Dick Cappels' project pages http://www.projects.cappels.org

Avalanche Breakdown Photoemission and the Photoelectric Effect in Bipolar Transistors

Or
World's Simplest Positive to Negative Power Supply Converter.

By utilizing avalanche breakdown photoemission and the photoelectric effect a transistor can be made to supply a negative current source from a positive power supply. Current "gain" was measured.

Figure 1. The test fixture used to measure
the avalanche current and photoelectric current.

The Concept

Figure 2. Circuit that demonstrates the effect. Avalanche current for the emitter is taken from a positive power supply.
Notice the polarity of the current meter.

Light is generated in the base-emitter junction of bipolar transistors when the reverse biased junction is avalanched.  Some of this light causes the photoelectric effect in the base-collector junction, which can be observed as either a voltage across the base-collector junction or a current through the junction. The photoelectric current in the base flows in the opposite direction of the avalanche current through the base, thus enabling the generation of negative voltage or a negative current from the collector with respect to the base with a positive power supply.

The maximum observed open circuit output voltage was a little more than 0.3 volts.

The measurement fixture

Figure 3. Measurement fixture.

Photo currents were measured in the fixture shown in Figure 3. The reverse breakdown characteristics varied according to individual transistors, so the input voltage was adjusted to obtain approximately 20 milliamps for each transistor.

TABLE 1. NPN TRANSISTORS, SMALL SIGNAL

Table 1 shows the voltage measured across the 220 ohm resistor, the calculated avalanche emitter current, the measured photoelectric collector current, the emitter voltage for those transistors I thought to measure that voltage,  photo beta (pBeta), and calculated emitter power for each transistor.

Of the transistors tested, the NPN transistors with the highest Photo Beta were some Fairchild KSP-10 transistors manufactured in approximately 2014.

 input input input input calculated calculated calculated Type V/220 Iout Ve Iin pb Emttr. Power NT3906 NS 4.48 1.41E-11 7.22 20.4E-3 692.4E-12 147.0E-3 2SA1015GR 4.52 1.50E-06 11.19 20.5E-3 73.0E-6 229.9E-3 KTA1266 4.42 2.10E-06 10.87 20.1E-3 104.5E-6 218.4E-3 2N5401YTA 4.53 2.50E-06 9.2 20.6E-3 121.4E-6 189.4E-3 KSP2907 FAIR 4.66 3.00E-06 9.42 21.2E-3 141.6E-6 199.5E-3 KSP2907 FAIR 4.77 3.20E-06 9.33 21.7E-3 147.6E-6 202.3E-3 S8550D 4.56 3.30E-06 8.93 20.7E-3 159.2E-6 185.1E-3 MPSH55 MOT 4.44 6.10E-06 7.05 20.2E-3 302.3E-6 142.3E-3 2N3906 MOT 4.5 6.30E-06 7.26 20.5E-3 308.0E-6 148.5E-3 NT3906 NS 4.55 1.14E-05 7.29 20.7E-3 549.8E-6 150.8E-3 NT3906 NS 4.43 1.22E-05 7.41 20.1E-3 605.9E-6 149.2E-3 NT3906 NS 4.45 1.41E-05 7.21 20.2E-3 697.1E-6 145.8E-3 7 August 2015 Dick Cappels

TABLE 2. PNP TRANSISTORS, SMALL SIGNAL

Data for several PNP transistors, including calculated emitter power at approximately 20 milliamps is shown in Table 2.  Of the transistors tested, the best performing PNP transistors were some National Semiconductor NT3906 transistors from the 1970's. More modern 2N3906 transistors made later did not perform nearly as well.

It is clear that the Photo Beta performance can vary by manufacturer. Since neither base-emitter avalanche voltage not Photo Beta are specified by manufacturers, in the absence of further analysis and testing it must be assumed performance can vary considerably from transistor-to-transistor.

Chart 1. Photo Beta vs emitter current for a Fairchild KSP-10.

Chart 1 shows the Photo Beta as a function of emitter current. The change in efficiency might be partly a thermal effect given the power dissipation in the emitter rises from a few milliwatts to approximately 140 milliwatts.

Throgh-The-Rails_DC_Amplifier

## High-speed avalanche light emitting diode (ALED) and related apparatus and method US Patent US8344394 B1

Semiconductor light-emitting device  US 5107311 A (Google Search)

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