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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1 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 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). 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 diffuse from p-type region into n-type region while electrons diffuse 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. Owing to this one-direction conductance, current-voltage characteristic of p-n diode has a rectifying shape shown in Fig. 2.

2 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. 37

3 38 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 with temperature. An important parameter of p-n diode is rectification ratio η measured as the ratio of currents at forward and reverse voltages of the same magnitude. The value of η depends on voltage and commonly exceeds The ideal diode equation assumes that all the annihilation of the injected charge carriers occurs in the bulk areas of the device (not in the junction) via band-to-band or deep trap recombination (SRH recombination). Using this assumption the derivation produces the ideal diode equation (1) below with the factor n = 1. II = II 0 ee eeee nnkkkk 1. (1) However recombination does occur in other ways and in other areas of the device. This additional recombination can be taken into account by adjusting the factor n, which is known as ideality factor. Thus, the ideality factor of a diode is a measure of how closely the diode follows the ideal diode equation. Usually, n varies from 1 to 2. The Table 1 below shows the value of ideality factor for different mechanisms of recombination. Table 1. Mechanisms of recombination of charge carriers in p-n diode with corresponding ideality factors. Recombination Type n Description SRH, band to band (low level injection) SRH, band to band (high level injection) 1 Recombination limited by minority carrier. 2 Recombination limited by both carrier types. Auger 2/3 Two majority and one minority carriers required for recombination. Depletion region (junction) 2 Two carriers limit recombination. At reasonably high voltages (over 0.5 V for Si and Ge diodes), the current through a diode is given by the equation (2): II II 0 ee eeee nnnnnn, (2) where II 0 is inverse saturation current:

4 39 II 0 AAee EE gg kkkk, (3) where A is a constant and n is the ideality factor. By combining (2) and (3) together we obtain: II BBee EE gg kkkk + eeee nnnnnn (3) By taking a logarithm on both sides we can rearrange this formula to the expression: VV = kkkk ee TT + EE gg (4) where CC = llll II BB and EE gg is now represented in Volts As long as current remains constant, then C is constant, and the activation energy E g can be obtained from the intersection of the linear fit (y = ax+b) of equation (4) with vertical axis. The ideality factor n can be found as an inverse of the slope from the IV characteristic of the diode presented in coordinates lni versus V: lnii lnii nn eeee. (5) kkkk 6.3. Materials Global Specialties 1310 DC Power Supply BNC-2120 accessory board Hotplate 2x Beaker with water and dry ice respectively Type-K Thermocouple wires 1n4005 Silicon Diode (or equivalent) in rubber shielding 1n60 Germanium Diode (or equivalent) in rubber shielding Decade resistor box Connecting wires

5 Procedure Part A: Temperature dependence 1) Connect the diode and the shunt resistor as shown below: 2) Connect the thermocouple wires to ai1 BNC input 3) Channel ai2 should be used for current, and ai3 should be used for voltage in this setup 4) Connect the diode such that it is forward biased 5) Set the DC power supply to 5.0V 6) Set the Shunt Resistor to 100kΩ, and set Current Max/Min to ±50μμAA in LabView 7) Run the VI and make a graph of diode voltage vs temperature 8) Place the diode/thermocouple in the test-tube to be headed in boiling water, and then cool it down to dry ice temperature in a beaker of dry ice 9) Repeat steps 1-6 with the 1N60 Germanium diode Procedure B: Ideality factor 1) Connect the function generator in place of the DC power supply, and set to ±5V, 1Hz, triangle wave 2) Make a graph of I vs V in LabView to see characteristic curve 3) If the graph is upside down, reverse polarity of the diode 4) If more range is required, reduce Shunt Resistor to 10kΩ or 1kΩ (and adjust LabView Max\Min when doing so) 5) Repeat for both silicon and germanium diodes 6.5. Calculations and Discussion 1. Using current-voltage characteristics calculate rectification ratio for voltages in the range 0.1 to 1 V for each diode. Discuss the change in the rectification ratio with voltage. 2. Find linear fit for the measured dependences voltage versus temperature and determine the voltage at which the fitting lines intersect vertical axis. 3. Determine the bandgap of the semiconductor material the diodes are made of. 4. Present the current-voltage characteristics in the linear form of equation (5) and find their slope. 5. Calculate ideality factor of the measured diodes. Discuss the obtained data with using the information from Table Discuss possible ways of the increase of the maximum working temperature of p-n diode.

6 41 5. Discuss possible ways of the the 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? 4. At what voltages the measured diodes can be used as effective rectifiers?

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. 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