in opposite direction (reverse current under reverse bias) (Fig. 1). a Fig. 1. Transport of electrons and holes in p-n diode under bias. (a) Forward bias. Most of majority charge carriers flow through the junction: holes drift from p-type region into n-type region while electrons drift from n-type region into p-type region. (b) Reverse bias. Only minority electrons can diffuse from p-type region into n-type region and minority holes can diffuse from n-type region into p-type region. b Owing to this one-direction conductance, current-voltage characteristic of p-n diode has a rectifying shape shown in Fig. 2.

The forward current I F is determined by the concentrations of majority charge carriers n(t) and p(t) in n- and p-regions of the diode respectively.

2 34 Fig. 2. Current-voltage characteristic of p-n diode. Forward bias (voltage) generates great current of majority charge carriers, while reverse bias can drag only little current of minority charge carriers. The forward current I F is determined by the concentrations of majority charge carriers n(t) and p(t) in n- and p-regions of the diode respectively. These concentrations depend temperature T and their values can be found from the following expressions (1): (1) where N D, N A, E D and E A are the concentration of donors, the concentration of acceptors, the activation energy of donors and the activation energy of acceptors correspondingly. Thus, forward current I F can be presented as a sum of currents of electrons and holes (2): where A and B are some constants showing the contributions of both currents. Experimentally, the measured current I F can presented as (3): (2)

35 where I F0 is a constant and E is a combined activation energy of donors and acceptors. Forward current is a function of temperature. The greater temperature, the greater forward current (Fig. 3).

3 35 where I F0 is a constant and E is a combined activation energy of donors and acceptors. Forward current is a function of temperature. The greater temperature, the greater forward current (Fig. 3). (3) Fig. 3. Dependence of current-voltage characteristic of p-n diode on temperature. (a) Forward bias. Forward current exhibits steeper increase and the onset voltage reduces as temperature rises. (b) Reverse current rises drastically with temperature. Thus measuring forward current I F1 and I F2 of a diode at two different temperatures T 1 and T 2, one can calculate effective activation energy of donors and acceptors used for the fabrication of the diode (4): ; (4) When p-n junction diode is biased reversely, a small current flows which is nearly independent of the bias. This current (saturation current) is a combination of the current caused by thermal generation of electron-hole pairs within the depletion region of the diode (generation current) and the diffusion of minority carriers (diffusion current). Although the saturation current is voltage independent, it does depend on temperature since both current constituents depend on the concentration of thermally generated charge carriers. The generation current I gen is proportional to the intrinsic carrier concentration in the depletion region: (5)

4 36 where E g is the bandgap energy. The generation current is the dominant contributor to the saturation current when the intrinsic carrier concentration is small compared to the dopant contributed carrier concentrations within the n and p regions and the minority carriers diffusing across the depletion region. In this case, the depletion region behaves like an intrinsic semiconductor. This condition is satisfied more perfectly for larger ratios E g /kt. The diffusion current I diff is given by the expression: So the total reverse current I R is given as a sum of these two currents: where A and B are constants. The experimental temperature dependence of reverse current can be written approximately in the form: where the parameter x varies between 1 and 2. If the diffusion current dominates, then x = 1. If the generation current dominates, then x = 2. The value of x can be found experimentally when measuring reverse current I R1 and I R2 of diode at two different temperatures T 1 and T 2 : (6) (8) (7) ; (9) That is, experimenting with a p-n junction diode at different temperatures, it is possible to determine the bandgap energy of the semiconductor material this diode is made of and the activation energy of the dopants used for fabrication of n- and p-type regions of the diode. ADDITION TO THEORY The current through an ideal diode is given by this equation Where is a constant determined by the reverse bias current (1) And B is some constant. By combining (1) and (2) together we obtain (2) By taking a logarithm on both sides, and isolating T, we can rearrange this formula to this (3)

5 37 (4) Where As long as current remains constant, then C is constant, and the activation energy obtained by a linear fit of, such that can be (5) Materials Global Specialties 1310 DC Power Supply SC-2075 Signal Conditioning Board Hotplate 2x Beaker with ice water Type-J Thermocouple wires with banana plug terminals 1n4005 Silicon Diode (or equivalent) in rubber shielding 1n60 Germanium Diode (or equivalent) in rubber shielding Resistance decade box Connecting wires Procedure LabView 1) Open the Characteristic Curve VI 2) Double click on the DAQ Assistant to open it 3) Add another input channel from ai0 and set range to ±50mV 4) Increase the size of the Split Signal block to make 3 outputs 5) Insert the Temperature Formula VI, wire the new input from the split signal to the V input, and place a graph on the Result output 6) Rewire the Build XY Graph so that X input is Device Voltage, and Y input is Result from Temperature Formula 7) Rewire the Error to pass through the Temperature Formula VI 8) Replace the Numeric Constant for resistance to a Numeric Indicator

