1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a semiconductor device having a PIN (Positive Intrinsic Negative) diode.
2. Description of the Background Art
In these years, inverters are used in those fields such as the field of industrial power units. For the inverter, usually a commercial power source (AC power source) is used. Thus, the inverter includes a converter unit first converting an AC voltage into a DC voltage (forward conversion), a smoothing circuit unit and an inverter unit converting the DC voltage into an AC voltage (inverse conversion). As a main power device of the inverter unit, an insulated gate bipolar transistor (hereinafter referred to as “IGBT”) capable of performing switching operation at a relatively high speed is chiefly employed.
In most cases, the load of the inverter is an electric induction machine (motor which is an inductive load). The inductive load is connected to a point of an intermediate potential between an upper arm element and a lower arm element, and electric current flows to the inductive load in both of the positive and negative directions. Therefore, in order to direct the current flowing in the inductive load from the end where the load is connected back to the power supply of a high potential and to direct the current from the end where the load is connected to the ground, a freewheel diode for circulating the current between the inductive load and the closed circuit of the arm elements is necessary. One of such freewheel diodes is a PIN diode.
In the inverter, usually the IGBT is operated as a switch to repeat the OFF state and the ON state so as to control the power energy. Regarding the switching of the inverter with an inductive load, the ON state is reached through a turn-on process while the OFF state is reached through a turn-off process. Here, the turn-on process refers to a change of the IGBT from the OFF state to the ON state while the turn-off process refers to a change of the IGBT from the ON state to the OFF state.
While the IGBT is in the ON state, current does not flow through the PIN diode and the PIN diode is in the OFF state. In contrast, while the IGBT is in the OFF state, current flows through the PIN diode and the PIN diode is in the ON state. In order to improve the switching characteristic of the inverter, it is necessary to change the PIN diode from the ON state to the OFF state as fast as possible. Therefore, the PIN diode is required to have a shorter lifetime. Meanwhile, a shorter lifetime results in a problem that the resistance in the ON state (ON resistance) is higher. Therefore, in order to lower the ON resistance while ensuring the switching characteristic of the PIN diode, it is necessary to precisely control the lifetime of the PIN diode. The PIN diode is disclosed for example in Japanese Patent Laying-Open Nos. 11-026779 and 2000-323724.
The above-described semiconductor device, however, has the following problem. Conventionally, the lifetime of the PIN diode is controlled using electron beam radiation or platinum diffusion to form a crystal defect or impurity trap. These methods change the lifetime of the whole substrate. In other words, the lifetime of the PIN diode is conventionally controlled by changing the lifetime of the whole substrate, thereby controlling the lifetime of the PIN diode. It is therefore a problem that optimization of the lifetime of the PIN diode adapted to characteristics of an applied apparatus is difficult.