Categorized |Design Basics, General Basics, Power Electronics Basics

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Posted on 11 February 2020

Charge carrier lifetime refers to the average time taken for a minority charge carrier (electron or hole) to recombine with one of its counterparts (with opposite charge). The ability to calibrate charge carrier lifetime in semiconductors provides a means of controlling certain properties when producing power semiconductor devices. Charge carrier lifetime in semiconductors is adjusted by inducing defects in the semiconductor's crystal lattice.

Charge Carrier Recombination

Recombination refers to reuniting of a free electron with a defect electron (hole) such that both charge carriers vanish.

Figure 1. Charge Carrier Recombination in Silicon

The recombination rate is proportional to excess concentration of the charge carriers compared to the equilibrium state caused, for example, by forward current. The recombination rate is determined by the concentration of minority charge carriers. The number of excess charge carriers decreases exponentially:

$n \propto e^{\frac{-t}{\tau}}$

The charge carries (electrons and holes) recombine either directly with one another or are gathered in a recombination center where they then proceed to recombine. The strongest recombination takes place on the silicon surface where there is also a break down of the crystal grid.

Storage charging is reduced by shortening charge carrier lifetime. Reducing charge carrier lifetime leads to short switching time trr in diodes, shorter circuit commutated turn-off time tq for thyristors (fast thyristors), and shorter turn-off times toff for IGBTs. In general, this means that the turn-off losses in the component are reduced. The charge carrier lifetime has no effect on the turn-on losses within the component. Reducing charge carrier lifetime goes hand in hand with an increase in forward voltage as well as an increase in reverse current. Trigger current in thyristors also increases following a  reduction in charge carrier lifetime.

Figure 2. Additional switching losses in the IGBT due to return current in the diode

In a series connection of IGBT and free wheeling diode (half-bridge), the diode presents a short circuit upon commutation of the current from the diode to the IGBT during the storage time ts. The diode return current flows as additional current in the IGBT (but not within the load) which increases the turn-on losses in the IGBT.

Calibrating Charge Carrier Lifetime by Inducing Crystal Defects

Charge carrier lifetime can be calibrated using various methods all of which involve creating crystal defects on the silicon. Crystal defects can be caused by high doping. High numbers of foreign or impurity atoms lead to crystal defects or at the very least to tension within the crystal lattice. This in turn reduces charge carrier lifetime.

Crystal defects caused by high temperature

Crystal defects can also be inflicted on the crystal lattice using very high temperatures. When exposed to extremely high temperatures, atoms within the crystals move at high speed relative to one another. This creates empty spaces within the lattice (solids expand at high temperatures). If the temperature applied on the crystal drops drastically, the empty spaces and other defects become frozen in place. The resulting reduction of charge carriers can be reversed through tempering (heating) and slow gradual cooling of the crystal.

Another way of creating crystal defects is using electron radiation. Very fast electrons, accelerated by high electric voltages amounting to one or more million Volts have enough energy to deflect or remove silicon atoms from their space on the crystal lattice. These displaced atoms find space elsewhere on the lattice, mostly as interstitial atoms with reduced lifetime. Electron radiation is the only method which creates a homogeneous reduction of lifetime within the whole crystal. The defects can be reversed using high temperature treatments. This also applies for ion radiation.

Ion implantation

Ion radiation, also referred to as ion implantation, has the same effect as electron radiation on the crystal lattice. Ions, however, have limited ion type and energy dependent depth of penetration. Crystal defects are inflicted at this depth.

The defect distribution is inhomogeneous. The best components are created when the depth of penetration coincides with the P-N Junction, for example in a CAL diode (Controlled Axial Lifetime).

Heavy metal diffusion

Crystal defects can also be created by diffusing heavy metals into silicon. The heavy metals create recombination centers within the silicon, where electrons and holes are held and recombine. Gold and platinum are frequently used. Application of heavy metals on silicon is done through deposition, sputtering, or in a liquid dope. Gold and platinum diffuse very fast into silicon and spread in proportion to the doping profile. Diffusion temperatures range from 700°C to 900°C. Gold causes conductivity to decrease a little but causes reverse current to increase tremendously. Platinum has the opposite effect. The greatest disadvantage of using heavy metal diffusion, however, is that it is rather difficult to control.

What is a Semiconductor?

Semiconductor Doping

P-N Junction

Silicon Production

Neutron Transmutation Doping of Silicon Rods

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