Semiconductor device having vertical bipolar transistor

A bipolar transistor used in an oscillation circuit, comprising:

• a first semiconductor region of a first conductivity type, the first semiconductor region serving as a collector region;
• a second semiconductor region of the first conductivity type, the second semiconductor region being provided on the first semiconductor region and serving as a collector region whose impurity concentration is lower that of the first semiconductor region;
• a third semiconductor region of a second conductivity type, the third semiconductor region being provided in the second semiconductor region and serving as a base region;
• and
• a fourth semiconductor region of the first conductivity type, the fourth semiconductor region being provided in the third semiconductor region and serving as an emitter region,
• wherein the first and second semiconductor regions have an impurity-concentration profile set such that a depletion layer formed in the collector region reaches the first semiconductor region when a potential difference between the base region and the emitter region is substantially equal to a potential difference between the collector region and the emitter region.

A bipolar transistor used in an oscillation circuit, comprising:

• a first semiconductor region of a first conductivity type, the first semiconductor region serving as a collector region;
• a second semiconductor region of the first conductivity type, the second semiconductor region being provided on the first semiconductor region and serving as a collector region whose impurity concentration is lower than that of the first semiconductor region;
• a third semiconductor region of a second conductivity type, the third semiconductor region being provided in the second semiconductor region and serving as a base region;
• and
• a fourth semiconductor region of the first conductivity type, the fourth semiconductor region being provided in the third semiconductor region and serving as an emitter region,
• wherein the second semiconductor region is thinner than a depletion layer formed in the collector region when a potential difference between the base region and the emitter region is substantially equal to a potential difference between the collector region and the emitter region, and the second semiconductor region has an impurity-concentration profile set such that impurity concentration increases in a depth direction.