Punch through float-zone silicon phototransistors with high linearity and sensitivity

In this paper, we propose, analyze and demonstrate a high-purity float-zone (FZ) silicon phototransistor operating at the punch through state with high linearity and sensitivity. Those phototransistors were fabricated on high-purity FZ silicon substrates; the dependence of the sensitivity on incident optical power and bias voltages has been investigated to light with a wavelength of 0.83 μm from a laser diode. In accordance with theoretical prediction, it shows that the linearity reaches the highest when the base of the phototransistor is completely depleted (punch through), and the fitting goodness of output R2 is 0.9954 over a 40 dB range from 0.15 to 1500 nW. The sensitivity of the phototransistor can be as large as 40–70 A/W at the punch through state, which is equivalent to an internal current conversion gain of about 75–130. The stability of 1% in the sensitivity for the punch through phototransistor with an internal current conversion gain of 130 can be obtained if the bias voltage and operating temperature can be stable to about 2.5% (1 in 40 V) and the temperature to ±2 °C.