Transistors. "The Transistor was probably the most important invention of the 20th Century The American Institute of Physics

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1 Transistors "The Transistor was probably the most important invention of the 20th Century The American Institute of Physics The Nobel Prize in Physics 1956 William Bradford Shockley John Bardeen Walter Houser Brattain 1/3 of the prize 1/3 of the prize 1/3 of the prize USA USA USA Semiconductor Laboratory of Beckman Instruments, Inc. Mountain View, CA, USA b (in London, United Kingdom) d University of Illinois Urbana, IL, USA b d Bell Telephone Laboratories Murray Hill, NJ, USA b d. 1987

2 Transistors Zhores I. Alferov, Russian, 70, A.F. Ioffe Physico-Technical Institute, St. Petersburg Herbert Kroemer, German-born American, 72, Univ. of California, Santa Barbara Jack S. Kilby, American, 76, Texas Instruments, Dallas The prize is being awarded with one half jointly to: ZHORES I. ALFEROV, and HERBERT KROEMER for developing semiconductor heterostructures used in high-speed- and opto-electronics and one half to: JACK ST. CLAIR KILBY for his part in the invention of the integrated circuit

3 Transistors First Transistor, 1947 (Schokley, Bardeen, Brattain) First Integrated Circuit, 1958 (Jack Kilby) Intel s 1.7 Billion Transistor Chip 2004

4 What are transistors for? Transistor is a solid-state switch Mechanical switch 3V 120V Solar triggered switch (opto-switch) Thermo-switch 4

5 What are transistors for? Transistor is a logical gate a heart of computer 5

6 What are transistors for? Transistor is an amplifier (audio, video, microwave) 6

7 Bipolar Junction Transistors (BJT) A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined back-to-back. This forms a sort of a sandwich where one kind of semiconductor is placed in between two others. 7

8 BJT operation n p p n Forward polarity is applied to the left diode. Positive battery polarity attracts electrons. If the battery voltage exceeds the built-in voltage, there is a high current flowing through. Reverse polarity is applied to the right diode Positive battery polarity attracts electrons. There is no electrons on the p-side of the diode. The current is zero. 8

9 BJT operation n p p n Battery 1 Battery 2 Forward polarity is applied to the left diode. There is a high current flowing through it. The electrons entering p-region recombine with holes to form a current loop. Reverse polarity is applied to the right diode There is still no electrons on the p-side of the diode (they all move into battery 1. The current is zero. 9

10 BJT operation p n n Battery 1 Battery 2 The P-layer is made VERY THIN Forward polarity is applied to the left diode. Electrons from the left n-region are driven to the positive battery 1 terminal through the layer p. However, as the p-layer is very thin, most of the electrons miss the holes and shoot through the second barrier, toward the positive terminal of the battery 2. There is a high current 10 flowing through it.

11 n-p-n bipolar transistor Device structure Circuit symbol Emitter Base Collector C n p n B E p-n-p bipolar transistor Device structure Circuit symbol Emitter Base Collector C p n p B E 11

12 BJT Current-Voltage Characteristics V CE, V Very small base current (~5-75 µa) causes much higher collector current (up to 7.5 ma). The current gain is ~

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14 Transistor amplifiers characteristics Amplifiers GAIN = 10 An amplifier makes an electrical signal bigger and mirror precisely every small change of the input level. Vin Iin Vout Iout Amplifiers are used for analog signals, such as speech or music and for digital signals such as radar transmitters or communication system repeaters. 14

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16 BJT amplifier circuit analysis: Operating point R L V CE, V Collector current depends on two circuit parameters: the base current and the collector voltage. At high collector voltage the collector current depends on the base current only. For I base = 40 µa, I coll = 4 ma for any E C-E greater than 1.5 V 16

17 BJT amplifier circuit analysis: Operating point V CC R L V CE V CE, V For an arbitrary collector voltage, collector current can be found using the KVL. The KVL for the collector emitter circuit, V CC = I RL R L + V CE ; I V V CC CE RL = The R L current depends linearly on the collector voltage V CE RL Resistor R L and the C-E circuit of BJT are connected in series, hence I RL = I C For I b = 40 µa and V CC = 13V, the collector current I C = 4 ma For I b = 75 µa and V CC = 14V, the collector current I C = 4 ma 17

18 BJT amplifier gain analysis: 1 1. Input circuit The input voltage has two components: the DC bias and the AC signal V in AC signal amplitude DC bias Time DC voltage component biases the base-emitter p-n junction in the forward direction AC component is the input signal to be amplified by the BJT. 18

19 BJT amplifier gain analysis: 2 V CE, V 2. Output circuit The collector current has two components too. Base current I C AC current amplitude DC current Time DC and AC collector currents flow through the BJT in accordance with its I-V characteristics 19

20 BJT amplifier gain analysis: 3 3. Amplifier gain The resistance of the forward-biased B-E junction is very low. V CE, V Hence the base current I B V in /R 1 = (V indc +V inac )/R 1 The collector current I C does not depend on the collector voltage if the latter is high enough. Hence, I C β I B ; The voltage drop across the load resistance R 2 : V 2 = I C R 2 ; The output voltage V out = V CC - V 2 = V CC - I C R 2 ; V out = V CC - I C R 2 = V CC - β I B R 2 = V CC - β R 2 (V indc +V inac )/R 1 ; In most amplifiers only AC component of the output voltage is important: V outac = - β R 2 V inac /R 1 ; The amplifier voltage gain: k V = V outac /V inac = - β R 2 /R 1 ; 20