COURSE NAME: SEMICONDUCTORS Course Code: PHYS 473 Week No. 5

Transistor A three lead semiconductor device that acts as: (1)an electrically controlled switch (2)a current amplifier. 2

Bipolar Junction Transistor (BJT) A bipolar junction transistor (BJT) is a type of transistor that relies on the contact of two types of semiconductor for its operation. (1) NPN (2) PNP 3

BJT types A transistor consisting of two n- and one p-type layers of material called NPN transistor. A transistor consisting of two p- and one n-type layers of material called PNP transistor. 4

Bipolar Junction Transistor Fundamentals 5

Types of configurations Common Base Configuration: has Voltage Gain but no Current Gain. Common Emitter Configuration: has both Current and Voltage Gain. Common Collector Configuration: has Current Gain but no Voltage Gain. 6

Types of configurations 7

NPN BJT: How it works 8

NPN Transistor in a Complete Circuit (1) (2) 9

Transistor consists of two pn-junctions: emitter-base junction (EBJ) collector-base junction (CBJ) Operating mode depends on biasing. active mode – used for amplification cutoff and saturation modes – used for switching. A simplified structure of the npn transistor 10

Current Flow 11

Current Flow Forward bias on emitter-base junction will cause current to flow. This current has two components: electrons injected from emitter into base holes injected from base into emitter. 12

Basic principle of the BJT The basic principle of the BJT is: “The voltage between two terminals controls the current through the third terminal”. 13

14 Basic formulae of BJT

Question No. 1 An NPN Transistor has a DC current gain, (Beta) value of 200. Calculate the base current required to switch a resistive load of 4mA. Solution: 15

Question No. 2 If the collector current Ic = 7.95 mA and the emitter current Ie=8mA, then calculate current gain alpha. Solution: 16

Input characteristics of NPN transistor 17

Transistor Operation and Characteristic I-V curves The three terminals of the transistors and the two junctions, present us with multiple operating regimes. In order to distinguish these regimes we have to look at the I-V characteristics of the device. The most important characteristic of the BJT is the plot of the collector current, Ic, versus the collector-emitter voltage, V CE, for various values of the base current, I B as shown on the circuit. 18

Transistor Operation and Characteristic I-V curves Cutoff region: Base-emitter junction is reverse biased. No current flow. 19

Transistor Operation and Characteristic I-V curves Saturation region: Base-emitter junction forward biased. Collector-base junction is forward biased. Ic reaches a maximum which is independent of I B and β. V CE < V BE 20

Transistor Operation and Characteristic I-V curves Active region: Base-emitter junction forward biased Collector-base junction is reverse biased V BE < V CE < V CC 21

Transistor Operation and Characteristic I-V curves Breakdown region: I C and V CE exceed specifications damage to the transistor 22

Modes of Operation Depending on the bias condition on its two junctions, the BJT can operate in one of three possible modes: cut-off (both junctions reverse biased) active (the EBJ forward-biased and CBJ reversed) saturation (both junctions forward biased) 23

Summary of Bipolar Junction Transistors BJT is a three layer device constructed form two semiconductor diode junctions joined together, one forward biased and one reverse biased. There are two main types of BJT, the NPN and the PNP transistor. Transistors are "Current Operated Devices" where a much smaller Base current causes a larger Emitter to Collector current, which themselves are nearly equal, to flow. The most common transistor connection is the Common-emitter configuration. 24