Study of unidirectional conductivity on the electrical discharge machining of semiconductor crystals

• The circuit of semiconductor EDM is a typical circuit with reverse-biased diode and linear resistance in series. This circuit has three typical parameters: breakdown point, conduction angle, and breakdown angle.
• Two diodes exist in the equivalent circuit with two metal electric feeders. One diode is forward-biased and the other is reverse-biased. The loop current will improve if the diode with larger contact area is under reverse bias.
• The discharge voltage of semiconductor EDM is almost the same with the no-load voltage. The body resistance also decreases, which results in the continuous increase of the discharge current.
• The nature of the plasma channel of semiconductor EDM is similar to that of metal EDM, which can be regarded as a 20 V voltage regulator tube. If the work piece is connected to anode in semiconductor EDM, the diode in the conduction side is reverse-biased and the avalanche voltage is only 42 V. If the work piece is connected to the cathode in semiconductor EDM, the diode in the discharge side becomes reverse-biased. The temperature in the arc plasma side is high, causing the breakdown voltage to be much higher than the theoretical calculation value 88.5 V.
• Favorable conditions for spark generation could be achieved and the process efficiency could be improved if positive polarity discharge is adopted for P-type silicon.

This paper investigates the unidirectional conductivity of semiconductor crystals machined by electrical discharge machining (EDM) by analyzing the properties of current–voltage (I–V) curves of the equivalent circuit. The simplified equivalent circuit of a semiconductor EDM consists of reverse-biased diodes and linear resistance. The I–V curve has three typical parameters, namely, conduction angle, breakdown angle, and breakdown point. The values of the conduction angle and the breakdown point are determined by the contact area of the reverse-biased diode, and the breakdown angle is determined by the value of linear resistance. Two diodes exist in the model with two metal electric feeders. To increase the current in this model, the diode with larger contact area should be reverse-biased. If the work piece is connected to anode in semiconductor EDM, the diode in the conduction side is reverse-biased and the avalanche voltage is only 42 V. If the work piece is connected to the cathode in semiconductor EDM, then the arc plasma, which is a termination with a small area, becomes reverse-biased. The temperature in the arc plasma side is high, causing the breakdown voltage to be much higher than the theoretical calculation value 88.5 V. As a result, when the work piece is connected to the cathode, spark production is difficult. Holes are bored on the P-type semiconductor crystals by positive and negative polarity, which could prove that machining with positive polarity is suitable for P-type semiconductor crystals during EDM. When the no-load voltage is set to 150 V, the penetration speed by positive polarity can reach 533 μm/min.

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