The heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor (BJT) which uses different semiconductor materials for the emitter and base regions, making a heterojunction. The HBT can handle signals of much higher frequencies, (up to several hundred GHz) than BJT. HBT is commonly used in modern ultrafast circuits, mostly radio-frequency (RF) systems, and in applications requiring a high power efficiency, such as RF power amplifiers in mobile phones. The idea of using a heterojunction is as old as the conventional BJT, dating back to a patent from 1951.
The principal difference between the BJT and HBT is in the use of different semiconductor materials for the emitter and base regions, making a heterojunction. This limits the injection of holes from the base into the emitter region, since the potential barrier in the valence band is higher than in the conduction band. Unlike BJT technology, this allows a high doping density to be used in the base. The high doping density reduces the base resistance while maintaining gain. The efficiency of the heterojunction is measured by the Kroemer factor.
Wet chemistry
Wet chemistry studies chemical reactions that occur in the liquid phase. It is also called bench chemistry because many of the tests performed are done at a lab bench.
Wet chemistry uses laboratory glassware, such as beakers and flasks, without quantitative chemical analysis using instrumentation. Many high school and college laboratories teach students basic wet chemistry methods.
Before the age of theoretical and computational chemistry, it was the main form of scientific discovery in the chemical field. So, some people call wet chemistry classic chemistry or classical chemistry. Today's society and quality control requirements need a high volume of wet chemistry. So, many wet chemistry methods have been automated and computerized for faster analysis.
Wet chemistry techniques can be used for qualitative chemical measurements, such as changes in color (colorimetry), but often involves more quantitative chemical measurements, using methods such as gravimetry and titrimetry. Some uses for wet chemistry include tests for:
Wet chemistry is also used in environmental chemistry settings and is used for to test:
It can also involve the elemental analysis of samples, e.g., water sources, for items like:
Korean Canadian
Korean Canadians are Canadians of Korean descent. According to South Korea's Ministry of Foreign Affairs and Trade, there are 223,322 Koreans or people of Korean ancestry in Canada. This is the fourth-largest population of Koreans outside of Korea (after Koreans in Japan and ahead of Koreans in Russia). Also, the government of South Korean guessed that there are more than 100,000 Koreans living in Canada illegally.
The first Koreans who came to Canada were Christians. They were sent by Canadian missionaries to study Christianity in Canada. Not many stayed in Canada though; and in 1965, the total Korean population of Canada was only about 70 people. However, after 1966's change of Canadian immigration laws, South Korean immigration to Canada grew. Between 1970 and 1980, 18,148 Koreans immigrated to Canada, and another 17,583 came in the 1990s. In the late 1990s, South Korea became the fifth-largest source of immigrants to Canada. Toronto has the most Koreans in Canada, but Vancouver has the highest growth rate of Koreans. They have increased 69% since 1996. Montreal has the third-most Korean immigrants at this time. In 2001, More Koreans went to Canada than to the United States. The number of temporary residents also grew after the Canadian government made visas not necessary; South Korea sent the most international students to Canada in the late 1990s. Many of the Koreans who came to Canada, came from the Korean population in China.
In the 1990s, many Koreans came to Canada because unemployment was high and income growth was low. It was the opposite in the United States at this time. They also came because the Canadian dollar was very weak, so they could get a lot of Canadian money from their Korean money. In the USA, they could get less American dollars. So they had more money if they came to Canada Lastly, in Korea at the time there was lots of anti-American feelings, and also many local native teachers came from Canada, so Koreans knew Canadians very well. .
Many Koreans in Canada are well educated, and many new immigrants and their children are skilled workers such as—doctors, professors, engineers, computer technicians—or businessmen.
Korean Canadians are usually one of three types: first generation immigrants, the “1.5” generation who began their schooling in Korea but moved to Canada in their childhood or teen years, and the second generation born in Canada. Each group is very unique, but Korean Canadians are very loyal to their Korean culture. They do this by local culture activities, university and alumni groups, seniors’ groups, language schools and business contacts. Vancouver and Toronto have Korean-language newspapers, and Korean television and radio programs. Korean dance, music art, as well as martial arts and Korean cuisine, are very popular in Canada.
Today, many Korean Canadians are Christians, and often attend Korean churches. A small number are Buddhist or are not religious.
Statistics were researched in 2007 by the South Korean Ministry of Foreign Affairs and Trade. The statistics showed that among Koreans living in Canada, 86,084 are Canadian citizens, 72,077 are permanent residents, 20,738 people have student visas, and 19,271 other people are temporary residents. The Canada 2001 Census said that there are 101,715 Canadians of Korean descent, but Korean community leaders and the media think that there are more than this. A big group could be temporary students studying English. According to the Canada 1996 Census, 53.6% of Korean immigrants to Canada graduated from University, compared to 23% of other Canadians. However, because Canadian employers prefer degrees from Canada, Korean immigrants often do jobs that are not what they studied in Korea; 40% worked in family-owned businesses, and their average income is only 67% of other Canadian's income.
Placodont
Placodonts were a group of Triassic marine reptiles which had heavy flat teeth. These teeth were used for crushing shellfish. Placodonts are in the Order Sauropterygia, the group that includes Plesiosaurs. Placodonts were mostly between in length, up to a maximum long. They are found in marine sediments of the Middle and Upper Triassic, 235–250 million years ago.
The first specimen was discovered in 1830, and the group was very widely distributed. Shellfish were common throughout the Mesozoic period, but this group of shellfish-eaters went extinct at the end of the Triassic. They were clearly a victim of some event which disturbed their lifestyle. The end-Triassic had several extinction events which are not well understood.
The earliest forms like Placodus, in the early to middle Triassic, looked rather barrel-shaped. Later placodonts had bony plates on their backs which protected their bodies while they were feeding.
By the Upper Triassic, these plates had grown so that some placodonts of the time looked like the sea turtles of modern day. Other placodonts developed plates that resembled the shells of horseshoe crabs and trilobites rather than those of sea turtles. These adaptations are examples of convergent evolution.
Because of their dense bone and heavy armour plating, these creatures would have been too heavy to float in the ocean. They would have used a lot of energy to reach the water surface. For this reason, and because of the type of sediment found with the fossils, it is suggested placodonts lived in shallow waters and not in deep oceans.