Influencing the Depletion Region: Biasing the Device

At normal temperatures, the thermal

energy of the semi- conductors is sufficient to produce a depletion region around the np-junction. However, the size of this region can be manipulated by adding electric voltages to each side of the np-device. In order to "forward-bias" the device and decrease the size of the depletion region, one should set up an electric field such that a positive voltage is in contact with the p-type end of the device and a negative voltage is in contact with the n-type semi-conductor as shown in Fig. 19. In this biasing mode, it is easier for current to flow because less energy is required to cross the depletion region and get from one side to the other.

The result of this applied voltage is that the holes in the p-type side of the device and the electrons in the n- type side of the device are both repelled from the applied voltages and "pushed" towards the depletion region. This results in a decrease in the width of the depletion region and, consequently, the energy needed to cross that barrier. This makes is easier for current to flow and, if the applied voltages are large enough (typically 0.6 V for silicon), the np-device will start to conduct freely.

Reverse-Biasing

In order to increase the size of the depletion region and thereby make it tougher for current to flow (larger energy required to cross the depletion region and get from one side to the other), one should "reverse-bias" the device. To do this, electric voltages are applied such that a positive voltage is in contact with the n-type end of the device, and a negative voltage is placed in contact with the p-type semi-conductor as shown in Fig. 20. When this electric field is set up, the positive voltage will attract the negative electrons from the n-type semi- conductor, drawing them away from the depletion region. Conversely, the negative voltage will attract the positive holes away from the depletion region. These new forces of attraction result in an enlargement of the depletion region and, consequently, the energy gap between regions.

This reverse-biasing results in a larger sensitivity for detecting radiation (as will be shown in the discussion of the SSBs), and this sensitivity will increase with increasing external voltages. Unfortunately, there is a limit to the sensitivity and the amount of external voltage that can be applied. This voltage is determined by the resistance of the particular semi-conductors. At some maximum applied voltage, the semi-conductor device will breakdown and will start to conduct freely.

Diodes

The fact that the np-junction can be fully conducting when forward-biased and relatively non-conducting when reverse-biased has led to a number of important devices. A diode works on the basic principles of the np-junction: fully conducting in one direction of current while non- conducting in the other.

General Principles of Operation

Using the principles of solid state physics, Silicon Surface Barrier (SSB) detectors operate on principles analogous to those used in gas ionization chambers. In gas ionization chambers, incoming ionizing radiation creates electron-ion pairs which are collected by an electric field. In SSBs, the energy lost in the detectors by ionizing radiation ultimately results in the creation of electron- hole pairs which are collected by an electric field. By using electrical contacts that are placed in the SSB, a current proportional to the ionization can be detected. This current can then be converted to a collection of electric pulses for analysis.