Electric current in semiconductors
1. The thermistor has a resistance of 50kΏ at a temperature of 200C and at a temperature of 250C its resistance decreases to 42.5kΏ. Determine the average value of the temperature coefficient of resistance in this temperature interval.
Solution:
Analysis:
Δt = 50C = 5K, R20 = 50 000Ώ, R25 = 42 500Ώ, α = ?
The temperature coefficient of resistance of the thermistor is α = –0.03K–1
2.The average value of the temperature coefficient of resistance of a thermistor is α = –0.05K–1. By how much must the temperature of the thermistor increase so that its resistance decreases to half?
Solution:
Analysis:
The temperature of the thermistor must increase by 100C.
3.A thermistor connected in an electric circuit is heated by the flame of a gas burner. The dependence of the thermistor's resistance on temperature is given by a table. What is the temperature of the thermistor if the reading on the voltmeter is 3V and on the ammeter 15mA?
Solution:
Analysis:
U = 3V, I = 15.10–3A
According to the table, the thermistor has a resistance of 200Ώ when heated to a temperature of 3000C.
4.An NPN transistor is connected in an electric circuit. If the base current changes from 0.2mA to 0.3mA, the collector current changes from 5mA to 20mA, at a constant collector voltage of 4.5V. Determine the current gain factor β of the transistor.
Solution:
Analysis:
The current gain factor of the transistor at a voltage of 4.5V is β = 150.
5.A semiconductor sample cut from a silicon single crystal has the shape of a cuboid with a length of 5mm and a base area of 4mm2. On two opposite bases with an area of 4mm2 metallic contacts are formed. If a voltage of 4.5V is applied between the contacts, a current of 1.2mA flows through the sample at a temperature of 200C. After heating the sample to a temperature of 3000C the current increases to 2.16mA. What is the resistivity of silicon at temperatures of 200C and 3000C? Also determine the temperature coefficient of electrical resistance α.
Solution:
Analysis:
l = 5.10–3m, S = 4.10–6m2, U = 4.5V, I20 = 1.2.10–3A, I300 = 2.16.10–3A, ρ20 = ?, ρ300 =?
The resistivity of silicon is ρ20 = 3Ώ.m and ρ300 = 1.67Ώ.m.
The temperature coefficient of electrical resistance of silicon is α = –0.0016K-1
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