6 38

39 Procedure - Breadboard 1) Connect the diode and the resistance decade box to the SC-2075 as shown below (same as characteristic lab) 2) Connect the thermocouple wires to CH0+ and CH0- on the

7 39 Procedure - Breadboard 1) Connect the diode and the resistance decade box to the SC-2075 as shown below (same as characteristic lab) 2) Connect the thermocouple wires to CH0+ and CH0- on the SC ) Place one end of the thermocouple in a beaker of ice water 4) Place the second beaker of ice water on the hotplate, submerge the diode and other end of the thermocouple in it 5) Set the DC power supply to 2.5V 6) Run the VI on Labview, and adjust the resistance on the decade box as well as on screen so that the current is at a constant 10µA 7) If initial temperature reading is higher than 273, then switch the two thermocouple leads, and restart the VI 8) Run the VI while heating water, and adjust resistance if necessary to keep constant 10µA 9) Repeat the experiment with the 1N60 Germanium diode END OF ADDITION 6.3. Experimental Equipment

8 40 - Silicon diode with connecting wires - Germanium diode with connecting wires - LabView measuring unit - Hot plate - Thermally insulated jar with dry ice, or liquid nitrogen - Signal generator - Power supply - Digital ammeter - Two beakers with transformer oil. -Thermometer (electronic temperature sensor) 6.4. Procedure 1. Adjust the LabView measuring set-up for measurements current-voltage characteristics using signal generator as a bias source. 2. Connect the measuring set-up to the germanium diode. 2. Set signal generator in linear sweep (triangle pulse) regime at a frequency of 1 Hz with amplitude ±3 V. 3. Obtain current-voltage characteristic of the diode on computer monitor, examine and record it. Make sure that it has a shape of rectifying characteristic shown in Fig Record current-voltage characteristic of the diode at room temperature. Place diode in beaker with oil, wait for half a minute, measure temperature of oil T 1 and take note of forward I F1 and reverse I R1 currents at voltages +2 V and -2 V. 5. Measure conductance of diode at high temperature. Heat beaker with oil to a temperature of 150 C. Immerse diode in oil, wait for half a minute, record current-voltage characteristic, measure temperature of oil and take note of forward and reverse currents at +2 V and -2 V bias. These is another set of data T 2, I F2 and I R2. 6. Measure conductance of diode at low temperature. Place diode in dry ice (liquid nitrogen) together with thermometer (temperature sensor), measure temperature of the cooled diode, record current-voltage characteristic and measure forward and reverse currents at bias +2 V and - 2 V. These is the third set of data T 3, I F3 and I R3. 7. Repeat the whole procedure for silicon diode Calculations and Discussion 1. For each diode, calculate effective activation energy of charge carriers E and coefficient x using formulae (4) and (9) for three combinations of temperature: - room temperature (T 1, I F1 and I R1 ) hot oil. (T 2, I F2 and I R2 );

9 41 - dry ice / liquid nitrogen (T 3, I F3 and I R3 ) room temperature (T 1, I F1 and I R1 ); - dry ice / liquid nitrogen (T 3, I F3 and I R3 ) hot oil (T 2, I F2 and I R2 ). 2. Compare the results. Discuss the reason of change of the activation energy and the coefficient x with temperature. Explain any discrepancies between your experimental results and the expected results. Based on your experimental results, discuss which contribution to the saturation current is dominant in each diode. 3. Discuss whether your measurements give an idea about the working temperature range of silicon and germanium diodes. 4. Discuss possible ways to increase the maximum working temperature of p-n diode. 5. Discuss possible ways to decrease the minimum working temperature on p-n diode. 6. Discuss if it is possible to make p-n diode which could work both at high and low temperatures Questions 1. Why germanium diode is inferior to silicon one in high temperature applications? 2. Why silicon diode is inferior to germanium on in low temperature applications? 3. Which of the two semiconductors (silicon and germanium) is more suitable for high voltage applications and why?

6. Bipolar Diode. Owing to this one-direction conductance, current-voltage characteristic of p-n diode has a rectifying shape shown in Fig. 2.

6. Bipolar Diode. Owing to this one-direction conductance, current-voltage characteristic of p-n diode has a rectifying shape shown in Fig. 2. 36 6. Bipolar Diode 6.1. Objectives - To experimentally observe temperature dependence of the current flowing in p-n junction silicon and germanium diodes; - To measure current-voltage characteristics