Basic Knowledge of Discrete Semiconductor Device Chapter III Transistors September 2018 Toshiba Electronic Devices & Storage Corporation © 2018 Toshiba Electronic Devices & Storage Corporation Types of Transistors Transistors are roughly classified into three types: bipolar, field effect and insulated gate bipolar. Bipolar transistors are current-driven devices. Field-effect transistors (FET) and insulated- gate bipolar transistors (IGBT) are voltage-driven devices. Transistors Bipolar transistors (BJTs) Small signal transistors Power transistors Bias Resistor Built-in Transistors (BRTs) Field-effect transistors MOSFETs Metal-oxide-semiconductor field-effect transistors (FETs) Junction FETs (JFETs) Junction field-effect transistors Insulated-gate bipolar transistors (IGBTs) 2 © 2018 Toshiba Electronic Devices & Storage Corporation Bipolar Transistors (BJTs) There are two types of bipolar transistors: NPN type and Collector PNP type. NPN-type products have low withstand voltage Collectorコレクタ( C(C)) to high withstand voltage. PNP-type products with N withstand voltage of 400 V or less, and particularly those Baseベース (B)(B) P IC with withstand voltage of 200 V or less, are mainstream. Base I There is an amplification function to convert small signals B N エミッタEmitter(E )(E) to large signals. The ratio of collector current IC and base current IB (IC/IB) is called DC current gain, denoted as hFE. Emitter When small current (IB) flows from base to emitter, Fig. 3-1(a) Symbol and structure of NPN transistor current of IB x hFE flows from collector to emitter. Collector BJTs are current-driven devices driven by base current. Collectorコレクタ( C(C)) Operation of NPN transistor P ベース Base current: Current from base to emitter Base (B)(B) N -IC Collector current: Current from collector to emitter Base -IB Operation of PNP transistor P エミッタEmitter(E )(E) Base current: Current from emitter to base Collector current: Current from emitter to collector Emitter Fig. 3-1(b) Symbol and structure of PNP transistor 3 © 2018 Toshiba Electronic Devices & Storage Corporation Bias Resistor Built-in Transistors (BRTs) BRTs are bias resistor built-in transistors. BJTs are often used together with resistors in electronic equipment. The mounting area can be reduced by using BRTs, which integrate a transistor and a resistor. Vcc LED C C (Ex. LED driving circuit) R1 R1 B B R2 R2 Vin Digital IC 2SC2712 E E NPN Type PNP Type Fig. 3-2(a) Application example of BJT Fig. 3-2(b) Equivalent circuit of BRT R1 R1 R1 R2 IB Noise Noise VBE Unstable operation Stable operation Tend to malfunction, Rare malfunction It is difficult to control It is easier to affected by noise of base. Malfunction is rare, without current control input Current directly flows to because noise goes limitation. current by inserting base of transistor, and it through R2, which works input resistance. turns on. as a bypass. Fig. 3-2(c) Why BJT needs resistance in its base circuit 4 © 2018 Toshiba Electronic Devices & Storage Corporation Junction Field-Effect Transistors (JFETs) <Operation of JFETs> JFET: Junction Field-Effect Transistor (1) In the N-channel junction field-effect transistor (Figure 3-3 (a)), when a voltage is applied between the drain and the source, electrons flow from the source to the drain. (2) When a reverse bias is applied between the gate and source, the depletion layer expands and suppresses the electron flow in (1). (Narrowing path of electron flow) (3) If the reverse bias voltage between the gate and source Fig. 3-3(a) Symbol and operation of N-channel- type JFET is further increased, the depletion layer blocks the channel and the flow of electrons stops. As shown above, voltage applied between gate and source controls the condition between drain and source. So FETs are voltage-driven devices. (Note: Direction of current flow is opposite to that of electron flow. The mechanism of widening of the depletion layer is the same as for diode.) Fig. 3-3(b) Symbol and operation of P-channel- type JFET 5 © 2018 Toshiba Electronic Devices & Storage Corporation Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is currently attracting the most D attention among transistors. ドレインDrain (D) (D) There are two types of MOSFET: N channel (See Fig.3- 4(a) Nch below) and P channel (See Fg.3-4(b) Pch n ID below). G Gate(G)ゲート (G) Nch is widely used for AC/DC power supplies, DC/DC n p converters, inverter equipment, etc., whereas Pch is used for load switches, high-side switches, etc. Sourceソース ( S(S)) The differences between the bipolar transistor and the S MOSFET are shown in Table 3-1. Fig. 3-4(a) Symbol and operation of N-channel MOSFET Table 3-1 Comparison of BJT and MOSFET D BJT MOSFET ドレインDrain(D) (D) (Current-driven device) (Voltage-driven device) p •Low input impedance •High input impedance -I G Gate(G)ゲート D •Large reverse transfer •Small reverse transfer (G) capacitance capacitance p n •Narrow safe operating •Wide safe operating region ソース (S) region •Low gate power Source (S) •Enables low-voltage consumption S operation (On voltage is 0.6- •Easy driving 0.7 V) Fig. 3-4(b) Symbol and operation of P-channel MOSFET 6 © 2018 Toshiba Electronic Devices & Storage Corporation Differences between BJT and MOSFET Explanation of the differences between ON/OFF operation of BJT and MOSFET. (1) Base current of BJT starts flowing when base voltage increases, and collector current is in proportion to this base current. This flow starts at about 0.7 V. This voltage is called the base-emitter threshold voltage (VBE). In order to make collector current flow, it is necessary to supply the base current and continuous driving power is required. (Low drive voltage, continuous driving power required) Fig. 3-5(a) Switching operation of BJT (2) Since the MOSFET forms a channel according to the gate- source voltage, this voltage must be a certain voltage or more. Once the channel is formed, the ON state continues and the drain current continues to flow, and so the power required for the driving is small. By discharging the charge accumulated in the gate and removing the channel, it shifts to the OFF state. (Driving voltage higher than BJT, small driving power) Fig. 3-5(b) Switching operation of MOSFET 7 © 2018 Toshiba Electronic Devices & Storage Corporation Structure and Operation of MOSFET Here we explain the operation of the MOSFET, referring to Fig. 3-6(a). (1) Apply voltage between drain and source with positive drain polarity. (Drain-source voltage: VDS) (2) Apply voltage between gate and source with positive gate polarity. (Gate-source voltage: VGS) (3) As a result, electrons are attracted to the p-type layer just under the gate insulator film, and part of the p-type layer is turned into n-type region. (This n-type region in this p-type layer is called the "inversion layer (channel)".) (4) As this inversion layer is completed, an n-layer path is formed from the drain to the source of the MOSFET. (n + ⇔ n - ⇔ inversion layer (n) ⇔ n +) (5) As a result, the MOSFET works at low resistance, and drain current is determined by applied VDS and load flows. Planar gate MOSFET Trench gate MOSFET Electrode Insulator film (π-MOS) (U-MOS) poly Si Electron ( N layer is formed. ) Fig. 3-6(a) Fig. 3-6(b) Structure and operation of planar gate MOSFET Structure and operation of trench gate MOSFET 8 © 2018 Toshiba Electronic Devices & Storage Corporation MOSFET Performance Improvement: Decision Factors of RDS(ON) 1) The MOSFET device structure is selected according to the required withstand voltage. The factors that determine the on-resistance RDS (ON) are as shown in Figure 3-7 and Equation 3- (1). Depending on the structure of the device, the ratio of factors determining on-resistance will change. 2) For example, many middle- and high-voltage MOSFETs (250 V or higher) have planar MOS (π-MOS) structure, and products with less than 200 V have more trench MOS (U-MOS). Therefore, when the withstand voltage VDSS = 600 V, Rdrift becomes the dominant factor, and in the case of 30 V, the ratio of Rch + RJFET is high. Planar MOS (π-MOS) Trench MOS (U-MOS) Fig. 3-7(a) Fig. 3-7(b) ON resistance decision factors of D-MOS ON resistance decision factors of trench MOS RDS(ON）= Rsub + Rdrift + RJ-FET + Rch + RN+ , RDS(ON）= Rsub + Rdrift + Rch + RN+ ・・・・・・・・・・・ Equation 3-(1) In the case of VDSS=600 V, the order is Rdrift >> Rch > RJ-FET , RN+ , Rsub and RDS(ON) depends on Rdrift In the case of VDSS=30V, the order is Rch >> Rdrift > RN+ , Rsub. Dependence of RDS(ON) on Rch can be minimized by fine patterning of trench MOS structure. 9 © 2018 Toshiba Electronic Devices & Storage Corporation MOSFET Performance Improvement: Approach to Low RDS(ON) Summary of approach to low ON resistance Rsub Rdrift Rch Methods Disadvantages Improvements Fine pattern Increment of Shorter channel Ciss Fine pattern Same as Trench & Trench above shorter channel Long Super junction production Single epitaxial Thin wafer TAT (High density Difficult to Thin-wafer of N++) produce technology We are pursuing the following countermeasures Source Gate for the biggest problem of MOSFET: ”How to effectively reduce on-resistance by effectively utilizing the element area“ 1. High voltage: Reduce resistance of Rdrift by the advanced super-junction process explained on the next page. 2. Low voltage: Minimize resistance of Rch by fine patterning of trench structure and reduce Drain resistance of Rsub by thinning wafer Fig. 3-8 Factors for ON resistance of MOSFET 10 © 2018 Toshiba Electronic Devices & Storage Corporation MOSFET Performance Improvement: Super-Junction MOSFETs (SJ-MOS) (1) SJ-MOS has pillar-shaped P layer (P pillar layer) in N layer.